cleaning, pretreatment & surface preparation

SURFACE PREPARATION OF VARIOUS

METALS AND ALLOYS BEFORE PLATING

AND OTHER FINISHING APPLICATIONS

HUBBARD-HALL, WATERBURY, CONN.

There are three basic considerations for selecting the right cleaning and activationsolutions: what to use, when to use, and how to use. These are supportedby specific guidelines to help us make the right choices:

• Identify the base metal (type, alloy, surface characteristics)

• Limitations (process line, chemistries, temperature, time)

• Rinsing characteristics (parts, equipment, process line)The next set of considerations addresses the concern for sufficient, complete

soil removal. Focus on condition of the parts, soils, and existing surfacecoatings.

• Types of soils (oils, grease, shop dirt, buffing and polishing compounds,smuts, scales)

• Existing finishes (chromates, electroplated coatings, phosphates, rustinhibitors)

This issue of the Metal Finishing Guidebook contains additional discussions,references, and suggestions for cleaning and activation, as well as moredetailed information regarding filtration, rinsing, analysis, testing, and relatedsubjects.

SOAK CLEANING

Practical soak cleaning should efficiently remove organic soils. But it shouldalso meet F006 sludge reduction mandates, OSHA safety regulations, facilitateanalysis control, and simplify waste treatment. More chemically diverse oilsin stamping, forming, extruding, and rust proofing, coupled with reductionin solvent cleaning, make the soak cleaner selection more challenging. Liquid

concentrates and powder blends are formulated to meet the specific demandsof most soak-cleaning requirements. This includes cleaning ferrous and nonferrousmetals in the same solution. In some cleaning applications strong alkalis,such as sodium and potassium hydroxide, are beneficial. Conversely, these maybe detrimental for removing certain soils, such as chlorinated paraffin oils, orchemically attack nonferrous metals. Factors influencing soak cleaning—time,concentration, and temperature—should be determined by appropriate trialand evaluation, adhering to any specific limitations of the cycle or process.Displacement and emulsification mechanisms remove oils, grease, and shop dirtin this first step of surface preparation. In recent years displacement cleaninghas become more preferred to extend cleaner bath service life and simplify waste treatment. Automatic skimming devices, such as belts, coalescers, ultrafiltratiotank weirs, and overflow dams, are mechanical aids to facilitate oil and greaseremoval from displacement and emulsifying cleaners. Most soak cleaners meetthe operating criteria shown in Table I. Aluminum requires a specialized, differentapproach to cleaning, which will be dealt with separately.

Bulk parts may be soak cleaned in line or off line in basket or barrel operations.Table II provides an example of general soak cleaner constituents andapplicable concentration ranges. Trial evaluation and testing is required todetermine which specific formulation meets the soak-cleaning requirementswithin the specified cycle limitations.These are some appropriate cleanliness tests to confirm removal of soils:

• Absence of water breaks on parts rinsed after a weak post acid dip

• Flash rusting of ferrous parts

• White towel wipe cleaned surface, confirming absence of smuts, oils, and grease

• Absence of UV light fluorescence on cleaned surface previously coated with UV fluorescingoils.

• Immersion bronze, copper, or tin deposits on the cleaned, active, appropriately reactivesubstrate

• Mechanical deformation, bending of finished part or grinding of plated deposit

• Measure the contact angle of a drop of water on the cleaned metal surface.

ELECTROCLEANING

This method uses a DC rectifier to provide current, generating gas bubblesthat mechanically scrub the part. This is a powerful cleaning method thatcomplements the previous soak-cleaning step. Parts are predominantly positivelycharged, resulting in anodic or reverse current cleaning. To a lesserdegree parts may be negatively charged, resulting in cathodic cleaning. A thirdoption is periodic reverse, which takes advantage of anodic and cathodic cleaningmechanisms. Electrocleaning can be classified into four groups, meetingmost cleaning applications.

1 .Anodic. If preceded by a soak cleaner the electrocleaner’s mainfunction should be effective removal of metallic fines and oxide deposits.

Oil and grease removal should be secondary since an effective soakcleaner removes these soils as a primary function. The electrocleaner

concentrate can be either liquid or powder. The main ingredientis either sodium or potassium hydroxide as the source of solution

conductivity. Desmutters, descalers, and water hardness conditionersare also present. Buffers and inhibitors control the surface action,

moderate pH, and protect the base metal against the harmful effectsof the process itself and buildup of solution bearing contaminants.

Wetters and surfactants provide secondary cleaning to remove organicsoils. They also form a light foam blanket to significantly suppress

the effects of corrosive fumes during electrolyzing. The bath mayalso contain reducing agents to control certain contaminants suchas hexavalent chromium.

2. Cathodic electrocleaning generates twice the volume of gas bubblesversus anodic electrocleaning. The scrubbing action on parts is essentiallydoubled. This method is preferred for highly buffed and polishednonferrous metals such as brass, other copper alloys, and whitemetal. It prevents oxidation, tarnish, and surface attack, which wouldmar or destroy the desired surface brightness, leveling, and luster.

3. Periodic reverse (PR) is a specialized treatment for descaling andderusting steel. This procedure uses a switch on the rectifier (automatic

or manual) that changes polarity on the work between anodicand cathodic in specific time cycles for optimum cleaning. Parts usually

exit the process bath anodic, deplating any metallic smuts depositedin the previous highly scrubbing cathodic mode. This oxidation/

reduction/oxidation surface treatment softens scales, rust, and oxides,permitting chelates and complexors to dissolve them. These electrocleanersare also referred to as alkaline descalers.

4. Combination soak/electrocleaners meet the requirements of soakand electrocleaning in one step, one tank, or in separate process tanks.

In many applications this provides three advantages: simplifies prod-uct inventory, eliminates a rinse between soak and electrocleaner, andaccomplishes both cleaning steps in one tank. A disadvantage wouldbe shorter service life of the electrocleaner due to oil and grease buildup.Based on the metals electrocleaned, the alkalinity level is critical relative tothe caustic (sodium or potassium hydroxide) content. Nonferrous metals, suchas copper alloys, brass, and zinc, are best suited to electrocleaning in low- tomid-range caustic solutions. These solutions must also contain inhibitors, suchas silicate in ratio with caustic, for optimum conductivity with sufficient inhibitionof the zinc surface to prevent etching; Borax buffer and silicate inhibitorfor copper alloys and brass to prevent dezincification of brass and excess oxidationof copper alloys; high caustic for steel electrocleaning requirements suchas conductivity. The optimum caustic level also dissolves the iron hydroxidesurface film that forms, preventing splotchy brown stains and burning due tolow conductivity.Current densities are related to the base metal and whether the application israck or barrel. (See Table III.) Double cleaning cycles are ideally suited to cleaningand activating welded parts, such as wire goods, or heat-treated parts. Typicaloperating parameters are given in Table IV.Sufficiently electrocleaned parts should be free of smuts, oils, and grease.Scales and rust can be removed or softened prior to removal in the acid.

ACID TREATMENT

A more comprehensive discussion of this subject is found in the chapter“Pickling and Acid Dipping.”The consideration of knowing the metal or alloys processed remains acritical factor in selecting the optimum acid solution. Sensitive metals (brass,copper alloys, and zinc) require milder acid treatments. (See Tables V andVI.) Steels can be scaled and rusted, needing more aggressive treatment, evencathodic action. The acids used can be grouped into inorganic (hydrochloricor sulfuric) and organic (sulfamic, citric, gluconic, etc.). Accelerators, suchas chloride and fluoride, provide extra “bite” to improve pickling. Fluoridesactivate brass by dissolving lead smuts. Inhibitors prevent over pickling steelthat would result in raising excessive surface smuts or detrimental hydrogenembrittlement. Pickle aids help two ways: lower solution surface tension toimprove wetting and increase contact action. Wetting agents generate a lightfoam blanket to minimize corrosive sprays and mist and emulsify residual oilson parts or dragged into the acid bath. Deflocculents prevent the redepositionof soils.

Double cleaning cycles may employ an aggressive first acid to meet picklingdemands. The second acid should be a milder type sufficient to neutralize thesecond electrocleaner film while activating the surface as a last step before plating.One note of caution! Hydrochloric acid or chloride salts in the first acidpresents a special problem. Insufficient rinsing and draining of parts after thisdip can drag chloride, a contaminant, into the anodic second electrocleaner.A sufficient buildup of chloride (measured in part per million levels) in theelectrocleaner results in corrosive pitting of parts during the reverse anodiccleaning cycle. Specially inhibited electrocleaners minimize this condition,

increasing solution tolerance to chloride. Alternatively, a chloride-free acid, ifappropriate, should be used before the second electrocleaner. Heavily scaledor rusted steel parts may benefit from cathodic acid treatment. (See Table VII.)This process combines scrubbing action with activity of the acid solution to dissolve scales and rusInhibitors are special amines, substituted ureas, and glycol-based organiccompounds. Wetters may be anionic or nonionic types. Some wetters andinhibitors provide a filming action to inhibit attack on the base metal. Goodrinsing is required to remove any films, or in a double cleaning cycle use aninhibited/wetted acid as the first acid, followed by a simple mineral acid asthe second acid.Some modifications are made to cathodically remove heavy scales and rust.Acid dipped or pickled parts should be free of any organic soils, rust, scale,and smuts. This is the last process treatment bath before plating, painting,chromating, or final topcoat application.

ADDITIONAL CLEANING OPERATIONS

Electropolishing

This is an electrolytic process by which the substrate’s surface can be improvedusing a specific solution. Burrs, belt lines, scratches, scales, and other imperfectionsanywhere on the surface that is immersed and anodically charged willbe polished and refined. Electropolishing is current-density specific. In thisregard surface improvement occurs more readily than by mass finishing. A wide variety of common metals and alloys are successfully electropolished,especially the nickel-rich 300 series stainless steels. The electrolyte is typicallya mixture of mineral acids. Parts are predominantly racked. The ranges in theoperating parameters shown in Table VIII reflect the use of more than one typeof electrolyte.The solutions are acidic, typically composed of the following inorganic acids:chromic, fluoboric, hydrochloric, phosphoric, and sulfuric, in varying combinationsand strengths. Organic additives, such as glycols, help to condition thesurface during electropolishing.

Spray Cleaning

A wide variety of ferrous and nonferrous metals are cleaned in this optionalprocedure. Spray cleaning can be accomplished off line, as a precleaning step,or in the process line operation. It provides the following benefits:

• Low foaming cleaning action with displacement of soils

• Mechanical action facilitates cleaning

• Lower temperature ranges for energy savingsThe alkalinity level of the spray cleaner may range from near neutral (approximately

8) to high pH (14). This accommodates cleaning many metals (aluminum,brass, copper alloys, steel, stainless steel, and zinc). A desired or effectivechemistry lifts soils. The concentration of surfactants and wetting agents can below since mechanical action of spraying helps to dislodge soils. Displacement ofoils and grease allows them to be collected in a side tank and removed by skimmingor other separation device. This extends service life of the cleaner. It’s a realbenefit considering the heavy oil loading some incoming parts have. Removingdisplaced soils also prevents them from being sprayed on to parts that are tobe cleaned. Water hardness conditioners in the spray cleaner are invaluable toprevent nozzle pluggage. Typical operating conditions shown in Tables IX and X.

Mass Finishing

This method helps with off-line capabilities. Cleaning, deburring, descaling,and burnishing are surface improvements accomplished by mass finishing.The base metal is conditioned prior to additional surface finishing. Criticalareas are rounded out and burnishing can result in low rms value or high luster.The process combines mechanical energy and chemical action. The mechanicalcontribution is by tumbling in horizontal or oblique barrels or by using vibratorybowls. Specially blended chemicals are added in dilute-liquid form or lowconcentration

powders. They wet and react with the surface of parts, allowingother parts or special media (e.g., plastic, ceramic, or stone) to work on the parts.(See Table XI.) Mass finishing is especially helpful to seal porosity of aluminumand zinc before transfer to the plating line. If parts are to be mass finished or ifthis is a feasible option, trial evaluations are recommended to determine bestsuited equipment, media, and optimum: media-to-parts ratio, flow rates, and cycle times

SURFACE PREPARATION FOR SPECIFIC METALS & ALLOYS

The selection of specific working solutions should be determined by first evaluatingcandidate baths to meet or exceed requirements while adhering to cycleand handling limitations. Information is given for the more commonly encounteredmetals and alloys.

ALUMINUM

Aluminum is in a class by itself. It requires special handling, using some uniquesteps and considerations. Because of its light weight, heat capacity, durability,and corrosion resistance, aluminum is the metal of choice for many applications.A surface preparation cycle for electroplating or electroless plating generallyconsists of soak clean, etch, desmut, zincate, optional double zincate, strikeplate, and plate.It may seem easy but aluminum demands we invest in a quality effort toobtain a quality finish. Knowing the alloy designation is critical to selecting theoptimum bath chemistries for each step in the surface preparation cycle. (SeeTable XII.)Soak cleaning denotes no etching or attack of the base metal. (See Table XIII.)The cleaner bath pH ranges from 8 to 9.5. Ultrasonic soak cleaners also have

a similar chemistry profile. They differ in containing higher detergency levelsalong with selected solvents.Etching is accomplished using acidic or highly alkaline solutions. (See TablesXIV and XV.) This is the primary method of removing the outer, passive aluminumoxide skin. Etching also cleans the surface by undercutting soils and

lifting them off.Etchants and preferences:

• Alkaline—aluminum alloy extrusions, and stampings.

• Acidic—castings, polished parts, and prior to electroless nickel.When etched, some alloys (in the 5000, 6000 series, and castings) tend to

generate heavy smuts. This can lead to incomplete desmutting, detrimentallyaffecting the zincate treatment. Acidic etchants, being less aggressive, raise lesssmut. Typical desmutters are given in Table XVI.Other desmutter baths for consideration:

• 50-100% v/v nitric acid

• 15-25% v/v nitric acid + 10-20% v/v sulfuric acid

• Iron salts (ferric sulfate 3-4 oz/gal + 5-10% v/v sulfuric acid

• Universal tri-acid. Mixture of 50% v/v nitric acid + 20-25% v/v sulfuric acid

+ 1-2 lb/gal ammonium bifluoride, balance water to 100%.

Aluminum die cast alloys (see Table XVII) are based on six major elements:silicon, copper, magnesium, iron, manganese, and zinc. An example of applyingthe preferred desmutting bath can be illustrated by the following castingcomparisons.Tips:

• The universal tri-acid is best suited to desmut both of these castings;however, the formula containing 2 lb/gal of ammonium bifluoride isrecommended for the series 413 casting. That’s because of its greatersilicon content (41% more).

• Usually, the aluminum part will exit the desmut bath white and smutfree. Close inspection may also indicate a very fine surface etch, which isactually beneficial for zincating or chromating. If the part fails a whitepaper towel wipe (smutty) chances are slim that subsequentprocessingwill be successful.

• If the part gasses while immersed in the zincate there is a good possibilityit hasn’t been properly desmutted.

• If the desmut bath contains nitric acid be certain that good operating,compliant exhaust is in use to safely vent off nitric oxide fumes.

Zincating

This is an immersion treatment where a coating of zinc or zinc alloy is depositedover cleaned and activated aluminum. It is over this tightly surface-adherent filmthat plating can occur. There are three common zincating solutions:

1. Conventional zincate. This solution contains one metal, zinc, which isimmersion deposited over aluminum. It also contains an oxidizer, suchas sodium nitrate, conditioning the aluminum surface by mildly etchingit. Tartrates are included as complexors. The viscous working solution isconcentrated in sodium hydroxide (forming the chemical zincate). Bathsprepared from powdered concentrates must be cooled for several hoursbefore they can be used. 11-13 oz/gal sodium hydroxide, 2-3 oz/gal zinc oxide, 0.6-0.8 oz/gal sodium nitrate, 75-85°F (24-29°C), 0.5-2 minute

2. Conventional alloy zincate. Similar to the conventional zincate but differsas follows: contains iron, which forms an Fe-Zn alloy immersion deposit.Chemistry and operation as previous plus 0.2-0.4 oz/gal ferric chloride

3. Modified alloy zincate. Similar to conventional alloy zincate but differingas follows: contains several metals (commonly from among copper, iron,nickel and zinc, forming a unique alloy immersion deposit. Copper andnickel control rate of zincate formation and enhance its tight, cross-linkedstructure. Gluconate complexors (small amounts of cyanide are optional)used in place of tartrates, and much less sodium hydroxide. The workingsolution is much less viscous, providing improved rinsing characteristics.In each zincate described, the type and concentration of complexors arecritical to maintain solubility of the alloying metals.Which zincate to use? The conventional zincate is a good process whenapplied to high-purity aluminum alloys. But, it doesn’t provide as strongadhesion over 5000 and 6000 series alloys as do conventional alloy and modifiedalloy zincates. The latter provide a far stronger bonding to a wider rangeof aluminum alloys. This is due to formation of less porous, denser, uniformfilms. They also protect sharpened corners and edges of zincated parts frombeing worn and abraded in barrel plating.Tips on zincating include:

• Rinse well before the zincate bath to prevent drag in of desmut acidsolution. For example, fluorides will detrimentally affect the zincatefilm.

• The zincate should be an even gray or blue-gray color. Splotchiness mayindicate zincate solution components are out of balance.

• Poor adhesion of zincate to basis aluminum may be due to bath temperatureout of range or poor cleaning and surface preparation.

• Spongy zincate (thickened) is usually a result of excess immersion timeor too high bath temperature.

• A good, adherent zincate film will pass a Scotch tape pull.

Strikes

Copper

This bath is designed to coat the zincated surface with a strong bond, whilenot attacking it in the process. (See Table XVIII.) The deposit serves as an activesite for reception of subsequent electrodeposits, some of which might be highlyaggressive toward the unprotected zincate.Both formulas operate at 4 A/ft2 for 5 minutes or at 25 A/ft2 for 10 seconds,110-125°F (43-52°C). pH of first bath at 10-10.5. pH of second bath at 11.5-12.0. A proprietary grain refiner and anode corroder may also be added

Electrolytic Nickel

The purpose is the same as the copper strike, protect and seal the zincatefilm, preparing the part for reception of additional deposits. (See Table XIX.)The bath is operated at the same current density as Watts nickel barrel andrack plating solutions. Time is just sufficient to cover the zincate. Bath pHshould be maintained at 4.4 to 4.6 to minimize attack of solution on the zincate.Proprietary wetting agent and zinc tolerant Class I brightener (carrier) arenormally added. Routine low current density (LCD) dummying at 5 to 10 A/ft2is recommended to plate out zinc contaminant.Where possible, live entry into any of the described strike baths is recommended.This can be accomplished by using an auxiliary cable, while parts arein transit “live” to the strike bath. Plating begins as soon as the parts contactthe solution, significantly minimizing attack on the zincate.

Alkaline Electroless Nickel

The benefit of this bath is total, even nickel thickness of all exposed surfacessince this is an immersion process. The zincate itself is catalytic towardthe electroless nickel solution. For a 10-min immersion the deposit thicknessmay range from 20 to 30 millionths of an inch, at 110°F (43°C). Bath pH is8.5 to 10.0.

Low Carbon Steels (e.g., stampings and extrusions)

Standard soak clean, electroclean, and acid dip, as described in process bathdescriptions.

High Carbon Steels (e.g., springs, fasteners, lock parts)

Classified as above 0.35% carbon. Base metal has higher smutting tendency.Preferred acid dip consists of 25 to 40% v/v hydrochloric acid with additionsof a pickle aid and wetting agent. The pickle aid minimizes attack on the basemetal, greatly reducing tendency for hydrogen embrittlement. Stress due tohydrogen embrittlement can be relieved by baking at 350 to 400°F (177-204°C)for to 3 hours.

Cast Iron

Standard alkaline soak clean, followed by alternate hot and cold rinsing topush solutions out of pores. Anodically electroclean in alkaline descaler. Parts exiting the electrocleaner should have a uniform light yellow cast. Dip in 15

to 20% v/v hydrochloric acid or 5 to 10% v/v sulfuric acid, to dissolve oxides,desmut, and form an active surface for plating.

High Strength Alloy Steels

These materials retain a Rockwell C hardness of 38 or higher. Hydrogenembrittlement can be avoided by using the acid dip as mentioned previously.Baking at 50 to 75°F (10-24°C) below the tempering temperature, 800°F maximum(427°C) is recommended.

Stainless Steel

Standard soak and electrocleaning followed by acid dip or pickle is not sufficientif the material is to be plated. Surface passivity must be overcome.This is accomplished by a treatment in the Wood’s nickel strike solution. (SeeTables XX and XXI.)

Beryllium Copper

This copper alloy typically contains 2% beryllium with 0.25% cobalt and 0.36%nickel.Surface preparation cycle:

1. Alkaline soak clean to remove organic soils. Mild tarnish is acceptable.

2. Electroclean in a specially buffered blend (refer to suggested formula forcopper), having moderate caustic at 20-40 A/ft2, anodic.

3. Activate in a mildly etching solution composed of peroxy derivatives,persulfates, or sulfuric acid with fluoride. Ex. 2% v/v of sulfuric acid and 4oz/gal ammonium persulfate.

4. Rinse well,. proceed to plating bath.

Cobalt

Surface preparation similar to stainless steel. The Wood’s nickel strike is veryimportant to develop a sufficiently active surface to accept subsequent plateddeposits.

LEADED BRASS (0.35-4.00% LEAD)

Red and Yellow Brasses Commercial Bronzes

Surface preparation cycle:

1. Soak or ultrasonically clean to remove buffing and polishing compounds.20-40 KHz/gal. Highly wetted, with solvents, soap optional.

2. Secondary soak clean. Moderate alkalinity, containing surfactants, someinhibition preferred.

3. Electroclean at 10-30 A/ft2, anodic. Buffered blend similar to applicationon copper alloys.

4. Activate. Sulfuric acid type containing fluorides, essential to dissolve leadsmuts.

5. Rinse well, proceed to plating bath.

Bright Dipping Brass

1. Mild to moderately alkaline soak cleaner.

2. 5% v/v sulfuric acid dip. Neutralizes and conditions the surface.

3. Chemically polish in either a peroxide-type or sulfuric acid/iron salts blend. Both solutions are wetted and specially inhibited

4. Tarnish inhibit in dip application using either a soap (mechanical tarnishinhibit film) or a benzotriazole (active surface antioxidant).

5. Optionally lacquer (dip or electrolytic) or apply electrolytic chromate.

Inconel

This alloy constituent typically contains 13.5% nickel and 6.0% chromium.(Note: one alloy type may contain 2% silicon.)Surface preparation cycle:

1. Alkaline soak clean. Mild to moderate alkalinity with sufficient detergency.

2. Acid dip. 20-30% v/v hydrochloric acid for primary oxide removal.

3. Anodically etch. Wood’s nickel strike, 100-120°F (38-49°C), 50 A/ft2,

20-30 sec.

4. Strike plate cathodic. Woods’ nickel strike, 100-120°F (38-49°C), 50A/ft2, 2-3 min.

5. Rinse well, proceed to plating bath.The above cycle is sufficient for Inconel X and Hastelloy C.

Nickel and Nickel Alloys

Require similar treatment as stainless steels. Anodically etch at 15 to 25 A/ft2for 1 to 3 minutes in a 25% v/v sulfuric acid solution. Next, cathodically conditionat 150 to 225 A/ft2 in the Woods’ strike, or at 40 to 60 A/ft2 in a sulfuricacid/fluoride/chloride solution. Parts not long aged may also be activated inan immersion dip consisting of 5 tp 10% v/v sulfuric acid and 2 to 4 oz/galof potassium iodide at 75 to 90°F (24-32°C). These treatments also apply forreplating aged nickel plated parts and rejects.

Powdered Metal

Same recommended surface preparation steps as for cast iron. Rinsing isvery important, to facilitate drainage and removal of previous contaminatingsolutions.

Silver

The metal and its alloys tarnish readily, forming a blackish oxide film. Aftersoak cleaning in an appropriate caustic containing cleaner, dip in 5 to 10%v/v sulfuric acid to neutralize surface. Next, chemically polish in a solutionconsisting of 20 to 25% v/v hydrogen peroxide, at 85 to 100°F (29-38°C).

Titanium

Activation is the critical factor. The following cycle may be appropriate with

Table XXII. Zinc Alloy Compositions

Alloy % Zinc % Aluminum % Magnesium % Copper % Lead

Pure 99.9+ — — — —

Zamak 3 Balance 4.0 0.04 — —

Zamak 5 Balance 4.0 0.04 1.0 —

Zamak 2 Balance 4.0 0.03 3.0 —

Slush Balance 4.75 — 0.25 —

Slush Balance 5.5 — — —

Drawn Balance — — — 0.08

sufficient testing beforehand.

Surface preparation cycle:

1. Alkaline soak clean.

2. Activate and pickle in a solution consisting of 20-25% v/v hydrofluoricacid 75-80% v/v nitric acid.

3. Etch in solution of sodium dichromate at 30-35 oz/gal (225-263 g/L)and 4-5% v/v hydrofluoric acid for 15-30 minutes.Thorough rinsing between each step.

Zinc and Zinc Alloy Die Castings

Zinc is molten and cast into many shapes and forms, comprising a wide varietyof consumer and industry relegated parts. Just like aluminum, zinc is availablein different alloys. (See Table XXII.) The casting operation does resultin surface defects, which must be corrected in an appropriate manner eitherbefore shipment to the plater or in the surface preparation cycle. Pores, cracks,“cold shut,” and roughness are some of these common problems. Mechanicaloperations, such as buffing and polishing, refine, and smooth the surface butleave accumulated buildup of related soils, grease, compounds, and rouges.The exceptionally high temperature of these mechanical finishing techniqueswill burn, harden, and drive contaminants into the metal surface. The soonerparts are cleaned the easier the surface preparation cycle becomes.Surface preparation: (refer to specific cleaner baths and operating parameters,as previously given)

1. Soak or ultrasonically clean. Removing buffing and polishing compounds.The cleaner may be wetted with glycol and cyclic pyrollidonetype solvents. A combination of high HLB and low HLB surfactants arehelpful. Soaps are also an option. The cleaner should be buffered to preventtarnish and etching of the zinc surface. Many buffing and polishingcompounds are effectively softened in the soak cleaner at 175-190°F(79-88°C). Ultrasonic conditioning uses 25-43 KHz/gal of power inthe solution to maintain effective standing waves, resulting in bubblesimploding on the surface for cleaning action. Temperature of the ultrasoniccleaner should be in the range of 160-180°F (71-82°C). Somewhatcooler to avoid higher temperatures, which distort the standing waves.

2. Secondary soak clean. Removes residual organic contaminants and anyinhibiting films that may have formed on the surface during the step#1 soak cleaning.

3. Electroclean. Moderate alkalinity, inhibited.

4. Acid dip.Zinc die castings may be treated in a specially blended acid solution, commonlyreferred to as immersion chemical polishing. This process facilitatessurface preparation by deburring, smoothening, leveling, and brightening.Common base metal defects, such as nodules and pores, are effectively workedout. A typical solution consists of: 42° Be` nitric acid (20-30%), 66° Be` sulfuricacid (20-25%), ammonium bifluoride (20-40%), and nonionic or amphotericsurfactant (>0.5%).Application: 65-115°F (18-46°C). Maintaining temperature is critical to

avoid etching or dulling the surface. Immersion time depends on particularsurface requirements. Organic soils (grease, oils, buffing compound, moldrelease, etc.) should be removed in a suitable soak or ultrasonic cleaner beforethe chemical polishing step.Thorough rinsing is understood between steps.Copper strike as per formulas given for zincated aluminum. Castings shouldbe sealed with at least 0.03 to 0.05 mil. Additional copper as plated to 0.08 to0.14 mil before application of nickel plating

CONSIDERATIONS IN THE FINISHING

EQUIPMENT SELECTION PROCESS

finishing equipment & plant engineering

CJI PROCESS SYSTEMS, SANTA FE SPRINGS, CALIF.

When budgeting for new finishing equipment or upgrading an existing line,

it is important to note that each requirement is unique and must be carefullyconsidered before estimating a price. Otherwise, when the real purchase ordermaterializes for the quoted system, all of the pre-engineering data must be available,as well as current costs, in order to build a particular line. This article willdescribe several key considerations in the selection process of a custom manualor automated plating, anodizing, or chemical process system.Beyond the obvious—selecting floor coating, secondary containment trays, orberming, power, air, and exhaust requirements—the equipment selection processmight proceed as follows:

• The equipment estimator must first collect all the data.

• Then, a determination of how many parts are to be finished peryear, month, week, day, must be broken down into hours per day,in order to size the process line.

• Pretreatment requirements, such as burnishing, tumbling, deburring,buffing, polishing, or degreasing, and selection of any specializedequipment, must be considered.

• Selection of the process, which will depend on whether the partsneed to be barreled or racked, is yet another factor.

• Determine a plating or anodizing process cycle for the particularbase material, as well as the configuration of the parts.

• Determine if the plating thickness requires electroplating,immersion, or autocatalytic (electroless) processing or Type I, II, or IIIanodizing, etc.

• Carefully calculate the surface area of a single part to determinehow many parts may be loaded per barrel, rack, or fixture.

• If the parts are to be barrel plated, then determine if the parts willnest, or stick together; and, if so, what type of barrels will be used.

• If the parts are to be racked, then each part needs to have a specialrack or fixture designed to accommodate that special part. If morethan one rack per flight bar is required, determine just howmany racks per load will achieve the best results.

• Masking considerations: Many parts will require masking withspecial tapes or waxes, as well as holes plugged with custom plugs.

• Reels of connector parts might require selective plating only insome areas, especially where precious metals are plated.Customized selective strip plating lines will be required for eachspecial application.

Once the production quantities are determined, then the plating facility mustbe sized accordingly. The plating tanks must be laid out, and the footprint of alllines and systems measured, with optional floor coating, double containmentof the tanks, with catwalk and grating provided. If a manual line is sufficientfor the desired production volume, with one or more operators, then it mustbe determined if anoverhead hoist will be needed—and if so, will it be a manualchain hoist, powered trolley with push button, or joy stick variable-speed motorized hoist.

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Automatic solar panel plating line.

(All images courtesy of CJI Process Systems)

Phosphate line.

If an automatic hoist line is needed,then you’ll need to determineprecisely how many hoists will berequired. Depending on the configurationof the line, there might beparallel lines, side by side, with load,unload at the same end, or load onone end, unload on the other end,and with either wet or dry shuttletransporting the barrels or racksfrom one side of the line to the other,or a U-shaped return line, and dryer.The PC software must be programmablein order to allow controlof all the process parameters, suchas solution operating temperatures;low-level shut off, alarms, auto-fillof tanks; variable or constant currentand voltage requirement of therectifiers; cathodic or anodic; automaticramp up of voltage for anodizing;historical process data recordedfor future records; hoist location,position, and speeds; pumps andfiltration operation; air blower pressure;and amp min/hr. Other parametersto consider are chemical dosing,and if any brightener feeders orchemical feeders are supplied withmetering pumps, etc.In order to design the plating line(s) correctly, key items must consideredfor every single tank in the line. The designer must go through each station ortank, one at a time, to decide which controls or accessories need to be installedon each tank. A manual line would need the same items as an automated line,except the automated line would have either single or multiple programmablehoists, which might be either a monorail type, sidearm, semi-bridge, bridge, or a“rail rider.” The hoist positioning might be laser-controlled encoder or manual,with random loading scheduling—or it could be time-way based. The line mightbe totally enclosed because of either clean room or other environmental circumstances,with the operator working inside the enclosure.All of the tanks must be sized to accommodate the barrels, or racks, with sufficientclearance for the heaters, sensors, coils, pumps, filters, spargers, level controls, anode baskets, etc. The tank materialmust be chemically compatible—withthe decision to either line the tank,or offer it without linings or innercoatings—for each solution, as wellas each individual component. Eachtank must be outfitted with a varietyof components, based on just whatthe tank is supposed to accomplish.The soak cleaner would needeither electric heaters or heatingcoils, temperature controllers, sensors,hi/lo level sensors, individualsolenoids for city water or deionizedwater feed, agitation sparger(with agitation either provided bylow-pressure, oil-free filtered air),or eductor/pump agitation. Othernecessities: oil skimmer, oil coalescer,pump and filter, and low-levelshut off of the heater.The rinse tanks might requireauto-fill city or deionized water solenoids,air sparger manifolds, drainvalves, overflow weirs, conductivitycontrollers, and possibly pump andfilter, depending on particulate drug into them. Electro-cleaner tanks wouldalso need a rectifier, anode/cathode bars, pump and filter, oil skimmer, heateror steam coil, solenoids for city and deionized water feed, etc.The process tanks would require similar components as the electro-cleaner, withan addition of rectifiers and other items, depending on the process. The rectifiersmight be chosen to accommodate a variety of controls, such as constant currentand/or constant voltage (pulsed, periodic reversed, or reverse pulsed; air, water, orconvection cooled), and might include analog or digital amp/volt meters mountedremotely. The designer must decide just what type of heaters, agitation, cooling,filtration, circulation, rectification, and materials of construction, as well as whatneeds to be exhausted and which tanks need exhaust plenums. CFM requirementsalso need to be calculated for the entire line in order to size the air scrubber.If the plating tank happens to be an electroless nickel process, then the decision

must be made as to how to heat the tank. For example, would it be morepractical to use heaters, steam, or hot water coils? Or does it make more sense to make the tank a double-boiler tank heated with coils in the lining of the tank?

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Rear view of large plating line.

Automatic electroless nickel plating line.

CONSIDERATIONS WHEN DESIGNING A TANK

There are many considerations when building the tanks, including size, quantity,and spacing of the girths around each tanks, as well as factoring in the weightcapacity of each solution. All of this depends on specific gravity, operating temperature,and geographical location. On the West Coast, for example, you mightrequire seismic calculations on the larger tanks.The plating lines might be either individual tanks sitting on a frame or mod- ules. Either way, the lines should be plumbed with valves, solenoids, city anddeionized water feeds, with separate drains to cyanide, acid/alkaline, and chromelines to the wastewater treatment system. Note: every plating facility will needsome type of treatment system, unless it’s all hauled away and treated off site.The plating line should offer single-point connections after arriving for hook upof the utilities, air, water, or steam, and electricity. Most plating lines are wired“three-phase” wherever possible for energy efficiency savings.Some plating lines are required to provide VFC (variable frequency controls)that vary the speed of the electric motors on the pumps, etc., depending on loadrequirements.The wastewater treatment system must have many components to accommodatethe plating line, and the plating line designer is usually asked to also quotethe wastewater system supporting the plating or anodizing line. Aside fromconsiderations regarding the wastewater treatment methodology of each platingline, the designer must determine just which type of system will be the mostefficient system for that particular line while satisfying the local permitting laws.

CONCLUSIONS

The aforementioned factors should offer readers just a few examples of themagnitude of calculations, researching, sizing, etc., that might be required whenestimating a new system. If the process line is designed properly to begin with, then the chemistry will have a much better opportunity of being successful.

FILTRATION AND PURIFICATION OF PLATING AND RELATED SOLUTIONS AND EFFLUENTS

environmental controls

BY JACK H. BERG

SERFILCO LTD., NORTHBROOK, ILL.; WWW.SERFILCO.COM

This introduction reflects the response needed by platers for quality control, tomeet just-in-time deliveries, and to achieve zero rejects. It also addresses the needfor platers to continue to reduce solid waste after neutralization and employfiltration wherever possible to recycle or lengthen the service life of cleaners,etchants, and rinses.Filtration usually includes the use of carbon for undesirable organic impurityremoval, which years ago also doubled as a filter media along with other formsof filter aids.Today’s acceptance of granular carbon in many situations has lessened theneed for powdered carbon and almost eliminated the weekly or monthly batchpurification treatment. There are, however, some occasions when powdered carbonmay be the only answer, and for that reason a separate piece of equipmentheld aside for such a need should be considered.Platers who appreciate the value of filtration must first understand that it isnot as much an art as it is a science. The requirement of a science is to have anorderly body of facts, facts that can be correlated and anticipated results yielded.Although there has been some work done in this area over the last 5-10 years,platers must still rely on experience to a great extent.In the past, it has been suggested that the plater decide the level of qualitysought and, using statistical quality control, determine if this goal has beenachieved. It is further recommended that the plater needs to know the partsper million of contamination (solids) so that the necessary size or dirt-holding(solids) capacity of the filter could be established. The plater must also knowthe nature of the solids, which would be critical to success. Slimy, stringy, or oilycontaminants blind a dense filter media surface quickly, whereas coarse, grainy,sandlike particles build a thick cake and still allow solution to pass, which providesfor continued solid/liquid separation.By first assessing these factors, platers can ascertain what results can beachieved. For example, slimy solids would require more surface area, whereasgritty particles could get by with less area (i.e., less solids-holding capacity).However, all filter media are not manufactured in the same manner, forinstance, filter paper, cloth, and plastic membranes provide a single junction tostop solids. Filter aids can enhance the ability of the filter media by creating aporous cake, which improves surface flow, but to really be successful a continuousmixing of filter aid and solids must be coordinated to maintain suitable porosity.Other types of filter media canprovide the necessary junction to stop solidsbut are built in such a manner as to achieve results from a combination ofsurfaces or juncture points, which achieve the solids retention by impedance.Thus, it is possible for continuous solid/liquid separation to be maintained overa longer period of time.Most filter media are rated according to the size of particles that they arecapable of stopping. Such a rating is based on laboratory tests and expressed inmicrometers. A coarse media would be 100 μm; a dense media would be 10, 5, or1 μm. The number suggests that at an efficiency level of 85 to 99%, all such particleswould be stopped, whereas if the micrometer retention level is expressed in“absolute” ratings, 100% of the stated micrometer size and larger sizes would beremoved. It further stands to reason that the coarser media will offer more solidsholdingcapacity, and the denser media will offer less solids-holding capacity.Next we discuss where these troublesome solids come from and how they canbe most effectively removed.

DIRT LOAD

The “dirt” (impurities) in a working plating bath can come from drag-in, anodes,water, and airborne sources. For their efficient removal, the system must bedesigned for the amount and type of contaminants present in the plating tank;these vary for each installation. Even without prior operating experience, anestimate of the dirt load can be made by reviewing the cleaning and platingprocesses to select and size the equipment needed.A filter with insufficient dirt-holding capacity will require frequent cleaningor servicing. The rapid pressurebuildup in the system as solids are retainedincreases the stress and wear of pump seals. By minimizing the dirt load, maintenanceof the filter and pump can be reduced considerably. Even after thoroughcleaning and rinsing, some solids and contaminants cling to parts, racks, andbarrels. Thus, they are dragged into the plating solution. The amount of drag-incontamination depends primarily on the type of parts, plating method (rack orbarrel), cleaning efficiency and rinsing cycles.In most plating plants, the type and amount of parts being processed mayvary considerably. For trouble-free operation, the filtration system should bedesigned for the heaviest work load and most difficult-to-clean parts. Drag-incontamination with barrels is high, due to incomplete draining of cleaners anddifficulty in rinsing of loads. Filtration and purification on automatic barrellines must be continuous, and equipment must be of sufficient size to minimizeservicing and work interruption.The amount of drag-in can often be reduced by improving the pretreatment.With the conversion of many vapor degreasing processes to aqueouscleaning, proper maintenance of cleaners and electrocleaners is of greaterimportance, particularly with machined or buffed parts carrying oil and lubricants.Recirculation and coalescing with an overflow weir on cleaner tanks willeffectively skim off oil and scum, which would quickly foul the filter mediumand carbon. More effective descaling will minimize the dirt load. Several countercurrentrinse tanks and a final spray rinse with clean water will also reducethe drag-in contamination. Due to the nature of the cleaning process, contaminationof the solution with organic soil (oil, wetting agents) and/or inorganic(metallic) compounds is sometimes unavoidable. These can generally be controlledby carbon treatment at the rinse tank before plating.Filterability depends on the nature, amount, and size of suspended particles,which, in turn, are contingent upon the type and chemistry of the plating solution.Generally, alkaline solutions, such as cyanide baths, have slimy or flocculentdifficult-to-filter insolubles, whereas most acid baths contain more grittysolids, which are relatively easy to filter even with a dense filter media. A quicktest of a representative sample with filter paper in a funnel will determine thenature and amount of solids present. This test will also indicate the most suitablefilter medium. Bagging of soluble anodes will materially reduce the amountof sludge entering the plating bath. Airborne dirt from ceiling blowers, motorfans, hoists, or nearby polishing or buffing operations may fall into the platingtank and cause defective plating. Good housekeeping and maintenance will, ofcourse, reduce dirt load and contamination of the plating solution.Prevention of deposit roughness is perhaps the foremost reason for filteringplating solutions. Better covering power with less chance of burning isalso achieved with a clean bath. In addition to suspended solids, the plateralso has to contend with organic and inorganic (metallic) impurities, whichare introduced into the solution primarily by drag-in. If this contamination isallowed to build up, it will affect deposit appearance. Continuous or periodicpurification of the solution with activated carbon and/or low-current-densityelectrolysis (dummying) will often remove these impurities before a shutdownof the plating line becomes necessary.The trend of Environmental Protection Agency (EPA) regulations is toseverely restrict the amount of suspended solids and dissolved metal impuritiesin wastewater discharged to sewers and streams. To comply, platingplants have had to resort to some chemical treatment of their effluents toprecipitate the metals as hydroxides. The filtration of these hydrated sludgesis difficult and requires special separation equipment. Closed-loop systems,recycling, and recovery are being employed and require greater attention tofiltration and purification.Most filtration systems consist of a filter chamber containing the filter mediaand a motor-driven pump to transfer or circulate the solution from the platingtank through the filter. The many filters and pumps on the market today makeit possible to select and justify a cost-effective filter system for each and everysolution, regardless of volume.When engineering a filter system for a plating installation, it is necessaryto first establish the main objectives, such as: high quality finish—maximumsmoothness and brightness; optimum physical properties—grain size, corrosion,

and wear resistance; or maximum process efficiency and control—coveringpower, plating rate, purification, and clarification.Then the following factors must be considered before selecting the size andmaterials needed for the filter media, chamber, pump, and motor:

1. Dirt load—suspended solids, size, kind, and amount; also soluble organicand inorganic impurities.

2. Flow rate—turnovers per hour for a given volume of solution necessaryto maintain clarity.

3. Frequency of filtration and purification—batch, intermittent, or continuousrequired to remove dirt and contamination and filter servicinginterval desired.When agitating solutions with air, a low-pressure blower is usually employed.This makes it virtually impossible to achieve good filtration of the air while keepingthe solution clean, because the plating solution then acts like a fume scrubber.If effluent regulations make it necessary to remove or reduce total suspendedsolids (TSS) from wastewater, the amount discharged per hour or shift can bereadily determined. For instance, a 100 gal/min (gpm) effluent containing 100ppm TSS (100 mg/L) will generate 5 lb of solids per hour, as calculated below:100 gpm 3.79 L/gal 100 mg/L 60 min/hr (1000 mg/g 454 g/lb) = 5 lb/hr (2.3 kg/hr)Therefore, the filter must have sufficient capacity to hold approximately40 lb of solids/8 hr of operation. A horizontal gravity filter would be the mostcost efficient for this dirt load and would operate automatically; however, ifdryness of the retained solids is to be achieved, then a filter press would berecommended.Filtration and/or purification during nonproductive hours makes it possibleto remove dirt at a time when no additional contaminants are being introducedinto the tank, such as insolubles from anodes, chemical additions, plus thatwhich would otherwise be dragged in from improper cleaning of the work.Again, individual tank operating characteristics and economics will determinethe ultimate level of acceptable quality.This brings up an important consideration. Contamination by organic compounds,inorganic salts, wetting agents, and oils is not removed by filtration,but by adsorption on activated carbon. Some plating solutions, such as brightnickel baths, generate organic byproducts during plating. It cannot be assumedthat both types of contamination increase at the same rate. A batch treatment,therefore, may eventually become necessary, either because of insoluble or solubleimpurities. A check of clarity, flow rate, and work appearance and a Hullcell test will indicate the need for transfer filtration and/or carbon treatment.If analysis shows that the concentration of insolubles (in ppm) has increased,it would indicate that the solution is not being adequately filtered. Therefore,transfer pumping of the solution through the filter should be employed as thequickest way of getting all the solids out at once and returning the clean solutionto the plating tank. Soluble impurities can be detected by inspection of thework on a Hull cell panel. Pitting, poor adhesion, or spotty appearance indicatesthe need for fresh carbon. Here again, it may be desirable to completely batchtreat the solution to restore it to good plating quality; however, since this necessitatesshutting down the plating line and requires considerable labor, everyeffort should be made to maintain solution clarity and purity continuously,without having to resort to such batch treatment.

FREQUENCY OF FILTRATION AND PURIFICATION

Since it is desirable to plate with a solution as free of suspended solids as possible,the quickest way to achieve clarification is by transfer pumping all of thesolution from one tank, through a filter, to another tank (batch treatment); however,to maintain both clarity and uniform deposit quality, continuous recirculationthrough a filter is most effective. Although continuous filtration is moredesirable, there are some plating installations that require only intermittentfiltration, because relatively small amounts of solids are present. In other cases,it is necessary to filter and purify the bath continuously, even when not plating.A high flow rate is essential to bring the particles to the filter as quicklyas possible and to prevent settling of dirt on parts being plated. Althoughplating in a solution completely free of solids would be best, this ideal can beapproached only in the laboratory. Some contamination always exists, and mustbe accepted. Continuous filtrationat a high flow rate can maintain a high levelof product quality by keeping suspended solids to a minimum. As Figure 1 indicates,four to five complete tank turnovers effectively remove 97% of all filterablematerials if no additional solids are introduced. Since, in many installations, therate at which contamination is introduced is higher than the rate at which it isremoved, the impurities and solids gradually increase with time unless filtrationis continued even during nonplating periods. The greater the turnover rate, the longer the plating bath can be operated

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Fig. 3. Typical flow versus pressure curve. Q represents the maximum open pumping against no

restriction, whereas P represents the pressure that the pump can develop at zero flow. A might

indicate the pressure drop across a depth type media or a bare support membrane, whereas points B

and C indicate the reduction in flow caused by the addition of filter aid and carbon, respectively.

before the reject rate becomes too high and batch (transfer) filtration is necessary.In practice, contaminants are not introduced at a steady rate; for instance,most are introduced with the parts to be plated and, therefore, at the momentof immersion the degree of contamination is sharply increased until it is againreduced by the action of the filters. It then increases again when more parts areput into the tank for plating.Figure 2 indicates the reduction in flow caused by the dirt buildup in thefilter on a day-to-day basis, where one week’s filtration would be effectedbefore service of the filter becomes necessary. This reduction in flow ratecould also have been representative of a longer time interval between filtercleanings. Graphically, it indicates why platers may experience roughness atvarying intervals in the plating filtration cycle. The amount of solids increasesin the tank as the flow rate decreases to a level that may cause rejects. After thefilter is serviced, the increased flow rate agitates any settled solids. Therefore,it is advisable to delay plating of parts until the contaminant level is againreduced by filtration to within tolerable limits. This phenomenon generallyoccurs in a still tank, since the dirt has more chance to settle. For this reason,when the solution is pumped into a treatment tank, sludge may be found onthe bottom of the plating tank.Dirt in an air-agitated tank can settle any time after the air is shut off. Ifcarbon and/or a filter aid is used in the filter during the continuous filtrationcycle, it should be borne in mind that, as these solids are collected on the media,the pressure increases appreciably, reducing the initial flow rate by almost 25%and the overall volume pumped through the filter by as much as 50% beforeservicing isnecessary (Fig. 3). Frequent laboratory checks will verify the amountof insolubles in the plating tank, which will tell whether a uniform degree ofclarity is being maintained or whether it is increasing slowly toward the rejectlevel. More frequent servicing of the existing filtration equipment will increasethe total volume pumped and, in turn, maintain the lowest possible level ofcontamination and minimize the need for batch treatment.It is, therefore, necessary for the plater to determine the particle size to be removed and then select themedia that provides the most solids-holdingcapacity. Then, knowing the efficiency of the media, multiply it by flow rateso that all of the solution passes through the filter in a certain period oftime, such as 1 hr or 1 min. Note the small amount of solution that is filteredin 5 min if a rate of one turnover per hour is used (Fig. 4) as compared withthe amount that would pass through at a rate of ten turnovers per hour(assume a 100-gallon solution):At one turnover per hour,At ten turnovers per hour,The point here is that if nearly the entire solutionis turned over every 5 min,then the plating bath will exhibit a high degree of clarity and purity. The netresult should be fewer rejects caused by occlusion of particulate matter in thedeposit.In modern electroplating, no area that can result in improved quality shouldbe overlooked. The plater can use the principles of high tank turnover and solutionvelocity to his advantage in his quest for zero rejects.During recent years the flow rate through the filter, or tank turnover as it isreferred to, has increased to two or three per hour or higher for most platingsolutions (see Table I). This means that 1,000 gallons require a flow rate of atleast 2,000 to 3,000 gallons per hour (7.6-11.5 m3/hr); however, platers shouldrecognize the need and employ turnovers of 10 or even 20 times per hour whenall solids must be removed (see Fig. 1).Alkaline solutions may require even higher flow rates for more effective solidsremoval by recirculation. Depending on the filter medium and its retentionefficiency, flow rates in the range of 0.5 to 2 gpm (2 to 8 Lpm) per square footof filter surface area are obtainable. Although 5 gpm per 10-in. (25-cm) cartridgeis permissible, flow rates under 1.5 gpm per cartridge offer better economy. Infact, at a given flow rate with a cartridge filter, servicing, cartridge cleaning, orreplacement can be reduced significantly by increasing the size of the filter.For example, if the size of the filter was multiplied by four the annual amount of filtercartridges consumed would be cut in half and the filter itself would operate unattended forat least four times as long before cartridge cleaning or replacement was necessary. This isan important consideration to reduce media consumption. It has also been found that the effective life of surface filters may often betripled by doubling the surface. By increasing the dirt-holding capacity andreducing the frequency of filter servicing and replacement, the cost of filtrationon a per month or per year basis is substantially reduced.

TYPES OF FILTER SYSTEMS

After estimating the dirt load and determining the flow rate and filtration frequencyrequired, a choice of filter method and medium must now be made. Themost common types of filters used in the plating industry are discussed below.These filters may be placed inside or outside the tank.In-Tank Considerations:Tank spaceMotors located over fumesLimited size of filter (less service life of media if used on pump suction)Out-of-Tank Considerations:Remote possibility for easy serviceEmploy sealless magnetically coupled pumps or direct-drive with singleor double water-flushed sealMore suitable for use with slurry tank for chemical or filter aid/carbonaddition or backwashingLarger dirt holding and flow capacity from cartridges or surface media

Cartridge Filters

Cartridges offer both surface and depth-type filtration characteristics, providingvarious levels of particle retention at different efficiencies (nominal andabsolute), manufactured in natural and synthetic (plastic) materials to providea wide range of chemical resistance, flow rates, and particle retention capacities.Pleated-surface media offer initially higher flow rates, are available with a choiceof porosities (usually in the denser range), and are sometimes given an absoluteparticle-retention rating.Depth-type media are available in 1- to 100-μm particle retention and, becauseof the variety of porosities available, they are sometimes best suited to handlehigh-dirt-load conditions. This is a result of the manner in which the depth-typecartridge filter is manufactured. Basically, it consists of a series of layers, whichare formed by winding a twisted yarn around a core to form a diamond opening.The fibers, which are stretched across the diamond opening, become thefilter media. Succeeding layers lock the previously brushed fibers in place and,. since there is the same number of diamond openings on each layer, the openingsbecome larger due to the increase in circumference; other fiber-bonded types alsoincrease density across the depth of the media.During filtration, the larger particles are retained on the outer layers of thecartridge where the openings are large, whereas the smaller particles are retainedselectively by the smaller openings on succeeding inner layers. This, then, makesit possible for an individual cartridge to have a dirt-holding capacity equal to 3.5ft2 of surface filter area of the same density. Cartridges having a 15- to 30-μmretention will often hold 6 to 8 oz of dry solids before replacement is necessary,whereas cartridges of 10 μm downto 1 μm will have a dirt-holding capacity ofperhaps 3 oz to less than 0.5 oz. These figures merely indicate that the coarsercartridges have greater dirt-holding capacity, are more economical to use, andcan be used longer before replacement.Also, as pointed out earlier, dirt loads vary from tank to tank, and cartridgesshould be selected according to the individual requirements. A dense cartridgehaving less dirt-holding capacity will load up more quickly, increasing the pressuredifferential and, therefore, reducing the flow (Fig. 5). Using coarser cartridges(greater than 30 μm on zinc, for example) that have greater dirt-holdingcapacity and a longer service life may make it possible to clarify the plating tankmore quickly because of the high obtainable flow rate. This will be accomplishedat less cost. Usually two cartridges (three on zinc, tin, and cadmium) are recommendedfor each 100 gal of tank capacity.The pump should provide a pumping rate of at least 100 gph (two tankturnovers per hour) for each cartridge. Usually, a cartridge life of 6 weeks onnickel or 4 weeks on zinc can be expected, with some tanks running as longas 12 weeks; however, much depends upon dirt load, hours of plating, and soon. With cartridges, a higher dirt load can be retained in the filter chamberbecause of the coarseness of the filter media. Higher flow rates can usually beemployed during the entire lifespan of the cartridge. This is due, in part, to thehigher head pressures of pumps employed without chancing the rupture of acartridge. Since all of the dirt is retained on and in the cartridge, the cartridgefilter can be turned off and on at will, unless the cartridges are precoated.Cartridges are changed with very little maintenance expense and no solutionloss; however, simplicity of use is perhaps the most predominant single factorin their selection.

Precoat Filters

Precoated filters consist of a membrane (leaf, sleeve, or screen) such as paper,cloth, ceramic, sintered metal, wire mesh, or wound cartridges. These membranessupport the diatomite or fibrous-type filter aid, which has been mixed ina slurry of water or plating solution and picked up by the membrane openings.The dirt is retained on the outer surface of the cake. When the pressure hasincreased and the flow rate has decreased to a point where filtration is no longerefficient, the dirt and cake are washed from the membrane. Paper membranesare discarded and replaced.The ability to obtain long runs is dependent upon proper selection of thefoundation media, coupled with a coarser-than-usual nonfibrous-type filteraid (to be used where possible). Periodic (daily, if necessary) additions of smallquantities of filter aid should be made to lengthen the cycle between servicing.The dirt-holding capacity of this type of filter is usually measured in squarefeet of filter surface. (If the standard 2.5 x 10-in. long cartridge is used, its outersurface when precoated would beequivalent to about 0.50 to 0.67 ft2 of area.)Flow rate and dirt-holding capacity of the various precoated membranes orcartridges would be about equal.Before precoating, the operator should know or determine the filtrationarea to be covered. The amount of filter aid used depends on its type and onthe solution being filtered. Generally, 0.5 to 2 oz/ft2 of filter is sufficient. Themanufacturer’s recommendations for type and amount of filter aid should befollowed if optimum results are to be obtained. A slurry of filter aid and platingsolution or water is mixed in a separate container or in a slurry tank, which maybe an integral part of the filtration system. The slurry is then caused to flowthrough the filter media and create a filter cake.Usual flow rates range from 0.5 to 2 gpm/ft2 of filter surface. A lower flowrate improves particle retention and smaller particles will be removed. It shouldbe pointed out that, although there may be a wide range in flow rate, the rangeof selectivity of particles being removed is between 0.5 and 5 μm, which is themost significant difference between precoat and depth-type cartridges and offersa wider choice of porosity.Buildup of cake should be gradual, and recirculation should continue untilthe solution runs clear. Cake should be dispersed uniformly across the mediabefore the plating solution is allowed to flow across the filter. A slurry tankpiped and valved into the filtration system becomes a convenient and versatilepiece of equipment. The slurry may be prepared with plating solution, ratherthan water, to avoid diluting critical mixtures. Via valving, the solution isdrawn into the slurry tank for sampling, preparation of slurry, and chemicaladditions. Similarly, the solution is returned to the plating tank. This methodeliminates the necessity of transfer hoses between tanks, and the subsequentrisk of loosening the cake or losing pump prime. The integral slurry tank isalso a convenient storage for backwash water.

 Precoat Backwash FiltersThese filters operate the same as, and have the same functional purpose as, ordinaryfilters with the further advantage that they can be cleaned quickly by reversingthe flow through the filter media. Backwashing the filter aid and dirt awaymakes the media available for prompt repeat precoating. The basic advantage isthat the filter chamber need not be opened each time the filter requires cleaning.Finer grades of filter aid may be precoated on top of the coarse filter aid whenfine powdered carbon is to be used continuously. Here again, periodic (daily, ifnecessary) additions of small quantities of filter aid should be made to lengthenthe cycle between backwashing. The media may be cleaned automatically withsluicing or using other devices. Iron hydroxide sludges can be dissolved by circulatingdilute hydrochloric acid from the slurry tank; additional manual cleaningmay also be required occasionally.Some disadvantages of precoat and backwashing are the possible loss of solution,increased waste treatment loading, and the possibility of migration of filteraid and carbon into the plating tank. The use of rinse water for backflushing willreduce waste treatment loading; however, if evaporation is used to control dragout,this may interfere with evaporator operation and the economies achievedby using this equipment.

Sand Filters

Using sand as the filter media, the pump and filter operate like a precoat surfacefilter and backwash like a precoat without the need of additional aid to achievefine particle retention. Performance can be acceptable based on recirculationturnover rates, with the basic disadvantage coming from a smaller surface area,which increases the need for frequent backwashing and resulting solution lossto maintain the desired flow rate (turnover required).

Horizontal Fabric and Screen Filters

These filters are especially well suited for the continuous dewatering of hydratedmetal sludges resulting from the neutralization of plating wastewater prior to

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Fig. 7. Skimmer, pump, and prefilter with carbon or free oil separator.

sewer discharge. They are also effective in removing accumulated iron sludgefrom phosphating tanks.In one such system (Fig. 6), the waste containing 1 to 3% solids is first allowedto settle in a cone-shaped tank. The supernatant liquid drains into a head box,which directs the flow across the filter medium (paper or plastic) supported bya motor-driven conveyor belt. The liquid passes through the disposable fabricby gravity flow into a receiving tank below. When the pores of the media becomeclogged, the liquid level rises and a float switch activates the belt drive. Freshmedia is fed over the tank and filtration is continuous. The cake on the fabricis allowed to drain before it is dumped into the sludge box. Gravity drain or animmersion pump empties the filtered water from the tank. Cycling and indexingof the filter are automatic. The occasional replacement of the filter fabric rollis the only labor required. The sediment in the bottom of the cone can also bedewatered periodically by filtration on the fabric. Other systems feature pressureor vacuum filtration. The sludge cake contains from 5 to 35% solids, dependingupon the equipment and type of cake. Cakes can be further treated by airevaporation or with heat for dry disposal. The filtrate can be discharged to thesewer if it meets local effluent regulations or can be recycled through the system.The performance of the unit can be improved greatly by the addition ofcoagulants and flocculating agents, such as polyelectrolytes, which increase theamount of solids, particle size, and settling rate. The flow rate is approximately1 gpm/ft2 with 90 to 95% solids retention; with coarse filter media, flow ratesincrease up to 10 gpm/ft2. Filter aid can also be precoated to improve retention.The filter media is available in porosities of 1 to 125 μm and rolls 500 yd long.Carbon-impregnated paper is used for purification and removal of organiccontaminants. The unit must be sized properly for each application to operateefficiently and with a minimum media cost. Steel, coated, stainless steel, orplastic units are available for corrosive solutions.

BATCH AND CONTINUOUS ACTIVATED-CARBON PURIFICATION

Virtually all plating solutions and some cleaners or rinses at some time will requirepurification via the adsorption of impurities on activated carbon. Those solutionsthat contain wetting agents require the most carbon; when oil is introduced intothe bath, the carbon is dispersed throughout the solution and clings to the parts,causing peeling or spotty work. Solutions that do not contain wetting agentshave a tendency to float oil to one corner, depending on the recirculation set upby the pump, and in this case the oil may be removed with a skimmer or coalescer(see Fig. 7).The choice of purification method depends on the size of tank and amountof carbon required and also on other available auxiliary equipment. Generally,carbon cartridges are used on small tanks (up to a few hundred gallons), andthe bulk or canister type or the precoat method is used for the very largest tanks.The canister type is also used on the larger tanks supplemental to surface ordepth-type cartridges or on certain automatic filters to supplement the amountof carbon.

Batch Treatment

The quality of the carbon is important and special sulfur-free grades areavailable. The average dosage is 10 lb of carbon to treat 500 to 1,000 gal ofwarm plating solution. At least sixty minutes contact time with agitationshould be allowed, followed by some settling before transfer clarificationcan be achieved.

Continuous PurificationA separate purification chamber holding bulk granular carbon, a carboncanister, or cartridges offers the most flexibility in purification treatment. Bymeans of bypass valving, the amount and rate of flow through the carbon canbe regulated to achieve optimum adsorption of impurities without completedepletion of wetting agents and brighteners in the plating bath. It providesfor uninterrupted production and fewer rejects. When necessary, the carboncan be changed without stopping filtration of the bath. Filtration shouldalways precede carbon treatment, to prevent dirt particles from covering thecarbon surfaces.

CONTINUOUS CARBON TREATMENT METHODS

Carbon Cartridges

Cartridges containing up to 8 oz of either powdered or granular carbon for every10 in. of cartridge length are available and will fit moststandard replaceablefilters that employ this type of media. They may include an outer layer, whichserves as a prefilter, and an inner layer, which serves as a trap filter. These handycartridges are ideal for small filter chambers because of the ease and convenienceof quickly replacing a conventional depth tube with the carbon tube when necessary.They may also be used with submersible filter systems, but in this case theflow rate could be greatly reduced.

Carbon Canister

Granular carbon may be used in ready-to-use chambers, each with a number ofcanisters holding up to 10 lb of granular carbon, and placed in line to the tank. Abuilt-in trap filter eliminates migration of the carbon. Prefiltration ahead of thepurification chamber will prevent solids from coating the surface of the carbonin the canister, assuring maximum adsorbency. The carbon in the canister can bereplaced when its adsorption capacity has been reached. This method of separatepurification offers the most flexibility. Any portion or all of the filtrate can betreated as needed by means of a bypass valve after the filter.

Bulk Carbon Method

Granular or bulk carbon is poured looselyaround standard depth-type cartridge filtersor sleeves, is poured into specific chambersdesigned for carbon, or is pumped betweenthe plates or disks of other surface media.Since no filter aid is used, fines breakingoff from the piece of carbon will have to bestopped by the surface media. Therefore, aninitial recirculation cycle without enteringthe plating tank or recirculation on the platingtank prior to plating is desirable. Thismethod does not alter the solids-holdingcapacity of depth-type cartridges, as mostof the carbon will stay on the outer surfacelayer; however, carbon removal is not easily accomplished.

؟؟؟

Fig. 8. Suction or dispersion piping

system with strainer and siphon

breaker. Drill a hole 2 in. below working

solution level as a siphon breaker

to prevent solution loss due to

unforeseen damage to piping, pump,

and so on. Chlorinated polyvinyl

chloride with screwed connections

offers maximum flexibility and ease

in installation and may also be used

on the return line by eliminating the

strainer and replacing it with a longer

length of pipe that is open along the

full length.

TIPS ON FILTER INSTALLATION

Filtration equipment should be installedas close to the plating tank as possiblein an area that affords access for servicing.Equipment that is not easy to servicewill not be attended to as frequently asrequired, and the benefits of filtration willnot be maximized. The suction line shouldalways have a larger diameter than the dischargeto avoid starving the pump (e.g.,1 in. versus in. or 2 in. versus 1.5 in.) Where it isnecessary to install theequipment more than 10 to 20 ft away, check the pump suction capabilitiesand increase the size of the suction piping (1.5 in. instead of 1 in., or 2 in.instead of 1.5 in.) to offset the pressure loss.Hoses made of rubber or plastic should be checked for compatibility withthe different solutions. Strong, hot alkaline and certain acid solutions such aschromium are especially aggressive. The use of chlorinated polyvinyl chloride(CPVC), polypropylene, or other molded plastic piping for permanent installationis becoming more common. Some plastics are available with socket-type fittings,which are joined with solvents. Their chemical inertness and temperaturecapabilities are excellent. Iron piping, lined with either rubber or plastic, is idealbut usually limited to use on a larger tank capable of justifying the investment.It should be pointed out that whenever permanent piping can be used in andout of the tank a more reliable installation will exist, since there is no shiftingto loosen fittings, and collapsing or sharp bending of hoses is eliminated. Thesuction should be located away from anode bags, to avoid their being drawn intothe line and causing cavitation. Strainers on the suction are always advisable.It is also desirable to drill a small opening into the suction pipe below thenormal solution operating level on permanent installations so that, shouldany damage occur to the system, the siphon action or suction of the pump willbe broken when the level reaches the hold (Fig. 8). This provides added safetyduring unattended operation. Whenever automatic equipment is operated,some provision must be made to protect against unforeseeable events thatcould cause severe losses. This includes some form of barrier or removablestrainer to prevent the suction of parts into the pump. The addition of a pressuregauge is strongly recommended to determine the initial pressure requiredto force the solution through the filter and also to determine when the filtermedia needs to be replaced.When starting up a new filter system, or after servicing an existing system,it is advisable to completely close the valve on the downstream side of thefilter; in this way, the pump will develop its maximum pressure, and one canimmediately determine whether thesystem is secure. Sometimes filtrationsystems are tested on a cold solution and, in turn, will leak on a hot solutionand vice versa. Therefore, a further tightening of cover bolts, flange bolts,and so on may be necessary after the filter has been operating at productiontemperature and pressure. If pump curves are not available, one may wishto check the flow at different pressure readings to determine a reasonabletime for servicing the equipment before the flow rate has dropped too low toaccomplish good dirt removal.

CONTROLCHEMICAL ANALYSIS OF PLATING SOLUTIONS*

troubleshooting, testing, & analysis 

BY CHARLES ROSENSTEIN

TESSERA-ISRAEL, LTD., JERUSALEM, ISRAEL

AND STANLEY HIRSCH

LEEAM CONSULTANTS LTD., NEW ROCHELLE, N.Y.

Plating solutions must be routinely analyzed in order to maintain the recommendedbath formulation and to preempt the occurrence of problems related toimproper levels of bath constituents. Contaminant levels in the solutions mustalso be monitored. Manufacturers of plating systems establish optimum specificationsto ensure maximum solution efficiency and uniformity of deposits.The various factors that cause the concentrations of bath constituents to deviatefrom their optimum values are as follows:

1. drag-out;

2. solution evaporation;

3. chemical decomposition; and

4. unequal anode and cathode efficiencies.

A current efficiency problem is recognized by gradual but continuous changesin pH, metal content, or cyanide content (see Table I).The techniques employed for the quantitative analysis of plating solutions areclassified as volumetric (titrimetric), gravimetric, and instrumental. Volumetricand gravimetric methods are also known as “wet” methods. The analyst mustselect the method that is best suited and most cost effective for a particularapplication.The wet methods outlined here are simple, accurate, and rapid enough forpractically all plating process control. They require only the common analyticalequipment found in the laboratory, and the instructions are sufficiently detailedfor an average technician to follow without any difficulty. The determinationof small amounts of impurities and uncommon metals should be referred toa competent laboratory, as a high degree of skill and chemical knowledge arerequired for the determination of these constituents.Hull cell testing (see the section on plating cells elsewhere in this Guidebook)enables the operator to observe the quality of a deposit over a wide currentdensity range.

VOLUMETRIC METHODS

When titrants composed of standard solutions are added to a sample that containsa component whose concentration is to be quantitatively determined, themethod is referred to as a volumetric method. The component to be determinedmust react completely with the titrant in stoichiometric proportions. From thevolume of titrant required, the component’s concentration is calculated. Thesimplicity, quickness, and relatively low cost of volumetric methods make themthe most widely used for the analysis of plating and related solutions.Volumetric methods involve reactions of several types: oxidation-reduction,acid-base, complexation, and precipitation. Indicators are auxiliary reagents,which usually signify the endpoint of the analysis. The endpoint can be indicatedby a color change, formation of a turbid solution, or the solubilization ofa turbid solution.Some volumetric methods require little sample preparation, whereas othersmay require extensive preparation. Accuracy decreases for volumetric analyses ofcomponents found in low concentrations, as endpoints are not as easily observedas with the components found in high concentrations.Volumetric methods are limited in that several conditions must be satisfied.Indicators should be available to signal the endpoint of the titration. The component-titrant reaction should not be affected by interferences from other substancesfound in the solution.

GRAVIMETRIC METHODS

In gravimetric methods, the component being determined is separated from othercomponents of the sample by precipitation, volatilization, or electroanalyticalmeans. Precipitation methods are the most important gravimetric methods. Theprecipitate is usually a very slightly soluble compound of high purity that containsthe component. The weight of the precipitate is determined after it is filtered fromsolution, washed, and dried. Gravimetric methods are used to supplement theavailable volumetric methods.Limitations of gravimetric methods include the requirement that the precipitatedcomponent has an extremely low solubility. The precipitate must also be ofhigh purity and be easily filterable.Species that are analyzed gravimetrically include chloride, sulfate, carbonate,phosphate, gold, and silver.

INSTRUMENTAL METHODS

Instrumental methods differ from wet methods in that they measure a physicalproperty related to the composition of a substance, whereas wet methods relyon chemical reactions. The selection of an instrument for the analysis of platingsolutions is a difficult task. Analysts must decide if the cost is justified and if theanalytical instrument is capable of analyzing for the required substances with ahigh degree of accuracy and precision. Instruments coupled to computers canautomatically sample, analyze, and record results. Mathematical errors are minimizedand sample measurements are more reproducible than with wet methods.Instrumental methods are also extremely rapid when compared with wet methods.Unlike humans, instruments cannot judge. They cannot recognize impropersample preparation or interfering substances. Erroneous results are sometimesproduced by electronic and mechanical malfunctions.Analytical instruments frequently used in the analysis of plating solutions canbe categorized as spectroscopic, photometric, chromatographic, and electroanalytical.Spectroscopic methods (flame photometry, emission spectrometry, X-rayfluorescence, mass spectrometry, and inductively coupled plasma) are based onthe emission of light. Photometric methods (spectrophotometry, colorimetry,and atomic absorption) are based on the absorption of light. Chromatographicmethods (ion chromatography) involve the separation of substances for subsequentidentification. Electroanalytical methods (potentiometry, conductometry,polarography, amperometry, and electrogravimetry) involve an electric current inthe course of the analysis.The instrumental methods, comprehensively reviewed below, are most applicableto plating environments.

SPECTROSCOPIC METHODS

Spectroscopy is the analysis of a substance by the measurement of emitted light.When heat, electrical energy, or radiant energy is added to an atom, the atombecomes excited and emits light. Excitation can be caused by a flame, spark, X-rays,or an AC or DC arc. The electrons in the atom are activated from their groundstate to unstable energy shells of higher potential energy. Upon returning to theirground state, energy is released in the form of electromagnetic radiation.Because each element contains atoms with different arrangements of outermostelectrons, a distinct set of wavelengths is obtained. These wavelengths, from atomsof several elements, are separated by a monochromator such as a prism or a diffractiongrating. Detection of the wavelengths can be accomplished photographically(spectrograph) or via direct-reading photoelectric detectors (spectrophotometers).The measurement of intensity emitted at a particular wavelength is proportionalto the concentration of the element being analyzed.An advantage of spectroscopy is that the method is specific for the elementbeing analyzed. It permits quantitative analysis of trace elements without anypreliminary treatment and without prior knowledge as to the presence of the element.Most metals and some nonmetals may be analyzed. Spectroscopic analysisis also useful for repetitive analytical work.Disadvantages of spectroscopic analysis include the temperature dependenceof intensity measurements, as intensity is very sensitive to small fluctuationsintemperature. The accuracy and precision of spectrographic methods is not as highas some spectrophotometric methods or wet analyses. Spectrographic methodsare usually limited to maximum element concentrations of 3%. Additionally, sensitivityis much smaller for elements of high energy (e.g., zinc) than for elementsof low energy (e.g., sodium).Applications of spectroscopy include the analysis of major constituents andimpurities in plating solutions, and of alloy deposits for composition.

Flame Photometry

In flame photometry (FP), a sample in solution is atomized at constant air pressureand introduced in its entirety into a flame as a fine mist. The temperatureof the flame (1,800-3,100OK) is kept constant. The solvent is evaporated and thesolid is vaporized and then dissociated into ground state atoms. The valenceelectrons of the ground state atoms are excited by the energy of the flame tohigher energy levels and then fall back to the ground state. The intensities of theemitted spectrum lines are determined in the spectrograph or measured directlyby a spectrophotometer.The flame photometer is calibrated with standards of known composition andconcentration. The intensity of a given spectral line of an unknown can then becorrelated with the amount of an element present that emits the specific radiation.Physical interferences may occur from solute or solvent effects on the rate oftransport of the sample into the flame. Spectral interferences are caused by adjacentline emissions when the element being analyzed has nearly the same wavelengthas another element. Monochromators or the selection of other spectral linesminimize this interference. Ionization interferences may occur with the highertemperature flames. By adding a second ionizable element, the interferences dueto the ionization of the element being determined are minimized.An advantage of FP is that the temperature of the flame can be kept morenearly constant than with electric sources. A disadvantage of the method is thatthe sensitivity of the flame source is many times smaller than that of an electricarc or spark.FP is used for the analysis of aluminum, boron, cadmium, calcium, chromium,cobalt, copper, indium, iron, lead, lithium, magnesium, nickel, palladium, platinum,potassium, rhodium, ruthenium, silver, sodium, strontium, tin, and zinc.

Emission Spectrometry

In emission spectrometry (ES), a sample composed of a solid, cast metal or solutionis excited by an electric discharge such as an AC arc, a DC arc, or a spark.The sample is usually placed in the cavity of a lower graphite electrode, which ismade positive. The upper counterelectrode is another graphite electrode groundto a point. Graphite is the preferred electrode material because of its ability towithstand the high electric discharge temperatures. It is also a good electricalconductor and does not generate its own spectral lines.The arc is started by touching the two graphite electrodes and then separatingthem. The extremely high temperatures (4,000-6,000OK) produce emitted radiationhigher in energy and in the number of spectral lines than in flame photometry.Characteristic wavelengths from atoms of several elements are separated bya monochromator and are detected by spectrographs or spectrophotometers.Qualitative identification is performed by using available charts and tables toidentify the spectral lines that the emission spectrometer sorts out according totheir wavelength. The elements present in a sample can also be qualitatively determinedby comparing the spectrum of an unknown with that of pure samples ofthe elements. The density of the wavelengths is proportional to the concentrationof the element being determined. Calibrations are done against standard samples.ES is a useful method for the analysis of trace metallic contaminants in platingbaths. The “oxide” method is a common quantitative technique in ES. A sampleof the plating bath is evaporated to dryness and then heated in a muffle furnace.The resultant oxides are mixed with graphite and placed in a graphite electrode.Standards are similarly prepared and a DC arc is used to excite the sample andstandards.

X-ray Fluorescence

X-ray fluorescence (XRF) spectroscopy is based on the excitation of samples by anX-ray source of sufficiently high energy, resulting in the emission of fluorescentradiation. The concentration of the element being determined is proportional tothe intensity of its characteristic wavelength. A typical XRF spectrometer consistsof an X-ray source, a detector, and a data analyzer.Advantages of XRF include the nondestructive nature of the X-rays on thesample. XRF is useful in measuring the major constituents of plating baths suchas cadmium, chromium, cobalt, gold, nickel, silver, tin, and zinc. Disadvantagesof XRF include its lack of sensitivity as compared with ES.X-ray spectroscopy is also used to measure the thickness of a plated deposit.The X-ray detector is placed on the wavelength of the element being measured.The surface of the deposit is exposed to an X-ray source and the intensity of theelement wavelength is measured. A calibration curve is constructed for intensityagainst thickness for a particular deposit. Coating compositions can also be determinedby XRF.

Mass Spectrometry

In mass spectrometry (MS), gases or vapors derived from liquids or solids arebombarded by a beam of electrons in an ionization chamber, causing ionizationand a rupture of chemical bonds. Charged particles are formed, which may becomposed of elements, molecules, or fragments. Electric and magnetic fields thenseparate the ions according to their mass to charge ratios (m/e). The amount andtype of fragments produced in an ionization chamber, for a particular energy ofthe bombarding beam, are characteristic of the molecule; therefore, every chemicalcompound has a distinct mass spectrum. By establishing a mass spectrum ofseveral pure compounds, an observed pattern allows identification and analysisof complex mixtures.The mass spectrum of a compound contains the masses of the ion fragmentsand the relative abundances of these ions plus the parent ion. Dissociation fragmentswill always occur in the same relative abundance for a particular compound.MS is applicable to all substances that have a sufficiently high vapor pressure.This usually includes substances whose boiling point is below 450OC. MS permitsqualitative and quantitative analysis of liquids, solids, and gases.

Inductively Coupled Plasma

Inductively coupled plasma (ICP) involves the aspiration of a sample in a streamof argon gas, and then its ionization by an applied radio frequency field. The fieldis inductively coupled to the ionized gas by a coil surrounding a quartz torch thatsupports and encloses the plasma. The sample aerosol is heated in the plasma, themolecules become almost completely dissociated and then the atoms present inthe sample emit light at their characteristic frequencies. The light passes througha monochromator and onto a detector.The high temperature (7,000OK) of the argon plasma gas produces efficientatomic emission and permits low detection limits for many elements. As withatomic absorption (AA),ICP does not distinguish between oxidation states (e.g.,Cr3+ and Cr6+) of the same element—the total element present is determined.Advantages of ICP include complete ionization and no matrix interferences asin AA. ICP allows simultaneous analysis of many elements in a short time. It issensitive to part-per-billion levels.Disadvantages of ICP include its high cost and its intolerance to samples withgreater than 3% dissolved solids. Background corrections usually compensate forinterferences due to background radiation from other elements and the plasmagases. Physical interferences, due to viscosity or surface tension, can cause significanterrors. These errors are reduced by diluting the sample. Although chemicalinterferences are insignificant in the ICP method, they can be greatly minimizedby careful selection of the instrument’s operating conditions, by matrix matching,or by buffering the sample.ICP is applicable to the analysis of major components and trace contaminantsin plating solutions. It is also useful for waste-treatment analysis.

PHOTOMETRIC METHODS

Photometric methods are based on the absorption of ultraviolet (200-400 nm)or visible (400-1,000 nm) radiant energy by a species in solution. The amount ofenergy absorbed is proportional to the concentration of the absorbing species insolution. Absorption is determined spectrophotometrically or colorimetrically.The sensitivity and accuracy of photometric methods must be frequentlychecked by testing standard solutions in order to detect electrical, optical, ormechanical malfunctions in the analytical instrument.

Spectrophotometry and Colorimetry

Spectrophotometry involves analysis by the measurement of the light absorbed bya solution. The absorbance is proportional to the concentration of the analytein solution. Spectrophotometric methods are most often used for the analysisof metals with concentrations of up to 2%.Spectrophotometers consist of a light source (tungsten or hydrogen), a monochromator,a sample holder, and a detector. Ultraviolet or visible light of a definitewavelength is used as the light source. Detectors are photoelectric cells thatmeasure the transmitted (unabsorbed) light. Spectrophotometers differ fromphotometers in that they utilize monochromators, whereas photometers use fil-ters to isolate the desired wavelength region. Filters isolate a wider band of light.In spectrophotometric titrations, the cell containing the analyte solutionis placed in the light path of a spectrophotometer. Titrant is added to the cellwith stirring, and the absorbance is measured. The endpoint is determinedgraphically. Applications of this titration include the analysis of a mixture ofarsenic and antimony and the analysis of copper with ethylene diamine tetraacetic acid (EDTA).The possibility of errors in spectrophotometric analyses is increased whennumerous dilutions are required for an analysis.Colorimetry involves comparing the color produced by an unknown quantityof a substance with the color produced by a standard containing a known quantityof that substance. When monochromatic light passes through the coloredsolution, a certain amount of the light, proportional to the concentration of thesubstance, will be absorbed. Substances that are colorless or only slightly coloredcan be rendered highly colored by a reaction with special reagents.In the standard series colorimetric method, the analyte solution is diluted toa certain volume (usually 50 or 100 ml) in a Nessler tube and mixed. The colorof the solution is compared with a series of standards similarly prepared. Theconcentration of the analyte equals the concentration of the standard solutionwhose color it matches exactly. Colors can also be compared to standards via acolorimeter (photometer), comparator, or spectrophotometer.The possible errors in colorimetric measurements may arise from the followingsources: turbidity, sensitivity of the eye or color blindness, dilutions, photometerfilters, chemical interferences, and variations in temperature or pH.Photometric methods are available for the analysis of the following analytes:Anodizing solutions: Fe, Cu, agents.

Atomic Absorption

Metals in plating and related solutions can be readily determined by AA spectrophotometry.Optimum ranges, detection limits, and sensitivities of metals varywith the various available instruments.In direct-aspiration atomic absorption (DAAA) analysis, the flame (usually air-acetyleneor nitrous oxide-acetylene) converts the sample aerosol into atomic vapor,which absorbs radiation from a light source. A light source from a hollow cathodelamp or an electrodeless discharge lamp is used, which emits a spectrum specificto the element being determined. The high cost of these lamps is a disadvantageof the AA method. A detector measures the light intensity to give a quantitativedetermination.DAAA is similar to flame photometry in that a sample is aspirated into a flameand atomized. The difference between the two methods is that flame photometrymeasures the amount of emitted light, whereas DAAA measures the amount oflight absorbed by the atomized element in the flame. In DAAA, the number ofatoms in the ground state is much greater than the number of atoms in any of theexcited states of the spectroscopic methods. Consequently, DAAA is more efficientand has better detection limits than the spectroscopic methods.Spectral interferences occur when a wavelength of an element being analyzedis close to that of an interfering element. The analysis will result in an erroneouslyhigh measurement. To compensate for this interference, an alternate wavelengthor smaller slit width is used.When the physicalproperties (e.g., viscosity) of a sample differ from those of thestandard, matrix interferences occur. Absorption can be enhanced orsuppressed.To overcome these interferences, matrix components in the sample and standardare matched or a release agent, such as EDTA or lanthanum, is added.Chemical interferences are the most common interferences encountered in AAanalysis. They result from the nonabsorption of molecularly bound atoms in theflame. These interferences are minimized by using a nitrous oxide-acetylene flameinstead of an air-acetylene flame to obtain the higher flame temperature neededto dissociate the molecule or by adding a specific substance (e.g.,lanthanum) torender the interferant harmless. Chemical interferences can also be overcome byextracting the element being determinedor by extracting the interferant fromthe sample.The sensitivity and detection limits in AA methods vary with the instrumentused, the nature of the matrix, the type of element being analyzed, and the particularAA technique chosen. It is best to use concentrations of standards andsamples within the optimum concentration range of the AA instrument. WhenDAAA provides inadequate sensitivity, other specialized AA methods, such asgraphite furnace AA, cold vapor AA, or hydride AA, are used.In graphite furnace AA (GFAA), the flame that is used in DAAA is replacedwith an electrically heated graphite furnace. A solution of the analyte is placedin a graphite tube in the furnace, evaporated to dryness, charred, and atomized.The metal atoms being analyzed are propelled into the path of the radiationbeam by increasing the temperature of the furnace and causing the sample tobe volatilized. Only very small amounts of sample are required for the analysis.GFAA is a very sensitive technique and permits very low detection limits. Theincreased sensitivity is due to the much greater occupancy time of the groundstate atoms in the optical path as compared with DAAA. Increased sensitivity canalso be obtained by using larger sample volumes or by using an argon-hydrogenpurge gas mixture instead of nitrogen. Because of its extreme sensitivity, determiningthe optimum heating times, temperature, and matrix modifiers is necessaryto overcome possible interferences.Interferences may occur in GFAA analysis due to molecular absorption andchemical effects. Background corrections compensate for the molecular absorptioninterference. Specially coated graphite tubes minimize its interaction withsome elements. Gradual heating helps to decrease background interference, andpermits determination of samples with complex mixtures of matrix components.The GFAA method has been applied to the analysis of aluminum, antimony,arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead,manganese, molybdenum, nickel, selenium, silver, and tin.Cold vapor atomic absorption (CVAA) involves the chemical reduction of mercuryor selenium by stannous chloride and its subsequent analysis. The reduced solutionis vigorously stirred in the reaction vessel to obtain an equilibrium betweenthe element in the liquid and vapor phases. The vapor is then purged into anabsorption cell located in the light path of a spectrophotometer. The resultantabsorbance peak is recorded on a strip chart recorder.The extremely sensitive CVAA procedure is subject to interferences from someorganics, sulfur compounds, and chlorine. Metallic ions (e.g., gold, selenium),which are reduced to the elemental state by stannous chloride, produce interferencesif they combine with mercury.Hydride atomic absorption (HAA) is based on chemical reduction with sodiumborohydride to selectively separate hydride-forming elements from a sample.The gaseous hydride that is generated is collected in a reservoir attached to ageneration flask, and is then purged by a stream of argon or nitrogen into anargon-hydrogen-air flame. This permits high-sensitivity determinations of antimony,arsenic, bismuth, germanium, selenium, tellurium, and tin.The HAA technique is sensitive to interferences from easily reduced metalssuch as silver, copper, and mercury. Interferences also arise from transitionmetals in concentrations greater than 200 mg/L and from oxides of nitrogen.

Ion Chromatography

In ion chromatography (IC), analytes are separated with an eluent on a chromatographiccolumn based on their ionic charges. Because plating solutionsare water based, the soluble components must be polar or ionic; therefore, IC isapplicable to the analysis of plating and related solutions.Ion chromatographs consist of a sample delivery system, a chromatographicseparation column, a detection system, and a data handling system.IC permits the rapid sequential analysis of multiple analytes in one sample. Thevarious detectors available, such as UV-visible, electrochemical, or conductivity,allow for specific detection in the presence of other analytes. IC is suitable for theanalysis of metals, anionic and cationic inorganic bath constituents, and variousorganic plating bath additives. It is also used for continuous on-line operations.Interferences arise from substances that have retention times coinciding withthat of any anion being analyzed. A high concentration of a particular ion mayinterfere with the resolution of other ions. These interferences can be greatlyminimized by gradient elution or sample dilution.IC has been applied to the analysis of the following analytes in plating andrelated solutions:Metals: Aluminum, barium, cadmium, calcium, trivalent and hexavalent chromium,cobalt, copper, gold, iron, lead, lithium, magnesium, nickel, palladium,platinum, silver, tin,zinc.Ions: Ammonium, bromide, carbonate, chloride, cyanide, fluoborate, fluoride,hypophosphite, nitrate, nitrite, phosphate, potassium,sodium, sulfate, sulfide,sulfite.Acid Mixtures: Hydrofluoric, nitric, and acetic acids.Organics: Brighteners, surfactants, organic acids.

ELECTROANALYTICAL METHODS

Electroanalytical methods involve the use of one or more of three electrical quantities—current, voltage, and resistance. These methods are useful when indicatorsfor a titration are unavailable or unsuitable. Although trace analysis may be donequite well by spectroscopic or photometric methods, electroanalytical methodsoffer ease of operation and relatively lower costs of purchase and maintenance.

Potentiometry

Potentiometry involves an electrode that responds to the activity of a particulargroup of ions in solution. Potentiometric methods correlate the activity of anion with its concentration in solution.In potentiometric titrations, titrant is added to a solution and the potentialbetween an indicator and reference electrode is measured. The reaction mustinvolve the addition or removal of an ion for which an electrode is available.Acid-base titrations are performed with a glass indicator electrode and a calomelreference electrode. The endpoint corresponds to the maximum rate of changeof potential per unit volume of titrant added.Advantages of potentiometric titrations include its applicability to colored,turbid, or fluorescent solutions. It is also useful in situations where indicatorsare unavailable.The sensitivity of potentiometric titrations is limited by the accuracy of themeasurement of electrode potentials at low concentrations. Solutions thatare more dilute than 10-5 N cannot be accurately titrated potentiometrically.This is because the experimentally measured electrode potential is a combinedpotential, which may differ appreciably from the true electrode potential. Thedifference between the true and experimental electrode potentials is due to theresidual current, which arises from the presence of electroactive trace impurities.The direct potentiometric measurement of single ion concentrations is donewith ion selective electrodes (ISEs). The ISE develops an electric potential inresponse to the activity of the ion for which the electrode is specific. ISEs areavailable for measuring calcium, copper, lead, cadmium, ammonia, bromide,nitrate, cyanide, sulfate, chloride, fluoride, and other cations and anions.Cation ISEs encounter interferences from other cations, and anion ISEsencounter interferences from other anions. These interferences can be eliminatedby adjusting the sample pH or by chelating the interfering ions. ISEinstructions must be reviewed carefully to determine the maximum allowablelevels of interferants, the upper limit of the single ion concentration for the ISE,and the type of media compatible with the particular ISE.Some of the solutions that can be analyzed by potentiometric methods are:Anodizing solutions: Al,

Anodizing solutions: Al, H2SO4, C2H2O4, CrO3, Cl

Brass solutions: Cu, Zn, NH3, CO3

Bronze solutions: Cu, Sn, NaOH, NaCN, Na2CO3

Chromium solutions: Cr, Cl

Cadmium solutions: Cd, NaOH, NaCN, Na2CO3

Acid copper solutions: Cl

Alkaline copper solutions: NaOH, NaCN, Na2CO3

Gold solutions: Au, Ag, Ni, Cu

Lead and tin/lead solutions: Pb, Sn, HBF4

Nickel solutions: Co, Cu, Zn, Cd, Cl, H3BO3

Silver solutions: Ag, Sb, Ni

Acid tin solutions: Sn, HBF4, H2SO4

Alkaline tin solutions: Sn, NaOH, NaCO3, Cl

Conductometry

Electrolytic conductivity measures a solution’s ability to carry an electric current.A current is produced by applying a potential between two inert metallicelectrodes (e.g., platinum) inserted into the solution being tested. When othervariables are held constant, changes in the concentration of an electrolyte resultin changes in the conductance of electric current by a solution.In conductometric titrations, the endpoint of the titration is obtained from aplot of conductance against the volume of titrant. Excessive amounts of extraneousforeign electrolytes can adversely affect the accuracy of a conductometrictitration.Conductometric methods are used when wet or potentiometric methodsgive inaccurate results due to increased solubility (in precipitation reactions)or hydrolysis at the equivalence point. The methods are accurate in both diluteand concentrated solutions, and they can also be used with colored solutions.Conductometric methods have been applied to the analysis of Cr, Cd, Co, Fe,

Ni, Pb, Ag, Zn, CO3, Cl, F, and SO4.

Polarography

In polarography, varying voltage is applied to a cell consisting of a large mercuryanode (reference electrode) and a small mercury cathode (indicator electrode)known as a dropping mercury electrode (DME). Consequent changes in currentare measured. The large area of the mercury anode precludes any polarization.The DME consists of a mercury reservoir attached to a glass capillary tube withsmall mercury drops falling slowly from the opening of the tube. A saturatedcalomel electrode is sometimes used as the reference electrode.The electrolyte in the cell consists of a dilute solution of the species beingdetermined in a medium of supporting electrolyte. The supporting electrolytefunctions to carry the current in order to raise the conductivity of the solution.This ensures that if the species to be determined is charged, it will not migrateto the DME. Bubbling an inert gas, such as nitrogen or hydrogen, through thesolution prior to running a polarogram, will expel dissolved oxygen in order toprevent the dissolved oxygen from appearing on the polarogram.Reducible ions diffuse to the DME. As the applied voltage increases, negligiblecurrent flow results until the decomposition potential is reached for the metalion being determined. When the ions are reduced at the same rate as they diffuseto the DME, no further increases in current occur, as the current is limited bythe diffusion rate. The half-wave potential is the potential at which the currentis 50% of the limiting value.Polarograms are obtained by the measurement of current as a function ofapplied potential. Half-wave potentials are characteristic of particular substancesunder specified conditions. The limiting current is proportional to the concentrationof the substance being reduced. Substances can be analyzed quantitativelyand qualitatively if they are capable of undergoing anodic oxidation orcathodic reduction. As with other instrumental methods, results are referred tostandards in order to quantitate the method.Advantages of polarographic methods include their ability to permit simultaneousqualitative and quantitative determinations of two or more analytes inthe same solution. Polarography has wide applicability to inorganic, organic,ionic, or molecular species.Disadvantages of polarography include the interferences caused by large concentrationsof electropositive metals in the determination of low concentrationsof electronegative metals. The very narrow capillary of the DME occasionallybecomes clogged.Polarographic methods are available for the following solutions:

Anodizing solutions: Cu, Zn, Mn

Brass solutions: Pb, Cd, Cu, Ni, Zn

Bronze solutions: Pb, Zn, Al, Cu, Ni

Cadmium solutions: Cu, Pb, Zn, Ni

Chromium solutions: Cu, Ni, Zn, Cl, SO4

Acid copper solutions: Cu, Cl

Alkaline copper solutions: Zn, Fe, Pb, Cu

Gold solutions: Au, Cu, Ni, Zn, In, Co, Cd

Iron solutions: Mn

Lead and tin-lead solutions: Cu, Cd, Ni, Zn, Sb

Nickel solutions: Cu, Pb, Zn, Cd, Na, Co, Cr, Mn

Palladium solutions: Pd, Cr3+, Cr6+

Rhodium solutions: Rh

Silver solutions: Sb, Cu, Cd

Acid tin solutions: Sn4+, Cu, Ni, Zn

Alkaline tin solutions: Pb, Cd, Zn, Cu

Acid zinc solutions: Cu, Fe, Pb, Cd

Alkaline zinc solutions: Pb, Cd, Cu

Wastewater: Cd, Cu, Cr3+, Ni, Sn, Zn

AMPEROMETRY

Amperometric titrations involve the use of polarography as the basis of an electrometrictitration. Voltage applied across the indicator electrode (e.g., DME orplatinum) and reference electrode (e.g., calomel or mercury) is held constantand the current passing through the cell is measured as a function of titrantvolume added. The endpoint of the titration is determined from the intersectionof the two straight lines in a plot of current against volume of titrant added.Polarograms are run to determine the optimum titration voltage.Amperometric titrations can be carried out at low analyte concentrations atwhich volumetric or potentiometric methods cannot yield accurate results. Theyare temperature independent and more accurate than polarographic methods.Although amperometry is useful for oxidation-reduction or precipitationreactions,few acid-base reactions are determined by this method.Some of the reactions that can be analyzed by amperometric methods aregiven in Table II.

ELECTROGRAVIMETRY

In electrogravimetry, the substance to be determined is separated at a fixed potentialon a preweighed inert cathode, which is then washed, dried, and weighed.Requirements for an accurate electrogravimetric analysis include good agitation,smooth adherent deposits, and proper pH, temperature, and current density.Advantages of electrogravimetry include its ability to remove quantitativelymost common metals from solution. The method does not require constantsupervision. Disadvantages include long electrolysis times.Some of the metals that have been determined electrogravimetrically arecadmium, cobalt, copper, gold, iron, lead, nickel, rhodium, silver, tin, and zinc.

SAMPLING

Analyses are accurate only when the sample is truly representative of the solutionbeing analyzed. Each tank should have a reference mark indicating the correct levelfor the solution, and the bath should always be at this level when the sample istaken. Solutions should be stirred before sampling. If there is sludge in the tank,the solution should be stirred at the end of the day and the bath allowed to standovernight, taking the sample in the morning.Solutions should be sampled by means of a long glass tube. The tube isimmersed in the solution, the thumb is placed over the upper open end, and afull tube of solution is withdrawn and transferred to a clean, dry container. Thesolution should be sampled at a minimum of 10 locations in the tank to ensure arepresentative sample. A quart sample is sufficient for analysis and Hull cell testing,and any remaining solution can be returned to its tank.

STANDARD SOLUTIONS, REAGENTS, AND INDICATORS FOR WET

METHODS

Standard solutions, reagents, and indicators can be purchased ready-made fromlaboratory supply distributors. Unless a laboratory has the experience and highdegree of accuracy that is required in preparing these solutions, it is recommendedthat they be purchased as prepared solutions. Preparations for all the solutions aregiven here to enable technicians to prepare or recheck their solutions.A standard solution is a solution with an accurately known concentration ofa substance used in a volumetric analysis. Standardization of standard solutionsrequires greater accuracy than routine volumetric analyses. An error in standardizationcauses errors in all analyses that are made with the solution; therefore,Primary Standard Grade chemicals should be used to standardize standard solutions.The strengths of standard solutions are usually expressed in terms of normalityor molarity. Normalities of standard solutions and their equivalent molarities arelisted in Table III. The methods to standardize all the standard solutions requiredfor the analysis of plating and related solutions are listed in Table IV.Indicators are added to solutions in volumetric analyses to show color changeor onset of turbidity, signifying the endpoint of a titration. The indicators requiredfor all of the analyses and their preparations are listed in Table V. Analytical Gradechemicals should be used in preparing analytical reagents (Table VI) and ReagentGrade acids should be used (Table VII). When chemicals of lesser purity are used,the accuracy of the results will be diminished.Tables VIII through XII provide specific methods for testing the constituentsof electroplating, electroless, and anodizing baths, as well as acid dips and alkalinecleaners.

???

Fig. 1. Test setup for determination of cathode efficiency. Use 500-ml beakers and

1 ´ 2-in. brass cathodes. The anodes for the test solution should match that used in the

plating bath. Use copper anodes for the coulometer.

SAFETY

As with any laboratory procedure, the accepted safety rules for handling acids,bases, and other solutions should be followed. Acids are always added to water,not the reverse. Mouth pipettes should not be used for pipetting plating solutions.Safety glasses should always be worn, and care should be exercised to avoidskin and eye contact when handling chemicals. A fume hood should be usedwhen an analytical method involves the liberation of hazardous or annoyingfumes. Laboratory staff should be well versed in the first-aid procedures requiredfor various chemical accidents.

DETERMINATION OF CATHODE EFFICIENCY

The procedure for determining cathode efficiency, using the setup pictured inFig. 1, is as follows:

1. Connect the copper coulometer in series with the test cell.

2. The copper coulometer solution should contain 30 oz/gal copper sulfatepentahydrate and 8 oz/gal sulfuric acid.

3. Use the same anodes, temperature, and agitation in the test solution thatare used in the plating bath.

4. Plate at 0.4 A (30 A/ft2) for a minimum of 10 minutes.

5. Rinse both cathodes, dry in acetone, and weigh.% Cathode Efficiency =weight in grams of test metal X valence of test metal in bath X 3177weight in grams of copper metal « atomic weight of test metal

*Editor’s note: To view this article in its entirety, including corresponding tables,

please consult the online Guidebook archive.

493

ZINC & ZINC ALLOY PLATING

PROBLEMS WITH EN BATHS AND ACID COPPER

troubleshooting, testing & analysis

BY MATT STAUFFER, PAVCO, CHARLOTTE, N.C.

Q: Our nickel metal is high in our bright nickel bath. [My vendor] tells us the only way tobring it down is to decant. Is there a way to bring it down slowly without wasting solution?

A: The increase in nickel metal growth in most acidic plating processes is causedby the difference in anode and cathode plating efficiencies. In the case of Wattsnickel baths, you have 100% anode efficiency and approximately 93% cathodeefficiency. The remaining 7% is directed towards the reduction of hydrogen intohydrogen gas. This hydrogen is a common source of gas pitting. So basically,you are dissolving more metal than you are plating out. Most platers dilutetheir baths to correct them. Some platers look towards insoluble anodes. Caremust be taken to avoid the creation of harmful oxidation products when usinginsolubles.There are products on the market that utilize a membrane system to preventthis reaction. These can be expensive and do require maintenance.The other options to consider involve nickel recycling. This can be done byworking with a company that recycles nickel solutions (plating or stripping)into nickel metal or by finding another plater, usually a barrel plater, who has aregular need for nickel salts in his process. Usually, an arrangement can be madeto be beneficial to both parties.The final option would be what I would call “home-grown” recycling. Setup a small plate out tank with insoluble carbon anodes. A good alternative forcathodes is nickel anode chips, which can be readily barrel plated.Occasional adjustment of the pH may be required to raise the pH due to theuse of insoluble anodes. Fresh solution can be added as the nickel is depletedfrom the plate out cell. The plated anodes can now be reused in the anode baskets.

Q: I am getting excess sulphate in my chrome tank. Due to this excess, I’m getting dull plating.Can you please tell me how to resolve this problem? Also, in my cyanide copper tank thesolution becomes dark after 4-5 plating rounds. Can you offer some advice?

A: You mention a chrome bath and a copper bath, so I’ll go out on a limb andassume your plating copper-nickel-chrome. If so, the usual source for excesssulfate is insufficient rinsing after nickel plating. A typical Watts nickel bathwill contain upwards of 35 opg (263 g/l) of nickel sulfate. Even a simple sulfatecatalyzed hex chrome has only 0.32 opg (2.4 g/l) sulfate. Based on those relativequantities, it is pretty easy to get a fair amount of sulfate from a nickel bath intoyour chrome if rinsing is insufficient. Use barium carbonate to remove excesssulfate and consider additional rinse tanks (counterflowed) after nickel plating,and/or use of spray rinsing for blind holes, etc.As far as the dark copper, we need a little more information to go on, but Iwill throw out the general advice that more cleaning is always good. You may becontaminating your copper with oils or buffing compound due to insufficientcleaning. Carbon treatment of your cyanide copper is an effective way to removecontaminants. Make sure your parts have a water-break-free surface before youattempt to plate them.

Q: I’m doing an acid-copper plating process on zinc die cast material ( MAZAK ). Thefinish obtained from acid-copper plating is excellent, but after that, when I apply lacquerto it, the material tarnishes in few days. What’s the problem?

A: Copper and copper alloys are prone to tarnishing on exposure to the atmosphere.It does require a suitable lacquer with sufficient thickness to preventexposure to oxidation. A neutralizing step is recommended to remove the acidfrom the surface, which in itself can cause issues when coming directly from acidcopper into a lacquer. A 1% ammonia dip or a heavily silicated brass cleaner followedby a thorough, clean rinse would be suitable. Extra care should be takenhere to ensure adequate neutralization and rinsing of porous or rough surfaces.This will result in a “bleed out” type of tarnish pattern. Proper lacquers for yourapplication are specifically designed for copper alloys. For extra protection, a hexchrome passivation or a non-chrome passivate for copper is available to provideadditional protection as the lacquer cures.

Q: I m working with cyanide copper barrel plating. Parts are zip sliders made of zinc diecasting. The problem I’m facing is controlling this solution to get bright copper parts. Can Iuse any other alloy in this solution? If yes, then which metal and how much?

A: There are various brightener systems used in cyanide copper. Some are metallic,some are organic, and some are both. I would suggest you contact a local finishingsupplier to get information on common additives available in your area.Something to consider regarding cyanide material sources: Sulfur is a commoncontaminant found in certain sources of sodium and potassium cyanide.This can cause a dark/dull red low current density area. This is easily fixedby a small addition of zinc cyanide, (1-2 g/l) as the zinc reacts with the sulfurcompound. The small amount of zinc will give a very low co-deposition of zinc,which is not a problem when plating zippers.

Q: I am plating semi-brilliant nickel bath over steel; my customer is heating parts (afternickel plating) up to 1250 degress celsius, and they are having blistering problems. Whatcan I do to solve this problem?

A: This situation may be caused by either base metal preparation or conditions (i.e.,stress) in your nickel plate. The problem needs to be isolated in order to solve it.I would recommend that you plate a zinc-coated steel hull cell panel in thetank using the same semi-bright nickel solution. Strip the zinc with fresh hydrochloricacid, then remove the panel from the acid immediately after stripping iscomplete to avoid over pickling. Ensure that there is no water break film beforeyou plate in your nickel.Heat treat the plated panel and check for blistering. If you see blistering, itwould appear you have an issue with the semi-bright nickel. If you do not seeblistering, chances are your problem is related to surface preparation. You canrepeat the plating test for confirmation in the lab by running hull cell panels ofyour nickel and a newly made nickel, and heat treating both panels. If you confirmthat the existing solution blisters and the new solution does not, you willthen need to investigate several potential factors, using the hull cell to confirmappropriate corrective action.

1. Excess semi-bright brightener additive. Semi-bright nickel does use levelingagents. Excess class 2 nickel brightener will impact stresslevels. This canbe removed by electrolysis.

2. Organic contaminant caused by brightener breakdown or soils/oil frompoor cleaning. Look at peroxide/carbon treatment for improvement.

3. Stress can be monitored through stress tabs or spiral contractometer. Thiswill allow a more direct evaluation of treatments.

4. Certain metals can co-deposit and cause stress. Look for low current densitydarkness in the hull cell. These metals can be dummy plated to remove.

5. Check iron levels and peroxide treat if necessary. Iron can cause HCDdefects. Keep below 20 ppm for your application.

6. Always make sure basic chemistry is correct. Low nickel, low boric, high pHcan all cause high current density issues. Start here.If you find there is no blistering on a test panel, then it is likely that your issueis related to preparation. Investigate to ensure parts are free of water breaks.Check for sufficient oxide removal as well as excess pickling. As Yogi Berra usedto say about plating, “90 percent of the plating game is half preparation.”

Q: I have alkaline-free cyanide zinc plating baths; the temperature is now 35°C. How canwe cool down the solutions in order to have better conditions? Is there a product that worksat high temperatures, or what kind of equipment should we use?

A: Cooling coils in the plating tank or a heat exchanger connected to an industrialchiller are recommended for alkaline zinc plating. There are zinc brighteners on themarket that work well (enough) at elevated temperatures, but none are as brightacross all current densities as a lower temperature bath operated at 25 degreesCelcius. Different additive levels and bath parameters are required at the highertemperatures. You can expect to use more brightener, and you will find that somelow current density areas tend not to be as bright as the lower temperature process.This may not be objectionable once the parts are bright-dipped and chromated,as both steps will tend to polish out dullness in the deposit. A stronger(or longer) bright dip step will help compensate for poor low current densitybrightness in the zinc plating. High polishing blue chromates are also availableto further your cause.Your ability to produce acceptably uniform brightness may depend upon, tosome degree, the geometry of the parts. Large flat surfaces tend not to polishnearly as well as round surfaces. Air agitation during polishing and chromatingwill tend to help compensate, but areas of the part that remain unagitated(interiors) may remain dull.In general, much of your success will depend on the nature of the work youare doing, (small parts vs. large flat parts) as well as the degree of brightness oruniformity desired.

Q: What is best for cleaning parts with small cracks?, I am using 20 pounds, but it seems tonot be enough. What should I use? I am using 2 stage ( cleaner and rinse) and times around35 sec by stage, normal concentration betwen 3 to 4 % of alkaline cleaner and 80°C oftemperature in both tanks.

A: Cracks and other areas where solution exchange are not good are challengingto clean and may require equipment improvements to address these issues.High-pressure spray cleaning or ultrasonic cleaning are good options for thesetypes of applications.

Q: Do you know of any solution to prevent oxide after nickel plating? The coating is nickelon steel, and the thickness is 5 microns average. It is a bright nickel solution, and we needto protect uncoated parts of low current density.

A: The oldest answer to your question is the use of a chromic acid passivationstep after nickel plating. Use 20-40 g/l of chromic acid, preferably hot, 35-40degrees C. This has the added benefit of removing any flash rusting that mayhave occurred in any of the process steps, such as acid rinse or nickel rinse.There are options on the market for similar processes that are free of hexavalentchrome. There are also many water-based lacquers or topcoats that can be usedover nickel plating to supplement corrosion protection in thin coverage areas.

Q: Is Alkaline Non Cyanide Copper effective & successful on Zinc Die Casted (Zamak 5)components?

A: When plating over zinc diecast, there are generally two copper processesinvolved. I will discuss both old and new methods here:

1. A copper strike is traditionally used directly over diecast to provide optimaladhesion. When using cyanide processes, the proper chemistry is neededin terms of pH, free cyanide, and copper content to ensure optimal adhesion.To replace this without cyanide, there are a few commercial processesavailable. If you google “non cyanide copper” you will find them near thetop. These require more attention than the standard cyanide processes andhave a higher operating cost due to the use of insoluble anodes. There arerecommended conditions that should be followed to optimize adhesionover zinc diecast.

2. The copper strike is followed by a cyanide copper plate that is typically optimizedfor higher efficiency in order to provide a sufficient barrier layer priorto nickel plating. This helps improve the corrosion resistance of the finalcoating system as well as minimize contamination of the nickel processes.The same non-cyanide processes as mentioned above can also be used ascopper plate. Pyrophosphate copper is also a time-tested process that is fullybright, and works as a suitable replacement for a heavy cyanide copper. Itdoes use copper anodes, which keeps operating cost down. It also is superiorto acid copper for use over diecast due to the mildly alkaline operating pH.This prevents attack of the zinc base metal that is seen in acid copper inareas where the copper strike is thin or in unplated internal areas.In general, non-cyanide copper is a more common choice when a facility haslittle or no other sources of cyanide in the facility. This makes the increasedoperating cost easily justified.

Q: We own and operate a small nickel plating facility. We only have a nickel sulfate bath.Would it behoove us to also have a nickel “strike” or copper “strike?” Would there be anybenefits in this? Also, do you see manufacturers that honestly want to go green, even if itcosts more?

A: The need for a strike bath is dictated by the type of substrate you are plating.If you are plating brass, copper, or steel, then a strike bath is not necessary. Ifyou are plating zinc die cast or tin alloys, then a strike bath is considered necessary.It is usually a copper strike.There is one benefit of a strike that is worth considering for substrates thatdo not necessarily require it. That is, it does isolate your main bath from contaminants,especially those that are cleaning related. This could mean the oilsand soils themselves, or even the cleaners, can become contaminants whendealing with difficult-to-rinse part configurations, such as hollow interiors orcup-shaped areas that cause high drag-out.Having a separate strike tank takes the brunt of the contamination, and itacts as the “canary in the coal mine,” showing the effect of contaminants beforethey end up contaminating your larger plating tanks. They are much easier andless expensive to treat or replace due to their size.Regarding green initiatives, the most readily adopted green initiatives arethose that save money or are revenue neutral, but sometimes all costs are notfully considered when making these types of decisions. When making a casefor a green initiative, it is important to include all of the costs associated withboth scenarios. This should include, at aminimum: employee training, healthcare costs, insurance costs, rejection rates, productivity impact, chemical totalcost, waste treatment cost, hazardous waste cost, incoming chemical shippingcost, and outgoing waste shipping cost. Some form of risk assessment can alsobe factored in, but calculating a cost for this is more nebulous, although peaceof mind does have a value for many people. If you can cast a wide net with thesecost factors, it is usually much easier to make the math of the “green” decisionmore palatable.For more information, please visit www.pavco.com.

MICRO- AND NANO-INDENTATION TESTING OF PLATING THICKNESS

troubleshooting, testing, & analysis

BY RAHUL NAIR, FISCHER TECHNOLOGY, INC., WITH CO-AUTHORS: MATT

TAYLOR, FISCHER TECHNOLOGY, INC., AND BERND BINDER, HELMUT

FISCHER, GMBH.

Indentation Testing is the technique of using a harder material commonlyreferred to as an indenter to deform a softer material. The calculated hardness(H) is the applied force (F) divided by the corresponding area of contact (A); H =F/A. One of the first modern forms of this technique was implemented by JohanAugust Brinell in 1900 [1]. A very heavy load, up to 30,000 N, is applied througha 10mm diameter hard ball onto the test material. The hardness of the materialis calculated by measuring the diameter of the residual imprint.As materials increased in hardness over the years, new techniques had to bedeveloped to measure this property. Patented in 1914 the Rockwell Test employssmaller indenters; a diamond cone or a 1/16 inch diameter steel ball 1. A lowerfixed load in the range of 600 N to 1,500 N is applied, the penetration depthmeasured and the corresponding area of contact calculated.While the aforementioned techniques are used to measure hardness of metalsand ceramics, Durometers where developed to measure the hardness of softpolymeric materials. Developed in the 1920s, ‘Shore’ hardness of material ischaracterized through this technique using Durometers with different springconstants and a conical or spherical shaped indenter per ASTM D 2240 andISO 868.Surface treatments of soft steels like case hardening, carburizing and carbonitridingrequire the surface mechanical properties to be measured, not the bulk.In order to limit the stress field from an indent to the treated surface, lower loadshave to be applied through smaller indenters. The Vickers and Knoop hardnesswere developed in 1921 and 1939, respectively, to meet this need. Indentersused in these techniques are diamond pyramids where the four sides meet at apoint. Low loads of up to 5N are applied through these indenters, and the areaof the residual imprint is optically measured per ISO 6507-1, 2, ISO 4545-1, 2or ASTM E384.Developments in deposition technology have resulted in an increase in theuse of thin films and coatings for aesthetic, tribological as well as functionalpurposes. These materials are used for a wide range of applications like automotiveclear coatings, protective metallic coatings, cutting tools, integratedcircuits and biomaterials. While traditional indentation testing can be used tocharacterize bulk steel, micro/nano scale layers and components have broughtmore challenges.Until recently, measuring the Pencil hardness of thin films according to ISO15184 has been commonplace, especially in the automotive paint industry. Withthis method, pencils of different hardness are moved at a certain angle and witha certain force across the paint surface to be tested. The ‘pencil hardness’ of thecoating is defined by two consecutive levels of pencil hardness, where the softerpencil leaves only a writing track, whereas the harder pencil causes a tangibledeformation of the paint coating.While Pencil, Vickers and Knoop hardness are still in use, the reliability andreproducibility of these methods are contentious for reasons mentioned later in thisarticle. Due to stringent quality standards in the coating industry, it is necessary to beable to test the hardness of coatings with accuracy and repeatability. The hardness ofthin coatings on tool bits, the viscoelasticity of protective coatings on optical lenses,the low friction coatings in consumer products all require precision application ofmillinewtons of force and corresponding measurements of depth in nanometers.This has led to the development of nanoindentation.

Nanoindentation

Instrumented indentation testing, more commonly referred to as nanoindentation– or, in simpler terms, depth-sensing indentation employs high-resolutioninstrumentation to continuously control and monitor the loads and displacementsof an indenter as it is driven into and withdrawn from a material. Theanalysis of the measured force-displacement curves described in ISO 14577 isbased on work by Doerner and Nix and Oliver and Pharr 2, 3.Developed in the mid-1970s, nanoindentation is used to characterize a varietyof mechanical properties of any material that can be measured in a uniaxial tensionor compression test. While nanoindenation is most often used to measurehardness, it is also possible to calculate the modulus and creep using the datacollected in this test. Methods usingnanoindentation testers have also beendevised for evaluating the yield stress and strain-hardening characteristic ofmetals, the storage and loss modulus in polymers, and the activation energyand stress exponent for creep. The fracture toughness of brittle materials canbe estimated as well using optical measurement of the lengths of cracks thathave formed at the corners of hardness impressions made with sharp indenters.

Construction of Testing Equipment

Equipment used to perform nanoindentation consists of three basic componentsas shown in Figure 1:(a) An indenter mounted onto a rigid column(b) An actuator for applying the force(c) And a sensor for measuring the indenter displacementsSmall forces are generated either electromagnetically with a coil and magnetassembly or electrostatically using a capacitor with fixed and moving plates or withpiezoelectric actuators. Displacements may be measured by eddy current sensors,capacitive sensors, linear variable differential transducers or laser interferometers.A diamond is typically used tomake indenters because it has highhardness and elastic modulus. Thisminimizes thecontribution to themeasured displacement as comparedto those that are made of other lessstiffmaterials like sapphire or tungstencarbide in which case the elasticdisplacements of the indenter mustbe accounted for. Vickers geometryindenter, a four-sided pyramid, ismost commonly used in higherload nanoindentation tests for itsdurability. The Berkovich geometryindenter is used for measurementsof a few nanometers for two reasons; they are very sharp, thus they cause

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Figure 1: Schematic of typical nanoindentation tester

with a force actuator and displacement sensor.

plastic deformation even at very small loads, and they are easier to manufactureprecisely as they have only three sides. Cube corner indenters are even sharperthan the Berkovich, causing higher stresses and strains. They can be used to estimatefracture toughness at relatively small scales. While using spherical indentersproduces only elastic deformation at low loads, they could be used to examineyielding and work hardening, and to generate the entire uniaxial stress-straincurve 4.

Hardness, Modulus and Creep

During a nanoindentation measurement the indenter is driven into the materialas shown in Figure 2, both elastic and plastic deformation processes occur. Thisproduces an impression with a projected area Ap and surface area As of contactthat depends on the shape of the indenter to a contact depth, hc.The nanoindentation measurement includes a loading and unloading cycle.Figure 3 shows indentation load (F) plotted against the displacement (h) relativeto the surface before deformation, where the data was obtained for one completeindentation cycle. The important quantities are the maximum depth (hmax) ofpenetration, the peak load (Fmax), and the final depth after unloading (hr). Theslope of the upper portion of the unloading curve, S is known as the contactstiffness. The contact depth and stiffness are determined using the Oliver-Pharrmethod as described in ISO 14577 and ASTM E2546. The hardness and elasticmodulus are derived from these quantities.In nanoindentation the Martens Hardness is determined from the loadingportion of the load-displacement curve and includes the materials resistance toboth plastic and elastic deformation. The Martens Hardness can be plotted as a function the indentation depth. Martens Hardness is given by,

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Figure 2: Schematic of indenter (blue) deforming test

material (green).

Instrumented IndentationHardness correlates to traditionalforms of hardness as it is ameasure of the resistance to plasticdeformation. InstrumentedIndentation Hardness is given by Reduced elastic modulus, Erthat is indicative of the stiffness of the sample is given by

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Figure 3: Load-displacement curve measured on a

nanoindentation tester.

is a constant that dependson the geometry of the indenter.The reduced elastic modulus accounts for the elastic displacement that occurs in both the indenter and thesample. For a test material with elastic modulus EIT it can be calculated by Here is the Poisson’s ratio forthe test material, and Ei and ?i arethe elastic modulus and Poisson’sratio of the indenter, respectively.Creep can be used tocharacterize material behaviorat a constant load. IndentationCreep is defined as an increase inpenetration depth under constantload. As shown in Figure 4 theselected final load is kept constantfor defined time duration and theindentation depth is measured.Indentation Creep, CIT is calculated as

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Figure 4: Load-displacement curve with defined creep

period at maximum load measured on a nanoindentation

tester.

h1: indentation depth at the start of the creep test

h2: indentation depth at the end of the creep test

Comparing Traditional Hardness Testing to Nanoindentation Hardness

As hardness is already being measured for most applications it is important tounderstand the correlation between these traditional forms of hardness andInstrumented Indentation Hardness.

Vickers Hardness vs. Nanoindentation HardnessSurface hardness of hard materials is commonly measured with Vickers orKnoop indenters with traditional microhardness testers. While these tests arestill reliable to characterize the hardness of most bulk materials they are not aseffective for coatings and thin films. The loads used in traditional microhardnesstesters are usually too high and results are affected by the properties of theunderlying layer. And because the indentation is measured optically, reproducibilityand accuracy of the data collected are affected by the quality of optics anduser’s definition of the diagonals of the residual indent. In nanoindentation themeasured depth is used to calculate the area of contact. But there is still a relationshipbetween Instrumented Indentation Hardness and Vickers Hardness asa Vickers geometry indenter is used in both tests. Even the Berkovich geometryindenters that are also used in nanoindentation simulate the same strain ratesas a Vickers geometry indenter. Thus, the relationship between InstrumentedIndentation Hardness and Vickers Hardness is defined as

HV = 0.0945 HIT 5

Shore Hardness vs. Nanoindentation Hardness

A study measuring Martens hardness of ShoreA standards with the FISCHERSCOPE®HM2000 S, a nanoindentation tester shownin Figure 5, shows a very linear correlationat relatively low loads. The data in graph inFigure 6 are from indents with 50mN maximumload with loading and unloading timeof 60 seconds and a creep time of 10 seconds.These testing parameters are similar to thoseused for soft coatings and thin films whenshallow indentation depths are required toprevent substrate effects.

Pencil Hardness vs. Nanoindentation Hardness

In the following study the Martens hardnesswas measured for a set of graded pencils usedin Pencil hardness testing. The tests were carried out with the FISCHERSCOPE®HM2000 S. Figure 7 shows the results of multiple measurements on pencils ofvarious hardness levels. The large standard deviations of the individual test seriesshow the limitations of the pencil hardness method. Especially in the higherrange, the nominal hardness (B, HB, F, H, etc.) of pencils are not a dependableindicator of their actual hardness.With a nanoindentation tester the hardness of paint coatings can be measureddirectly and accurately. In addition, other characteristics can be determined, suchas creep and relaxation behavior, as well as the modulus of elasticity. All of theseparameters provide a true indication of the paint qualit

Example of Applications

Nanoindentation testersavailable in the market havea variety of features, loadand displacement ranges andresolutions. The followingexamples discuss two very differentcoatings that are commonlycharacterized with theFISCHERSCOPE® HM2000S nanoindentation tester. Keyfeatures and capabilities thatare essential for the nanoindentationtester in each applicationare described below.

Mechanical characterization oflacquer coatings in automotiveapplications

In the automotive industry,clear coatings for paint areused as protection from corrosionand external damage. These lacquers are exposed

؟؟؟

Figure 6: Martens Hardness (HM) of Shore A standards

performed with a FISCHERSCOPE® HM2000 S

.

؟؟؟

Figure 7: Comparison of the Martens Hardness of pencils of

different hardness, shown with the standard deviation of

the measurements

. to environmental influencessuch as extreme temperaturefluctuations or moisture andsalt. In addition, automotivecoatings must exhibit a certaintoughness to make themresistant to mars and scratches.This requires the right balancesbetween hardness and elasticity.A quick differentiation anddetermination of these coatingproperties is possible with thenanoindentation test.Influence from underlyinglayers or the substrate can be avoided by selecting a sufficiently low maximumload that keeps the penetration depth of the indent below 10% of the coatingthickness. At the beginning of the curing process, the clear coats are relativelysoft. One of the key features of a nanoindentation tester is a sensitive automatedsurface detection. As the measured mechanical properties polymers are influencedby rate of loading and unloading, a thermally stable nanoindentationsystem is also essential. Drift in the depth measurements caused by changes inenvironmental temperature must be avoided or accounted for.The Martens hardness (HM) and the Martens hardness after creeping (HMCR)are values which specify plastic and elastic properties of the paint coating. Theindentation hardness (HIT) considers only the plastic portion of the materialdeformation. The hardness parameters allow for better understanding of aging,curing, cross-linking, embrittlement through UV radiation, hardness changethrough temperature influences and the degree of polymerization of the lacquer.One of the most important advantages of the instrumented indentationtest is the determination of elastic properties. The indentation modulus (EIT),creep at maximum load (CIT) can be determined using this method and providesinformation regarding the visco-elastic properties of lacquer coatings. Theseproperties show the ability of the lacquer to resist weather degradation and healin case of scratches.

Nanoindentation on wear-resistant DLC coatings applied to engine components

In order to reduce emissions in combustion engines without sacrificing performance,manufacturers are continually working to improve the ability of themoving components (e.g. camshafts, valve lifters, piston rings and gears) toresist abrasion and reduce friction.Protective coatings such asdiamond-like carbon (DLC) areincreasingly used in such applications.As DLC coatings canhave a wide range of hardnessdepending on the depositionprocess it is important to measurthe fundamental mechanicalproperties of this hard, lowfriction coating.Traditional hardness measurements

would involve apply

؟؟؟

Figure 9: DLC-coated engine components.

Figure 10: The graph shows the depth-dependent profile of the Martens Hardness of the DLC coating.

ing a load though a sharp indenter and measuring the residual imprint under amicroscope. However, this is almost impossible due to the elastic nature and darkcolor of the DLC coating.As these coatings are only a few microns in thickness the nanoindentationtester should have high depth resolution to allow for shallow indents to beperformed, thus preventing the substrate material from influencing the measurements.And because ceramics have higher stiffness, the instrument musthave a rigid frame to eliminate instrument compliance and only deform thematerial being tested.In this example, the measurement results of a 3 μm thick DLC layer areshown. The values for indentation hardness (HIT), Martens Hardness (HM) andindentation modulus (EIT) for the coating is listed in Table 1. The convertedVickers hardness (HV) helps correlate these measurements with traditionalmicrohardness testers. The graph in Figure 10 maps the measured MartensHardness as a function of indentation depth. Minimal change in this measurementwith increasing depth indicates that even at maximum load there is noinfluence from the under lying substrate.

CONCLUSION

Improving the surface mechanical properties of materials boosts performanceand increases life cycle of products. New developments in coating and surfacetreatment technology has seen nanoindentation gain wider acceptance.Combination of ISO and ASTM standards for nanoindentation and availabilityof off-the-shelf options from different vendors has also contributed to adoptionof this technique in many industries.Given the limitations of traditional hardness testing techniques, nanoindentationtesters are viewed as tools that can give a better understanding ofthe interactions between surfaces or against abrasive elements. The wealth ofinformation about the mechanical properties derived from a nanoindentationtest defines the true strength of a material. Additionally, a single tool can beused to characterize a wide variety of materials ranging from soft polymers tohard ceramics. Most importantly, this technique removes the majority of theuser-influence and subjectivity from the test and allows one to quantitativelyanalyze a surface or coating.

REFERENCES

1. The Hardness of Metals, D. Tabor, Oxford University Press, Aug 3, 2000, ISBN0198507763, 9780198507765

2. A method for interpreting the data from depth-sensing indentation instruments,M.F. Doerner, and W. D. Nix, Journal of Materials Research, Vol. 1, No. 4,Jul/Aug 1986

3. Measurement of hardness and elastic modulus by instrumented indentation:Advances in understanding and refinements to methodology, W.C. Oliver andG.M. Pharr, , Vol. 19, No. 1, Jan 2004

4. A simple predictive model for spherical indentation, J.S. Field and M.V. Swain,Journal of Materials Research, Vol. 8, No. 2, 1993

5. The IBIS Handbook of Nanoindentation, Anthony C. Fischer-Cripps, ISBN

0 9585525 4 

Selective plating processes - Local plating, andizing and electropolishing

plating processes, procedures & solutions

CHROMATE CONVERSION COATINGS

BY FRED W. EPPENSTEINER (RETIRED) AND MELVIN R. JENKINS

MACDERMID INC., NEW HUDSON, MICH.; WWW.MACDERMID.COM

Chromate conversion coatings are produced on various metals by chemical or electrochemical treatment with mixtures of hexavalent chromium and certain other compounds. These treatments convert the metal surface to a superficial layer

containing a complex mixture of chromium compounds. The coatings are usually applied by immersion, although spraying, brushing, swabbing, or electrolytic methods are also used. A number of metals and their alloys can be treated; notably,

aluminum, cadmium, copper, magnesium, silver, and zinc.

The appearance of the chromate film can vary, depending on the formulation of the bath, the basis metal used, and the process parameters. The films can be modified from thin, clear-bright and blue-bright, to the thicker, yellow

iridescent, to the heaviest brown, olive drab, and black films. A discussion of specific formulations is not included in this article because of the wide variety of solutions used to produce the numerous types of finishes. It is intended to

present sufficient general information to permit proper selection and operation of chromating baths. Proprietary products, which are designed for specific applications, are available from suppliers.

PROPERTIES AND USES

Physical Characteristics

Most chromate films are soft and gelatinous when freshly formed. Once dried, they slowly harden or “set” with age and become hydrophobic, less soluble, and more abrasion resistant. Although heating below 150OF (66OC) is of benefit in hastening this aging process, prolonged heating above 150OF may produce excessive dehydration of the film, with consequent reduction of its protective value. Coating thickness rarely exceeds 0.00005 in., and often is on the order of several microinches. The

amount of metal removed in forming the chromate film will vary with different processes.

Variegated colors normally are obtained on chromating, and are due mainly to interference colors of the thinner films and to the presence of chromium compounds in the film. Because the widest range of treatments available is for zinc, coatings for this metal afford an excellent example of how color varies with film thickness. In the case of electroplated zinc, clear-bright and blue-bright coatings are the thinnest. The blue-brights may show interference hues ranging from red,

purple, blue, and green, to a trace of yellow, especially when viewed against a white background. Next, in order of increasing thickness, come the iridescent yellows, browns, bronzes, olive drabs, and blacks.

Physical variations in the metal surface, such as those produced by polishing, machining, etching, etc., also affect the apparent color of the coated surface. The color of the thinner coatings on zinc can also be affected indirectly by chemical polishing, making the finish appear whiter.

Corrosion Prevention

Chromate conversion coatings can provide exceptionally good corrosion resistance, depending upon the basis metal, the treatment used, and the film thickness.

Protection is due both to the corrosion-inhibiting effect of hexavalentchromium contained in the film and to the physical  arrier presented by the film itself. Even scratched or abraded films retain a greatdeal of their protective value because the hexavalent chromium content is slowly leachable in contact with moisture, providing a self-healing effect.

The degree of protection normally is proportional to film thickness; therefore, thin, clear coatings provide the least corrosion protection, the light iridescent coatings form an intermediate group, and the heavy olive drab to brown coatings result in maximum corrosion protection. The coatings are particularly useful in protecting metal against oxidation that is due to highly humid storage conditions, exposure to marine atmospheres, handling or fingerprint marking, and other conditions that normally cause corrosion of metal.

Bonding of Organic Finishes

The bonding of paint, lacquer, and organic finishes to chromate conversion coatings is excellent. In addition to promoting good initial adhesion, their protective nature prevents subsequent loss of adhesion that is due to underfilm corrosion.

This protection continues even thought he finish has been scratched through to the bare metal. It is necessary that the organic finishes used have good adhesive properties, because bonding must take place on a smooth, chemically clean surface; this is not necessary with phosphate-type conversion coatings, which supply mechanical adhesion that is due to the crystal structure of the coating.

Chemical Polishing

Certain chromate treatments are designed to remove enough basis metal during the film-forming process to produce a chemical polishing, or brightening, action. Generally used for decorative work, most of these treatments produce very thin,

almost colorless films. Being thin, the coatings have little optical covering power to hide irregularities. In fact, they may accentuate large surface imperfections. In some instances, a leaching or “bleaching” step subsequent to chromating is used to remove traces of color from the film.

If chemical-polishing chromates are to be used on electroplated articles, consideration must be given to the thickness of the metal deposit. Sufficient thickness is necessary to allow for metal removal during the polishing operation.

Absorbency and Dyeing

When initially formed, many films are capable of absorbing dyes, thus providing a convenient and economical method of color coding. These colors supplement those that can be produced during the chromating operation, and a great variety

of dyes is available for this purpose. Dyeing operations must be conducted on freshly formed coatings. Once the coating is dried, it becomes nonabsorbent and hydrophobic and cannot be dyed. The color obtained with dyes is related to

the character and type of chromate film. Pastels are produced with the thinner coatings, and the darker colors are produced with the heavier chromates. Some decorative use of dyed finishes has been possible when finished with a clear lacquer topcoat, though caution is required because the dyes may not be lightfast.

In a few cases, film colors can be modified by incorporation of other ions or dyes added to the treatment solution.

Hardness

Although most coatings are soft and easily damaged while wet, they become reasonably hard and will withstand considerable handling, stamping, and cold forming.

They will not, however, withstand continued scratching or harsh abrasion. A few systems have been developed that possess some degree of “wet-hardness,” andthese will withstand moderate handling before drying.

Heat Resistance

Prolonged heating of chromate films at temperatures substantially above 150OF (66OC) can decrease their protective value dramatically. There are two effects of heating that are believed to be responsible for this phenomenon. One is the insolubilization of the hexavalent chromium, which renders it ineffective as a corrosion inhibitor. The second involves shrinking and cracking of the film, which destroys its physical integrity and its value as a protective barrier.

Many factors, such as the type of basis metal, the coating thickness, heating time, temperature, and relative humidity of the heated atmosphere, influence the degree of coating damage. Thus, predictions are difficult to make, and  thorough performance testing is recommended if heating of the coating is unavoidable.

The heat resistance of many chromates can be improved by certain posttreatments or “sealers.” Baking at paint-curing temperatures after an organic finish has been applied is a normal practice and does not appear to affect the properties of the treatment film.

Electrical Resistance

The contact resistance of articles that have been protected with a chromate conversion coating is generally much lower than that of an unprotected article that has developed corroded or oxidized surfaces. As would be expected, the thinner

the coating, the lower the contact resistance, i.e., clear coatings have the least resistance, iridescent yellow coatings have slightly more, and the heavy, olive drab coatings have the greatest. If exposure of an article to corrosive conditionsis anticipated, the choice of a coating thickness normally involves a compromise between a very thin film—which, although having very low initial contact resistance, is likely to allow early development of high electrical resistance corrosion products—and a heavier film, with somewhat higher initial contact resistance, but which is likely to remain relatively constant for a longer period under corrosive conditions.

Fabrication

Resistance Welding. Thin chromate films do not interfere appreciably with spot, seam, or other resistance-welding operations. Aluminum coated with a thin, nearly colorless film, for example, can be spot welded successfully with no increase in welding machine settings over those required for bare metal. Metal coated with thicker, colored films also can be resistance welded. The increased contact resistance of thicker coatings, however, necessitates using slightly higher machine settings.

Fusion Welding. These operations, likewise, are not hampered by the presence of chromate films. It has been reported, in fact, that chromate treatments on aluminum actually facilitate inert gas welding of this metal and its alloys, producing contamination-free welds.

Soldering. Cadmium and silver surfaces coated with thin chromate films can be soldered without difficulty using a mild organic flux. Conflicting reports exist regarding the solderabilty of chromated zinc surfaces.

Mechanical Fastening. The assembly of chromated parts using bolts, rivets, and other mechanical fastening devices usually results in local damage to the chromate film. Corrosion protection in these areas will depend upon the effectiveness of the self-healing properties of the surrounding coating.

Summary of Common Uses Table I summarizes the most common applications of chromate conversion coatings.

MATERIALS OF CONSTRUCTION

Generally, suppliers of proprietaries recommend materials for use with their products, which are resistant to oxidants, fluorides, chlorides, and acids. Materials that have been found to be satisfactory for most chromating applications are stainless steels and plastics. Stainless steels such as 304, 316, 317, and 347 are suitable for tanks andheaters where chlorides are absent. Containers and tank linings can be made from plastics such as polyvinyl chloride (PVC), polyvinylidine chloride (PVDC), polyethylene, and polypropylene. Acid-resistant brick or chemical stoneware is satisfactory

for some applications, but is subject to attacks by fluorides.

Parts-handling equipment is made of stainless steel, plastisol-coated mild steel, or plastic.

Mild steel can be used for leaching tanks because the solutions are generally alkaline, whereas tanks for dyeing solutions, which are slightly acid, should be of acid-resistant material.

Usually, ventilation is not necessary because most chromate solutions are operated at room temperature and are nonfuming. Where chromating processes are heated, they should be ventilated.

FILM FORMATION

Mechanism

The films in most common use are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of other components, or “activators,” in an acid solution. The hexavalent chromium is partially reduced to trivalent chromium during the reaction, with a concurrent rise in pH, forming a complex mixture consisting largely of hydrated basic chromium chromate and hydrous oxides of both chromium and the basis metal. The composition of the film is rather indefinite, because it contains varying quantities of the reactants, reaction products, and water of hydration, as well as the associated ions of the particular systems.

There are a number of factors that affect both the quality and the rate of formation of chromate coatings. Of the following items, some are peculiar to chromating; many derive simply from good shop practice. A working understanding of these factors will be helpful in obtaining high-quality, consistent results. Different formulations are required to produce satisfactory chromate films on various metals and alloys. Similarly, the characteristics of the chromate film produced by any given solution can vary with minor changes in the metal or alloy surface. Commonly encountered examples of this follow.

Effect of Basis Metals

Aluminum Alloys. The ease with which coatings on aluminum can be produced, and the degree of protection afforded by them, can vary significantly with the alloying constituents and/or the heat treatment of the part being processed. In general, low alloying constituent metals that are not heat treated are easiest to chromate and provide the maximum resistance to corrosion. Conversely, wrought aluminum, which is high in alloying elements (especially silicon, copper, or zinc) or which has undergone severe heat treatment, is more difficult to coat uniformly and is more susceptible to corrosive attack. High silicon casting alloys present similar problems. The effect of these metal differences, however, can be minimized by proper attention to the cleaning and pretreatment steps. Most proprietary treatment instructions contain detailed information regarding cleaning, desmutting, etc., of the various alloys.

Magnesium Alloys. As in the case of aluminum, the alloying element content and the type of heat treatment affect the chromating of magnesium. With the exception of the dichromate treatments listed as Type III in Military Specification MIL-M-3171, all of the treatments available can be used on all the magnesium alloys

Zinc Alloys. Chromate conversion coatings on zinc electroplate are affected by impurities codeposited with the zinc. For example, dissolved cadmium, copper, and lead in zinc plating solutions can ultimately cause dark chromated films. Similarly, dissolved iron in noncyanide zinc plating solutions can create chromating problems. Furthermore, the activity of zinc deposits from cyanide and noncyanide solutions can differ sufficiently to produce variations in the chromate film character.

Variations in the composition of zinc die casting alloys and hot-dipped galvanized surfaces can also affect chromate film formation; however, in the latter case, the result is usually difficult to predict, due to the wide variations encountered in

spelter composition, cooling rates, etc. Large differences in the chromate coating from spangle to spangle on a galvanized surface are not uncommon. This is especially evident in the heavier films.

Copper Alloys. Since chromate treatments for copper and its alloys can be used to polish chemically as well as to form protective films, the grain structure of the part becomes important, in addition to its alloying content. Whereas fine-grained, homogeneous material responds well to chromate polishing, alloys such as phosphor bronze and heavily leaded brass usually will acquire a pleasing but matte finish.

In addition, treatment of copper alloys, which contain lead in appreciable amounts, may result in the formation of a surface layer of powdery, yellow lead chromae.

Effects of pH

One of the more important factors in controlling the formation of the chromate film is the pH of the treatment solution. For any given metal/chromate solution system, there will exist a pH at which the rate of coating formation is at a maximum. As the pH is lowered from this point, the reaction products increasingly become more soluble, tending to remain in solution rather than deposit as a coating on the metal surface. Even though the rate of metal dissolution increases, the coating thickness will remain low. Chemical-polishing chromates for zinc, cadmium, and copper are purposely operated in this low pH range to take advantage of the increased rate of metal removal. The chromate films produced in these cases can be so thin that they are nearly invisible. Beyond this point, further lowering of the pH is sufficient to convert most chromate treatments into simple acid etchants.

Increasing the pH beyond the maximum noted above will gradually lower the rate of metal dissolution and coating formation to the point at which the reaction, for all practical purposes, ceases.

Hexavalent Chromium Concentration

Although the presence of hexavalent chromium is essential, its concentration in many treatment solutions can vary widely with limited effect, compared with that of pH. For example, the chromium concentration in a typical aluminum treatment solution can vary as much as 100% without substantially affecting the film-formation rate, as long as the pH is held constant. In chromating solutions for zinc or cadmium, the hexavalent chromium can vary fairly widely from its optimum concentration if the activator component is in the proper ratio and the pH is constant.

Activators

Chromate films normally will not form without the presence of certain anions in regulated amounts. They are commonly referred to as “activators’ and include acetate, formate, sulfate, chloride, fluoride, nitrate, phosphate, and sulfamate ions.

The character, rate of formation, and properties of the chromate film vary with the particular activator and its concentration. Consequently, many proprietary formulations have been developed for specific applications and they are the subject of numerous patents. Usually, these proprietary processes contain the optimum concentrations of the activator and other components; therefore, the user need not be concerned with the selection, separate addition, or control of the activator.

OPERATING CONDITIONS

In addition to the chemical make-up of the chromating solution, the following factors also govern film formation. Once established for a given operation, these parameters should be held constant.

Treatment Time. Immersion time, or contact time of the metal surface and the solution, can vary from as little as 1 second to as much as 1 hour, depending on the solution being used and metal being treated. If prolonged treatment times are required to obtain desired results, a fault in the system is indicated and should be corrected.

Solution Temperature. Chromating temperatures vary from ambient to boiling, depending on the particular solution and metal being processed. For a given system, an increase in the solution temperature will accelerate both the film-forming

rate and the rate of attack on the metal surface. This can result in a change in the character of the chromate film. Thus, temperatures should be adequately maintained  to ensure consistent results.

Solution Temperature. Chromating temperatures vary from ambient to boiling, depending on the particular solution and metal being processed. For a given system, an increase in the solution temperature will accelerate both the film-forming

rate and the rate of attack on the metal surface. This can result in a change in the character of the chromate film. Thus, temperatures should be adequately maintained to ensure consistent results.

Solution Agitation. Agitation of the working solution, or movement of the work in the solution, generally speeds the reaction and provides more uniform film formation.

Air agitation and spraying have been used for this purpose. There are, however, a few exceptions where excessive agitation will produce unsatisfactory films.

Solution Contamination

Although the presence of an activator in most treatment solutions is vital, an excessive concentration of this component, or the presence of the wrong activator, can be very detrimental. Most metal-finishing operations include sources of

potential activator contamination in the form of cleaners, pickles, deoxidizers, and desmutters. Unless proper precautions are taken, the chromate solution can easily become contaminated through drag-in of inadequately rinsed parts, drippage

from racks carried over the solution, etc.

A common source of contamination is that resulting from improperly cleaned work. If allowed to go unchecked, soils can build on the surface of the solution to the point at which even clean work becomes resoiled on entering the treatment

tank, resulting in blotchy, uneven coatings.

Other contaminants to be considered are those produced by the reactions occurring in the treatment solution itself. With very few exceptions, part of the trivalent chromium formed andpart of the basis metal dissolved during the coating reaction remain in the solution. Small amounts of these contaminants can be beneficial, and “brokenin” solutions often produce more consistent results. As the concentration of these metal contaminants increases, effective film formation will be inhibited. For a certain period, this effect can be counteracted by adjustments, such as lowered pH and increased hexavalent chromium concentration. Eventually, even these techniques become ineffective, at which point the solution must be discarded or a portion withdrawn and replaced with fresh solution.

Rinsing and Drying

Once a chromate film has been formed satisfactorily, the surface should be rinsed as soon as possible. Transfer times from the chromating stage to the rinsing stage should be short in order to minimize the continuing reaction that takes place on

the part.

Although rinsing should be thorough, this step can also affect the final character of the chromate film and should be controlled with respect to time and temperature, for consistent results.

Prolonged rinsing or the use of very hot rinsewater can dissolve, or leach, themore soluble hexavalent chromium ompounds from a freshly formed coating, resulting in a decrease in protective value. If a hot rinse is used to aid drying, avoid temperatures over about 150OF (66OC) for more than a few seconds. This leaching effect sometimes is used to advantage. In instances in which a highly colored or iridescent coating may be objectionable, a prolonged rinse in hot water can be used as a “bleaching” step to bring the color to an acceptable level. Instead of hot water leaching, some systems incorporate dilute acids and alkalis to accelerate this step.

Solution Control

Because most chromate processes are proprietary, it is suggested that the suppliers’ instructions be followed for solution make-up and control. Even though specific formulations will not be discussed, certain general principles can be outlined,

which apply generally to chromate solutions. The combination of hexavalent chromium concentration, activator type and concentration, and pH, i.e., the “chemistry” of the solution, largely determines the type of coating that will be

obtained, or whether a coating can be obtained at all, at given temperatures and immersion times. It is important that these factors making up the “chemistry” of the solution be properly controlled. As the solution is depleted through use, it is replenished by maintenance additions, as indicated by control tests or the appearance of the work.

Fortunately, analysis for each separate ingredient in a chromate bath is not necessary for proper control. A very effective control method uses pH and hexavalent chromium analysis. The pH is determined with a pH meter and the chromium is

determined by a simple titration. Indicators and pH papers are not recommended because of discoloration by the chromate solution. Additions are made to the solution to keep these two factors within operating limits. The amount of controlactually required for a given treatment depends on how wide its operating limits are, and on the degree of uniformity of results desired. Control by pH alone is adequate in some cases.

COATING EVALUATION

Chromate conversion coatings are covered by many internal company standards and/or U.S. government and American Society for Testing and Materials (ASTM) specifications. These standards usually contain sections on the following methods of evaluation.

Visual Inspection

The easiest way to evaluate chromate conversion coatings is to observe the color, uniformity of appearance, smoothness, and adhesion. Type of color and iridescence is a guide to film thickness, which is considered proportional to protective value; however, visual inspection by itself is not sufficient to indicate the protective value of the coating, especially if the film has been overheated during drying.

Accelerated Corrosion Test

The salt spray test, ASTM B 117, is the most common accelerated test developed in specification form. Although some disagreement exists as to the correlation of salt spray tests to actual performance, it remains in many specifications. Variations in results are often obtained when tested in different salt spray cabinets, and even in different locations within the same cabinet. Coatings should be aged for at least 24 hours before testing, for consistent results. Generally,  pecifications require a minimum exposure time before visible corrosion forms. Typical salt spray test data are provided in Tables II to IV.

Humidity Tests

There appears to be no standard specification covering humidity tests for unpainted chromate conversion coatings. Evaluations are conducted under various conditions and cycles. Humidity tests may be more useful than salt spray tests, as they correspond to the normal environment better than the salt spray, except in marine atmospheres.

Water Tests

Immersion tests in distilled or deionized water have proven valuable in simulating such conditions as water accumulation in chromated zinc die castings, e.g., carburetors and fuel pumps.

Coatings applied on hot-dipped galvanized surfaces in strip mills are often tested by stacking wet sheets and weighing the top sheet. Periodic checks are made to determine when corrosion products first develop. The tests should be conducted at relatively constant temperatures to ensure consistent results.

Chemical and Spot Tests

The amount of hexavalent chromium in the film can be an indication of the corrosion protection afforded by the coating. Analytical procedures for small amounts of chromium on treated surfaces are comparatively rapid, quantitative,

and reproducible. Consequently, chemical analysis for the chromium content of the film appears to be a valuable tool. It would not be suitable, however, for predicting the performance of bleached, overheated, excessively dehydrated coatings.

Total coating weight is sometimes used as an indication of corrosion resistance.

It is derived by weighing a part having a known surface area before and after chemically stripping only the chromate film. Spot tests are used to test corrosion resistance by dissolving the chromate coating and reacting with the basis metal. The time required to produce a characteristic spot determines empirically the film thickness or degree of corrosion protection. It is advisable to use these tests as comparative tests only, always spotting an untreated and treated surface at the same time. Frequently, the spot tests are sufficient only to indicate differences between treated and untreated surfaces. Reproducibility is not good because aging affects the results.

Performance Tests for Organic Finishes

Paint, lacquer, and other organic finishes on chromate conversion coatings are tested in numerous ways to evaluate bonding and corrosion protection. These include pencil-hardness, cross-hatch, bending, impact, and tape tests with or without prior exposure to water or salt spray.

SPECIFICATIONS

A list of the more commonly used specifications covering chromate conversion coatings on different basis metals follows. Only the basic specification or standard number is listed, and reference should be made only to the appropriate revision of any particular document.

Aluminum

AMS 2473—Chemical Treatment for Aluminum Base Alloys—General Purpose

Coating

AMS 2474—Chemical Treatment for Aluminum Base Alloys—Low Electrical

Resistance Coating

ASTM D 1730—Preparation of Aluminum and Aluminum Alloy Surfaces for

Painting

MIL-C-5541—Chemical Films and Chemical Film Material for Aluminum and

Aluminum Alloys

MIL-C-81706—Chemical Conversion Materials for Coating Aluminum and

Aluminum Alloys

MIL-W-6858—Welding, Resistance: Aluminum, Magnesium, etc.; Spot and Seam

Cadmium

AMS 2400—Cadmium Plating

AMS 2416—Nickel-Cadmium Plating, Diffused

AMS 2426—Cadmium Plating, Vacuum Deposition

ASTM B 201—Testing Chromate Coatings on Zinc and Cadmium Surfaces

MIL-C-8837—Cadmium Coating (Vacuum Deposited)

QQ-P-416—Plating, Cadmium (Electrodeposited)

Magnesium

AMS 2475—Protective Treatments, Magnesium Base Alloys

MIL-M-3171—Magnesium Alloy, Process for Pretreatment and Prevention of

Corrosion on

MIL-W-6858—Welding, Resistance: Aluminum, Magnesium, etc.; Spot and Seam

Silver

QQ-S-365—Silver Plating, Electrodeposited, General Requirements for

Zinc

AMS 2402—Zinc Plating

ASTM B 201—Testing Chromate Coatings on Zinc and Cadmium Surfaces

ASTM D 2092—Preparation of Zinc-Coated Steel Surfaces for Painting

MIL-A-81801—Anodic Coatings for Zinc and Zinc Alloys

MIL-C-17711—Coatings, Chromate, for Zinc Alloy Castings and Hot-Dip

Galvanized Surface

MIL-T-12879—Treatments, Chemical, Prepaint and Corrosion Inhibitive, for

Zinc Surfaces

MIL-Z-17871—Zinc, Hot-Dip Galvanizing

QQ-Z-325—Zinc Coating, Electrodeposited, Requirements for

SPECIAL TREATMENTS

Solutions containing chromium compounds are used in some processes where

disagreement exists as to whether these form “true” chromate conversion coatings

or combination coatings, or act as passivating processes.

Electrolytic Processes

Although early chromate conversion coatings for zinc were electrolytically applied, this method has been largely replaced by immersion processes. More recently, the use of electric current has reappeared with solutions containing mixtures of chromates, phosphates, fluorides, etc., to produce “anodic coatings.” The coatings, however, are not similar to anodic coatings such as those produced on aluminum. The coatings on zinc surfaces are complex combinations of chromates, phosphates, oxides, etc. They are formed with 100-200 V AC or DC, and the fritted coating will withstand more than 1,000 hr of salt spray. The

process is used where outstanding corrosion resistance is needed. The coating also exhibits superior hardness, heat resistance, thickness, and dielectric strength when compared with normal chromate conversion coatings. Colors range from dark green to charcoal for different processes. Electrolytic treatments using chromium compounds are also applied to steel strip, where chromium along with oxides, etc., are deposited in a very thin, discontinuous film. These processes, which promote lacquer and paint adhesion, may be more chromium plate than chromate coating.

Coatings on Beryllium

It has been reported that chromate conversion coatings can be applied to beryllium to retard high-temperature oxidation in humid air.

Chromate-Phosphate Treatments

Chromate-phosphate treatments are based on chromate-phosphate mixtures and form a combination conversion coating on aluminum. The coating can appear practically colorless to a light-green hue. These treatments have been used to impact color for decorative purposes or to provide an imposed base for subsequent lacquer or paint operations.

Surface Preparation of Metals Prior to Plating (part 7: THE FACTORS LIMITING THE USEFUL LIFE OF AQUEOUS CLEANERS)

V. THE FACTORS LIMITING THE USEFUL LIFE OF AQUEOUS CLEANERS                            

Among others, one of the puzzling intangibles of metal cleaning art is the subject of useful "life”. Certainly, the first requirement of any metal cleaner is that it renders the surface both chemically and physically clean. The next requirement is longevity of acceptable performance before it must be replaced, and the final, obvious requirement is the total operating cost of the cleaner, including energy, reprocessing, maintenance, and waste disposal costs. The six main aspects responsible for changes occurring in cleaning solution, that will be further discussed, fall into two main categories: chemical changes and physical changes. The first two, saponification and carbonation, would be categorized as chemical effects, while the last four are physical effects. The seventh, indirect aspect is a proper maintenance.

1-.  Saponification. The saponification reaction is the basic mechanism of primary cleaning reaction. Any  saponifiable or soap forming oils present on the work will chemically combine (react) with alkalis in the bath, usually the caustic soda, (NaOH) or caustic potash, (KOH), to form soaps (R-ONa) and glycerol (C3H5 (OH)₃) according to the reaction:

The saponification reaction is actually a neutralization of an organic acid with a base with the formation of a salt, which, in this case, is the soap. The result is partial loss of free alkali and a gain in the solution of the crude surface active agent, such as, e.g.,  Na-stearate,  Na-oleate,  Na-rosinate, or some other metallic soap depending on the saponifying material.

There is a profit and loss balance resulting from these chemical reactions. The loss in free caustic reduces the electrical conductivity, an important parameter in electrocleaning, which in turn reduces the gas evolution at any given voltage. A full effect of the reduction in free caustic, which particularly applies to  anodic cleaning at high current densities, is the danger of  anodic polarization. This is caused by insufficient sodium or potassium ions present to neutralize the acid radicals, such as silicates, phosphates,  rosinates, etc. formed at the surface of the processed parts. The gain in soap formation, will promote cleaning to some extent except, obviously, if the soap content builds up too high, introducing gelatinous solutions and excessive foaming in electrocleaning baths.  Saponification can cause a further problem in regions where hard water is used. This is due to the formation of insoluble calcium and magnesium soaps which deposit on the parts as a curd and which are extremely difficult to rinse off.

3-  Carbonation.This second chemical change is also a simple acid-base neutralization. In this case, the weak acid is carbonic acid gas (carbon dioxide, CO2), present in the air, which combines with the stronger alkalis, such as NaOH, KOH, silicates, and phosphates to form sodium carbonate (soda ash), or potassium carbonate. Even, though it makes only 0.035 % of the atmosphere 89 , the large amount of air can be passed through the cleaner with, e.g., air agitation and neutralize the alkaline builders. Here again, the free caustic is the preferential victim of the reaction process, although other important ingredients, silicates and phosphates are also subject to conversion to carbonates. If an alkaline cleaner is allowed to stand for a prolonged period of time it will slowly absorb CO2 and will eventually become excessively high in carbonate content. This is true even though the solution is not used to clean dirty work. It is seen then that, after long periods of shutdown, one cannot expect the solution to be in the same condition as it was prior to the shutdown.

Treating this as a profit and loss balance situation follows that there is no profit but all loss. While it is true that formed sodium or potassium carbonate in itself will exert some cleaning action, it is relatively inefficient. The electrical resistivity of e.g., sodium carbonate solution is four times that of caustic soda and here again there is a loss of current carrying capacity at a given voltage. Furthermore, sodium or potassium carbonates are arelatively poor emulsifiers and dispersing agents. Consequently, this property will suffer proportionally to the carbonation occurring in the solution. As a consequence of carbonation, there is a loss in electrical conductivity, emulsifiability and dispersability.

3- Soil Load. This important and often neglected aspect is a physical build-up of  unsaponifiable oils and greases and a wide variety of solid soils incidental to buffing, handling and storage operations prior to the cleaning. As work is put through the cleaner solution, oils and greases are emulsified and distributed throughout the bath. Solid particles, such as abrasive grains, dust, dirt, metal chips, etc., either fall to the bottom if they are comparatively coarse particles or are suspended throughout the bath in a disperse condition.

The soil load probably limits the useful cleaner life before any of the other components reach their permissible limits. It is strictly a function of the amount and condition of the work put through the solution. It is almost impossible to correctly foresee how long the bath may be operational prior to replacing it.

In the case of the emulsified oils and greases, the older reasoning was that a cleaner of higher emulsifying power could disperse more of them, which would be an advantage. The newer opinion 57 is that a cleaner, which will separate the oils and greases but not emulsify them, gives better results. By skimming the accumulated oils from the surface of the solution with an overflow skimmer, the oil load in the solution is kept at a minimum and there is less danger of oil drag-out into the subsequent acid dips, activators, or plating solutions. The same reasoning does not apply to the solid soil, since it cannot be skimmed-off from the surface. Some is removed by settling, but in general, these solids should be dispersed so that the metal surface rinses free of all foreign matter.

4-  Drag-Out. This aspect has less to do with limiting the cleaner life than to wasteful extravagance.  Of course, some work is of such nature that recesses and pockets carry out a large volume of cleaning solution, as it also does all along the plating line in the acid dips, rinses, plating solutions, etc. Racking the work to the best advantage to give maximum drainage is the only practical means available to keep this loss at the lowest possible amount. Periodic titration checks should be made on the solution so that revitalizing additions can be made to overcome this waste. However, this compensation is obtained at the expense of increased cleaner consumption, in addition to waste treatment cost. The conscientious operator comparing the increased cleaner life against the amount of additions required must determine a cost balance. Heavy drag-out removes dirty cleaner solution and if compensated by regular additions, tends to oppose the deterioration resulting from soap, carbonate and soil build-up.

5- Solution Volume. This aspect pertains to the arithmetic relationship between area of work  leaned and volume of cleaner solution. Clearly, the larger the volume of solution for any given work loads, the longer it will require overloading with soil or grease. Conversely, a tank that is too small will very quickly reach its limit in soil tolerance, resulting in too frequent dumping of the solution. There is no fixed numerical value for this relationship. It might be argued that the volume of the cleaner solution cannot be too large, but in the interests of dimensional economy, it would not be good engineering to have the cleaner tank too cumbersome in size. It also might be possible to have a volume so large in relation to the amount of work that change of the active ingredients from evaporation, thermal degradation, drag-out, carbonation, etc. would reduce the cleaning efficiency before it had a chance to give maximum returns on the investment. Generally, the cleaner tank is designed as is the plating tank, i.e., to accommodate the largest piece to be treated.

6- Depletion.  By definition, the wetting agents used in cleaners are surface active materials. As such they distribute themselves at the surfaces of solid particles or at the interfacial surfaces of oils and greases in the processes of emulsification, dispersion and surface tension reduction. Eventually that they become depleted in the solution to the extent that the detergent and emulsification action is compromised. All proper cleaners also incorporate the sequestering agent(s) whose function is to keep soluble ‎metallic anions in the solution. ‎ However, the situation can arise, where metal accumulation can overpower the sequestrant’s capacity. The metal ions (that are cations), can in turn react with anionic surfactant, resulting in the loss of wetting abilities. These actions are inherent in all cleaners and are compensated for by the additions of fresh cleaner, as a whole or as a separate surfactant package. Cleaner vendors should provide sufficient surface active materials in their formulated products to have a reserve with the intention that the wetting action will be effective throughout the life of the cleaning solution.

7- Maintenance. Proper maintenance of the optimum cleaning action is necessary and almost mandatory in order to balance the six previous aspects, resulting in maximum results from the alkaline cleaning bath. The cleaner must be kept within allowable limits as regards the soil load in order to function properly. When this aspect becomes so high that the dragout contaminates subsequent solutions or the work cannot be rinsed soil-free, no amount of additions to the cleaner bath will restore its working efficiency. Once more, the limit is set by a slow build-up in the bath and not by any "break-down" of the cleaner. There is no simple way to determine this end-point in any particular application except by educated experience. It is often justifiable to make up a new cleaner more regularly, according to the dictates of past experience, given that they are far cheaper than the labor lost due to rejects.

The life of a cleaner is best determined by observation of results in an actual application. A log of analysis and performance of the cleaner is most helpful to anticipate and determine the cleaner’s productive life. Effort should be made for a scheduled, intermittent complete chemical analysis to determine the changes in composition with time. In addition, the measurements of specific gravity and conductivity have been found useful in some cases, since these are simple and rapid methods. A good log coordinated with observation of the effectiveness of the cleaner will help to establish the productive, usable life.

The volume should be sufficiently large to tolerate the soil load from one to four weeks. Longer periods may cause excessive carbonization and in some cases, excessive soap build-up. During these one to four week time periods, there must be sufficient active alkalinity maintained by regular additions of new material to support electrical conductivity and rapid emulsification, saponification and dispersion of the oils and greases. Shorter periods of operation before replacing the solution may lead to higher cleaner consumption. However, this item is much cheaper than the labor cost invested in rejects or lost time due to shutdowns in the midst of a day's production run.

If wetting agent is essential for good cleaning, then the wetting agent should be maintained by laboratory foam test, or observation of the foam blanket on the cleaner.

The cleaner will not suddenly lose cleaning power due to changes in the major chemical present. Since the cleaning power changes gradually, the loss of cleaning power can be observed over a longer periods by watchfully observing the elapsed cleaning time. If the normal cleaning time is one minute, the work should clean in half this time to maintain a margin for safe cleaning. A fresh cleaner may clean in ¼ minute. Empirically, the minimum cleaning time for a fresh cleaner can be estimated by hand dipping of production parts for various times. This test can be repeated at varying intervals as the cleaner is used. An increase in cleaning time will indicate unfavorable changes. If the cleaning time gradually increases, then the life of the cleaner will be anticipated by extrapolation to a cleaning time equal to the processing time.

The seven aspects presented above, indicate that the life limits of modern cleaner formulations are normally caused by deterioration ascribed to various chemical and physical changes and are not normally due to any sudden breakdown of the chemistry of the cleaner.

Surface Preparation of Metals Prior to Plating (part 6: TESTS & CONTROL METHODS FOR ‎CLEANLINESS)

Liquid cleaners for both spray and soak applications have been around for many years. The use of liquid cleaners to replace powders has been gaining wider acceptance in the industry. They are formulated to economically provide all the performance criteria of the powders 43. The advantages of liquid cleaners include the capability of automatic feeding tied to conductivity controllers. The automated system continually monitors the solution strength and makes additions on demand. Consequently, better bath control is achieved, eliminating wide swings in concentrations. As a consequence, liquid systems have substantially increased bath life in many installations.

Automatic recording capabilities of concentration and temperature can be achieved for statistical process control.

Tank additions of liquid concentrate eliminate the hazards associated with additions of alkaline powders to hot cleaner solutions.

 M. BIOLOGICALLY ACTIVE CLEANERS

Unique bacterias and enzimes 78-80 can help traditional surfactants to clean the parts. The baths with oil and greases is good habitat for oil-loving bacteria, as they appreciate organic soils as their meal. They can extend the bath life and significantly reduce the sludging of the bath.

 N. LOW TEMPERATURE CLEANING.

The reduction in energy consumption achievable by low-temperature cleaning (LTC) is very attractive from the standpoints of reduced energy costs 81 and increased flexibility during an energy crisis. 

While maintaining the temperature of a bath requires less energy than bringing a solution to its operating temperature, the energy reduction is very substantial at lower temperatures. On the other hand, LTC has limitations. The cleaner is more expensive and difficult to select for the following reasons:

1. A high concentration of surfactant is required, compared with conventional cleaners. Cleaning is improved rapidly as the concentration is increased—but only up to a point, after which the improvement in cleaning per unit of increase in concentration drops markedly. However, if LTC is adopted, we still may need to pay additional money for surfactant to achieve the desired degree of cleanliness, even at a lesser improvement per unit increase in surfactant.

2. More expensive surfactants may be needed, and a combinations of surfactants are required to obtain satisfactory cleaning and to overcome the loss of a "safety factor" available with high temperature cleaning; greater attention must be devoted to maintaining the proper conditions (e.g. solution concentration and level of agitation) to compensate for the normally better cleaning achievable at higher temperatures.

3. Solvents may be required to reduce the viscosity of the soil so that detergency is improved. The amount of solvent that can be justified in a commercial bath is too limited to dissolve soil but can be very significant in modifying the properties of the soil so that it can be removed by action of the builders and wetting agents.

IV. TESTS & CONTROL METHODS FOR CLEANLINESS

The degree of cleaning required for the surface of a part is a function of, and dictated by, operations to follow cleaning 82 .Test methods used to determine the cleanliness of a surface range from crude to highly sophisticated. Unfortunately, with few exceptions, these remain a research tool, rather than production tool. In actual practice, the cleaning process is in most cases, actually controlled by combination of visual water break and white glove tests plus solution analysis.

 i. The Eye as Analytical Instrument

The easiest, quickest and most straightforward method for recognizing that something is incorrect with the cleaning process is by looking intently at the parts themselves. If cleaning is marginal or insufficient, it will be more often than not indicated by a change in the appearance of the surface of the part or worse, in the plated deposit. The human eye is a critical instrument. It can detect minute changes in color and appearance better perhaps than some very elaborate and ‎expensive instruments. Unfortunately, however, in most cases Kushner's Third Law holds:  When you can see it, it's too late! The secret is to be able to see it before it makes trouble and this is no mean secret. ‎ In addition, there are certain changes in the deposit that are not visible to the human eye. An example of this is an improper cleaning that acts to decrease adhesion of the deposit. The deposit will look just about the same as it did before when it is removed from the plating bath. However, it can be that found, later in the field that the parts are peeling as a result of this visually indiscernible change in the deposit. Nevertheless, the eye is the first line of defense, so the eye should be trained to note even the slightest changes in color or appearance.

 ii. Water Break test

The water break test involves examination of a surface for the presence of a continuous water film that has “no water breaks.” The part will exhibit a surface that resembles the freshly waxed car after the rain.It is an indication that all organic soils are removed.

The test is subject to possible misinterpretation due to the retained surface film from inadequate rinsing or presence of hydrophilic smuts with possible oil trapped under the smut. If a water-break-free film of water is present, it is indicative of the absence of hydrophobic surface contaminants. Oils, greases, and water-insoluble organic compounds would be examples of hydrophobic contaminants. The water break test does not confirm the presence or absence of hydrophilic particulate contaminants or oxides. The key for the test is to use fresh uncontaminated water. Wetting agents or rinse aids, used in final rinse may hide poor cleaning.

iii. White Glove test

The quick and simple, time honored white glove test is used to show the presence of inorganic soil removal and, to a certain extent, organic contaminants on a surface after cleaning. The part may be tested while still wet from rinsing or after drying. The surface of the part to be tested is wiped with a white glove, cotton swab, or towel tissue. The material used to wipe the surface is then examined for the presence of black, gray, or off-white residue or oil staining. If contaminants are found to be present, microscopic examination or advanced chemical or surface analysis can be performed on the part surface or the item used to wipe the surface to more precisely determine the origin and the nature of the contaminants.

iv. Pumice Scrub test

This is popular and useful test to determine and differentiate problems associated with cleaning or plating sequence. The part is removed from the after cleaning and scrubbed with a soft, wet bristle dipped in pumice. After thorough water rinsing cleaned part is returned on the rack and plated. If the part is brighter than others after plating is indication of insufficient cleaning. If there is no improvement after plating the problem is in all probability related to the plating sequence.

v. Other test methods

Sophisticated physical and chemical analytical methods can be applied to test for residual contaminants on surfaces that have been cleaned 6-28 . Samples of parts that have been cleaned and dried can be immersed in a turbulent solution of a solvent. The solvent can then be analyzed for organic contaminants and insoluble particulate matter.

vi. Control methods.

There is still not readily available, especially for the small and medium size plating facilities, a quick, simple and reliable method for the determination of when the useful “life” of the cleaner is approaching its end.  In many plants, the “death” of the cleaner is first appreciated by a batch of rejects due to the insufficient cleaning. The procedures regarding the maintenance of the cleaners are still haphazard in many plants. The trouble is mainly to the difficulty in difficulty in analyzing proprietary compounded cleaners, especially as they age, and become loaded with carbonates, products of hydrolysis of complex phosphates, saponified oils, dissolved metal ions, etc. Methods for the routine quantitative control of cleaners are mostly semi-empirical, and analyses are often in terms of gram/liter or ounce/ gallon of the compounded cleaner rather than as the component chemicals.  Some procedures are available from the vendors or in the open literature 83-88. A single endpoint or a double end-point titration is usually made to estimate the effective alkalinity. The cleaner is titrated with a standard, usually 1 N acid. “Free” alkalinity is titrated with an appropriate indicator, e.g. Methyl Orange or Sulfoorange to about pH 10.5-11 for orange to yellow color change. “Total” alkalinity, which includes alkalinity contributed by all the alkaline chemical constituents of the cleaner, is titrated to a ph of 8.6 - 9.0 with phenolphthalein end point (pink to colorless). 

If sodium hydroxide, phosphates, and carbonates are present, these can be estimated with reasonable accuracy by titration to pH’s 10.5, 8.5 and 4.2. If silicates or borates are present, then titration for available alkalinity may be practical, but are only approximate. As the effective concentration of wetting agent(s) is reduced since the oil is accumulated in the cleaning bath, it may be advisable to control this critical ingredient, even with an indirect, approximate method. A simple method with affordable equipment is to check the surface tension with stalagmometer. The number or drops or time needed to empty the instrument can be compared with the fresh cleaner and utilized as simple guide for the state of surfactant used. In any event, the chemicals that are present must be known or determined in order to make a complete analysis. For many cleaners it is necessary to resort to gravimetric analyses for P₂O₅, Na₂O, CO₂, and SiO₂. In addition, it may be necessary to analyze for borates and wetting agents, so that a complete chemical analysis becomes impractical for routine purposes.

An alternative for complete analytical control is to maintain the bath by a recommended method for technical control and to make occasional cleaner tests. This consists of a standard procedure for measuring the cleaning time for a panel reproducibly contaminated with a standard soil. Many methods of this type have been used to evaluate cleaners, but they have not been applied for control, since it is difficult to produce a standard soil that will represent the production cleaning problem.

A commercial cleaner is accepted by application to a particular production need. Thus, the real trial is in the processing line or, at least, on a pilot plant basis. Thus, the obvious answer given to our question becomes the practical answer. Technical analyses are still used rather than complete chemical analyses, in a majority of the plants.

Surface Preparation of Metals Prior to Plating (part 5: introduction to ACID CLEANING & WATER RINSING)

H. ACID CLEANING ⁵⁹

Certain soils, especially those that are inorganic, either are removed in acid cleaners or are dissolved in the acids, or both. As distinct from pickling and derusting, the main emphasis is on cleaning effect, and solutions are therefore more mildly acidic. It is not the object of acid cleaners to remove heavy grease or oil deposits, such as removed by alkaline solutions or solvents, but rather to remove light grease, oxide films and similar inorganic films. They are generally used in final or near final preparation of metals prior to plating. A large number of compositions are available and can be classified in general terms as:

Mineral (inorganic) acids based, e.g. hydrochloric, sulphuric, phosphoric.

Solid acids, related to the sulfuric acid, e.g., Na- bisulfate, sulfamic acid; Ferric sulfate or chloride, monosodium phosphate, ammonium persulfate, and bifluoride salts.

Organic acids, e.g., gluconic, citric, tartaric, lactic, EDTA, acetic, oxalic,  hydroxiacetic among others..

The acid cleaners will have one or more of the above acids or their salts, appropriate low molecular weight acid-stable anionic surfactant, e.g., ethoxylated nonyl phenol, water soluble organic solvent, e.g., butyl cellosolve, ethylene glycol  monobutyl ether (EGBE), defoaming agents and in some applications a corrosion inhibitor. In some cases, chromic acid based solutions are used to clean iron and stainless steel (SS). For SS, a solution containing 60 gr/l of CrO₃+60 gr/l of H₂SO₄+60 gr/l of HCl is used at room temperature. Another solution for SS is HNO₃ (10-50 vol. %) + HF (1-3 vol%).

The advantages of using organic acids vs. mineral acids are numerous. Most of these acids are nonvolatile, have low corrosivity, safer to handle and can sequester dissolved metal.

Acid cleaners often suffer from rapid soil loading, particularly metal loading, that often requires decanting and dumping of the cleaner solution. This in turn leads to relatively high cost as compared to alkaline cleaning.

I. ACID DIPPING, ACTIVATION AND PICKLING

i. General. Acid treatments, prior to the plating step may be classified into three categories mostly used: acid dipping, activation and pickling. Diluted mineral acid is usually used, complemented with additives. However, mixtures of acids or acid salts are utilized to improve their reactions with metal. Their actions are increased by using higher acid concentration, temperature and agitation. They decrease in rate as solution becomes more concentrated in dissolved metal.

Surfactants and wetting agents are added to: a) reduce surface tension and emulsify residual oils, b)and provide a thin foam blanket to prevent the corrosive effects of mists, c) provide more uniform acid action, d) reduce  drag out losses, and e) as  dispersants they can be added to prevent redeposition of removed soils.

All acid solutions should be selected to meet the following qualifications:

The metal surface should not be disturbed any more than desired.

The metal salt formed from the reaction between the metal and the acid must be water-soluble.

To illustrate the later point, sulfuric acid would not be suitable for preparing lead or leaded brass for electroplating. This is due to the formation of insoluble lead sulfate, which would form a non-adherent film between the basis metal and the electrodeposited metal.  Sulfamic acid, citric acid or fluoroboric acid can be used.

When parts are acid treated, in order to minimize or prevent attack on the basis metal, acid inhibitors should be added to the acid bath. They function by suppressing the dissolution action of acid on basis metal. They also reduce hydrogen evolution and the likelihood of hydrogen embrittlement. They should be stable and not prone or susceptible to forming difficult to remove films. They reduce the roughening and pitting due to the excessive metal dissolution. This is extremely important to the parts that must maintain close tolerances on dimensions. This usually applies to the machined parts or on the parts that must retain a smooth, low micro-inch (RMS) finish. Inhibitors, consisting of various chemical compounds, can be quite specific to individual mineral acids. They also prevent the immersion deposits in aged and contaminated baths.

 ii. Acid Dips.

This is the last step prior to the plating sequence. This simple step has three purposes: a)to remove any residual alkalinity left on the cleaned and rinsed metal surface, b) to remove any oxide or superficial tarnish films formed due to the exposure to the cleaners or to the  air during previous rinsing or during the transfer time, and c) provide active and preferably micro etched surface for assuring good adherent  electrodeposits.

iii. Activation.

 Activation acid dipping is used to not only to neutralize the alkaline film and to dissolve any light oxide, scale and other acid soluble soils developed in the final cleaning, but also to activate the metal surface prone to form tenacious oxides, e.g., nickel or stainless steel. In immersion treatments, more concentrated acids, acid mixtures or acid salts are usually used with wetting agent. For difficult metals, activators are used, e.g., fluorides, persulfates, or proprietaries. For electro activation, the work is made cathodic. The evolution of hydrogen at the surface of the metal provides gas scrubbing, which increases the rate of oxide removal. The reducing action of hydrogen, decrease or eliminates passivity, e.g., on stainless steel or buffed nickel, which is to be electroplated.

 iv. Pickling.

Pickling is used to remove scale and oxides, usually resulting from heat-treatment, forging or casting operations. It can be performed as immersion dipping or cathodically. Steel sheets from which rolling scale is to be removed are normally immersed in warm sulfuric acid (H₂SO₄). Brass articles are generally treated in a mixture of nitric (HNO₃) and H₂SO₄ acids (except leaded brass), while a number of special solutions are used for various alloys ^{2, 59, 60}. When a metal that possess an oxide layer on the surface is dipped into an acid, the acid will first attack the oxide, initially producing metal salt and water. Next, however, the acid may attack the base metal, producing metal salt and liberating hydrogen, which will come to the surface as bubbles. Since the goal of pickling is simply to remove the oxide scale, inhibitors are added. With them present, the pickling time is usually increased, since dissolution rate of oxide as well of metal is decreased (albeit at slower rate). Their principal purpose is: a) to slow down attack on the metal by making it difficult for the hydrogen to be evolved from the surface, b) to reduce roughening and pitting due to excessive metal dissolution.

For pickling steel, cold hydrochloric acid ( HCl) or warm sulfuric acid is typically  used Phosphoric is outstanding, albeit more expensive, pickling acid as the resulting iron phosphate produced has rust preventative qualities. For pickling (etching) aluminum an alkali solution may be used. Tribasic sodium phosphate, Na₃PO₄, is suitable for light pickling and caustic soda with gluconate is added to prevent early precipitation, for deep etching (roughening). The parts may then be dipped in diluted HNO₃3, (1:3) to remove any smut. Cast aluminum is generally acid dipped in a hydrofluoric (HF) acid, mixture of nitric and HF acids, or in solution of fluorides salts such as ammonium bifluoride, in order to remove silicon. Copper and brass are treated with by H₂SO₄/sodium dichromate or H₂SO₄/HNO₃acid mixtures. Stainless steel is treated with an acid dip containing HCl or HF acid and/or ferric sulfate.

Electrolytic acid pickling is similar to electrocleaning. However the effect of the current is much more aggressive and should be checked in the laboratory before it is used on the production line. This method is most frequently beneficial if used for activation of nickel after stripping chromium for chromium replating.

Ether cathodic, anodic or periodic reverse DC current may be used to aid activation, increase scale removal, remove protruding metal slivers, or smoothen (“level”) rough surfaces.  Anodic (reverse current) is used to remove protruding metal slivers and to smoothen rough surfaces by electropolishing action. Highly concentrated acids are generally used for this application.

 J. WATER RINSING

i. Introduction. Rinsing is often the most overlooked aspect of surface preparation of metals. Long ago, in 1928 C.F. Nixon⁶¹presented a paper on drag-out showing that the substantial amount of valuable plating salts can be lost through this method, if rinsing is inadequate. Dragout losses and rinsing methods were advanced due to the studies by pioneer in this field, Dr. Joseph B. Kushner, starting in 1935 ⁶² and lasting through the next few decades ^63,64. Other practicing scientist, J. B. Mohler, contributed numerous papers on this subject ^65,66. Mooney ⁶⁷ defined rinsing as removal of a harmful clinging films of process solution from a work piece by substituting an innocuous film of water in its place.

 ii. Principles.

This vital step is necessary to: a. stop actions of chemicals from the cleaning solution, b. prevent contamination of the subsequent process, and c. prevents staining of the parts. Clearly, avoidance of excessive drag-in of alkalis from cleaning tanks into the rinse system, prior to plating must be implemented. Adequate part drainage above the cleaner tank prior to transfer to the rinsing operation is thus the first requisite for successful rinsing, and it was early recognized by Gustaf Soderberg ⁶⁹. He provided some experimental data and calculations on the amount of alkaline cleaner solution dragged into a rinse tank.

Obviously, cup shaped parts and other intricately shaped parts with blind holes and cavities can drag out significant volumes of solution. The time of drainage is important, and Soderberg recommended slow withdrawal and rapid transfer rates. The basic rinsing variables, which influence the amount of cleaner residue remaining on the cleaned part, are: the number of rinses, agitation, and type of rinse system and rinse temperature. Where practical, as a bare minimum, at least two running rinses arranged in a countercurrent or cascade fashion are recommended, particularly for the difficult-to rinse caustic-containing heavy-duty alkaline cleaners ⁶⁹. Self-evident, but not universally followed fact is that water consumption will be drastically reduced by cascading the final rinse into the previous rinse and that rinse to the previous one. Maximum efficiency of rinsing tanks design and accompanying controls is not difficult to achieve, as the principals involved are well understood.

Rinsing can be effective only if it reaches a part. Part orientation, loading, and rinse flow dynamics are important but often-overlooked considerations. For efficient rinsing, agitation is essential. Air agitation, sparging, ultrasonic, forced pumping, cathode rod movements are commonly used. In those cases where part geometry permits, an alternate hot-cold spray rinse is an efficient method of rinsing. For highly configured parts that have small crevices or blind holes, ultrasonic agitation is recommended to expedite the exchange of water for alkali in remote recesses.

 iii. Spray rinsing

Spray rinsing is what the name imply, i.e. rinsing with spray or fog of water with sufficient force to remove all or most of the carry-overs from preceding tanks. This is a valuable addition to overall rinsing operation, since, in effect, it is increasing the number or counter flowing rinsing stations. Sprays may be positioned to rinse racked parts as they leave any rinse station or as they emerge from the plating or processing tank. Each set of sprays should be supplied with water pumped from the following rinse tank. There are two advantages of spraying over the final rinse tank: city water line pressure eliminates the need for a pump, and any over spray beyond the rinse tank is free of drag out. Pumps or city water pressure should be activated only when processed parts are actually in the spray zone. This will conserve water and allow the use of higher flow rate nozzles ⁷⁰. Spray rinses are most efficient when they are turned off or on automatically when entering into and leaving the rinsing station.

iv. Rinse aids. Obviously, good rinsing involves intimate contact between the work surface and the rinse water. The use of a designated, surface tension reducing wetting agent in the rinse water which promotes intimate contact with and displacement of the concentrated solution film, will greatly improve the rinsing qualities of ordinary water. In addition, if air is being used for rinse tank agitation, the wetting agent will increase the number of air bubbles formed to further aid the process. Since only about (2% or less) of the appropriate wetting agents are needed to do such a job, a simple automatic liquid feeder can be used to meter the wetting agent into the rinse tank. It is quite economical to use such a scheme, particularly if three or more rinse tanks in series are used. 

Surface Preparation of Metals Prior to Plating (part 4: introduction to SOLVENT CLEANING & VAPOR ‎DEGREASING EMULSIFIABLE SOLVENTS & EMULSION CLEANERS)

SOLVENT CLEANING & VAPOR DEGREASING

Th eorganic solvents used for cleaning applications are the non-flammable chlorinated hydrocarbons: trichloroethylene (Trichlor), perchloroethylene (Perchlor) and 1,1,1 trichloroethane (Ethyl chloroform) 54-56.Most organic solvents are considered unacceptable for reasons of toxicity, flammability, effectiveness as ultrasonic mediums, or ease of recovery. required solvent properties as follows:

1. Effective cleaner for variety of organic soils.

2.Noncorrosive.

3. Nonflammable.

4.Low toxicity.

5.Mild or moderate odor.

6.Dries on ambient temperature.

7.Leaves no bondable surface.

8.Leaves no compounds with ODP, halogenated compounds, water,

ketones, aromatics or any of the 189 compounds listed by the US Clean

Air Act as Hazardous Air Pollutant.

Unlike aqueous cleaners, the solvent cleaners function by dissolving soil: oil, grease, wax, asphaltic materials, resins and gums.  They are used for hard surface cleaning of all metals, glass and certain plastics. Since these solvents are also relatively toxic and possess rapid evaporation rates when they are used in vapor degreasing equipment, the inhibited or stabilized forms are used. This is because these may be recovered by distillation and automatic control of these factors must be exercised.

These solvents may be used however, in properly designed degreasing tanks, as a last resort either for temperature sensitive applications or for those applications where particulate matter is the major contaminant. In the event that removal of solvent soluble residue is also involved, the contaminated solvent may be reclaimed by distillation in an auxiliary still.  Because of its low toxicity in comparison to Trichlor or Perchlor, ethyl chloroform (H₃C-CCl₃) is commonly selected for solvent cleaning in non-vapor degreasing applications.

Although one must neither recognize the chemical activity of a metal in the selection of a cleaning procedure, this factor is virtually non-existent in solvent vapor degreasing, since neither etching nor staining of the part will be experienced. Thus, when other factors are compatible to this method of cleaning, solvent vapor degreasing is a natural choice in cleaning a variety of materials. Neither the shape nor the fragility of a part is considering being a governing factor in vapor degreasing since the parts may be cleaned thoroughly without being subjected to a mechanical action. This method will realize good penetration even in the deepest recess. Where the soil is heavy or the part is so light as to limit the amount of flushing by vapors, immersion and/or spraying with liquid solvent can be performed within a degreasing unit prior to the vapor rinse. Since vapor degreasing may be performed on an automated basis, it is frequently preferred when high productivity is involved.

The operation that is to follow the cleaning step is also a selection factor. Thus, vapor degreasing is ideally suited to the cleaning of work prior to inspection or subsequent processing where the work is best handled dry. Where the work is to be wetted in the next operation, i.e., part will be plated, vapor degreasing is frequently used to remove soil economically and to minimize contamination of subsequent treating solutions. The normal cleaning sequence used in vapor degreasing consists of vapor clean, plus vapor rinse and dry. The initial vapor cleaning treatment is conducted in the vapor zone over the boiling sump. It offers the advantage of elevated temperature melting and dissolving actions plus the beneficial concentration of gross contaminants in one section of the degreaser.

Cleaning is next conducted in continuously distilled solvent for the final removal of greases, oils and particulate matter. Removal of particulate matter from the solvent is effected by either intermittent or continuous filtration. The temperature in the sump is maintained thermally well below the boiling temperature of the solvent to effect a second condensation of pure solvent on the work piece upon its withdrawal from the degreaser. For this treatment, the part is suspended in the hot vapor until it attains the vapor temperature, thus emerging from the degreaser as a clean dry part. Very slow exit speed, 3 ft/min, must be used to prevent vapor from escaping into air, and to ensure that no liquid solvent remains on the part.

The selection of a vapor degreasing solvent is made based on the physical properties of the soil, the solvent and the substrates involved. For example, Perchlor is specifically used where high temperature solvency is required as in the removal of high melting point waxes.  Trichlor on the other hand is the more popular of the two solvents because it offers greater solvency at a lower operating temperature; hence, a lower operating cost. For certain temperature sensitive operations, the use of ethyl chloroform may be justified because of its lower boiling temperature (74⁰C/165^oF).

Where selective solvency is a requirement, as in the cleaning of assembled devices containing plastic or painted components, the use of trichloro-trifluoroethane is recommended. Its low boiling point (48 ⁰C/118 ^oF), low Kauri Butanol value (31)( The Kauri-butanol value ("Kb value") is an international, standardized measure of solvent power for a hydrocarbon solvent, and is governed by an ASTM standardized test, ASTM D1133. The result of this test is a scaleless index, usually referred to as the "Kb value".) low surface tension (19.6 dynes/cm) and low toxicity (1000ppm) combine to fulfill a need not met by the chlorinated solvents. Regardless of the solvent selected, none will remove water-soluble contaminants. These must be removed in a solvent emulsion or in an aqueous cleaning treatment before or after vapor degreasing.

The popularity of vapor degreasing originates from the rapid solvent cleaning action achieved and the fact that subsequent drying operations are eliminated. The latter is particularly important for intricately shaped parts where the presence of residual water cannot be tolerated. Vapor degreasing by itself does not usually insure that the part is sufficiently clean for the final electroplating step.

EMULSIFIABLE SOLVENTS & EMULSION CLEANERS^57,58

There appears to be some reluctance toward the use of emulsion-type cleaners, as compared to the more widely accepted methods of solvent cleaning, vapor degreasing, and alkaline cleaning procedures. This may be due to a lack of knowledge of the characteristics displayed by this class of cleaning materials and the advantages gained, particularly when it is used as a pre-cleaner in the removal of stubborn contaminants.

EMULSIFIABLE SOLVENTS & EMULSION CLEANERS^57,58

There appears to be some reluctance toward the use of emulsion-type cleaners, as compared to the more widely accepted methods of solvent cleaning, vapor degreasing, and alkaline cleaning procedures. This may be due to a lack of knowledge of the characteristics displayed by this class of cleaning materials and the advantages gained, particularly when it is used as a pre-cleaner in the removal of stubborn contaminants.

i.Emulsion cleaners contain water, an emulsifier and a petroleum hydrocarbon, typically kerosene. The commercial products contain the solvent and emulsifier who is added to water in varying proportions prior to use. Emulsifier is usually a higher alcohol or wetting agent. Additional effectiveness may be obtained by addition of surface active agent. The normal dilution is one part concentrate to ten parts water. When used as an ultrasonic medium, the tap water makeup is first thoroughly degassed. The concentrate is then added to effect the desired dilution. The emulsion cleaners are used at an elevated temperature of 50-70 ^oC (~120-160 ^oF) and are preferably followed by a hot water rinse. These cleaners function well as ultrasonic mediums yielding bath uniformity difficult to achieve in either aqueous or solvent media alone.

Emulsion cleaners are formulated to combine the functions of both aqueous cleaners and solvent cleaners to yield a cleaner that possesses both detergency and solvency powers. For this reason, they are very well suited for the removal of gross contaminants that are otherwise difficult to remove in a single cleaning treatment. The cleaner residue from an emulsion cleaner is a thin oily residue that offers temporary rust protection to the parts after this cleaning operation. The presence of this oily residue is a distinct advantage for in-progress (intermediate) cleaning, but its presence serves to prohibit the use of emulsion cleaners for final cleaning operations.

ii. In the emulsifiable solvent cleaning, unlike with the emulsion cleaners, no water is involved. Water does enter into the cleaning operation. However, it is used in the subsequent pressure rinsing operation, which serves to emulsify and remove both the loosened soil and the emulsifiable solvent. These effects are made possible by virtue of the emulsifiable solvent formulation.

The commercial emulsifiable solvents may be used in the undiluted condition or diluted with the appropriate type of hydrocarbon solvent prior to use. Since the resultant solvent assumes the characteristics of the solvent base, both flammability and/or toxicity must be considered. Those based on petroleum hydrocarbons must be treated with caution when used as ultrasonic cleaners, as the flash points of the emulsifiable solvent are being the limiting factor.

The use of emulsifiable solvents based on the non-flammable chlorinated solvents is more acceptable for ultrasonic applications, since these may be thermally degassed prior to use as an ultrasonic medium without the danger of fire. The same considerations in respect to toxicity apply to the emulsifiable solvents as for the chlorinated hydrocarbon solvents.

 When cleaning prior to plating, alkaline cleaning should always be used after emulsion cleaning to remove thin oil film left by the emulsion.

Surface Preparation of Metals Prior to Plating (part 3: introduction to SPRAY CLEANING & ULTRASONIC CLEANING)

SPRAY CLEANING

Spray cleaning is a powerful, simple and effective cleaning method and it should be used whenever it is possible 44-45. Pressure from spray nozzles is adding mechanical energy to thermal energy given by heated spray chemicals and together with chemical energy supplied by chemical action of spray cleaner ingredients makes one effective combination. In general, it is highly effective on any surface that it can be “seen” directly by spray 46 .Different effects can be achieved by changing the pressure of the spray, spray pattern and the volume of the sprayed cleaner. The physical effect as provided by spray cleaning can enable one cleaner to perform many jobs under a wide variety of circumstances. An efficient spray washer makes possible the use of  cleaners which have desirable features of good rinsing and scale prevention characteristics, even though their soil removal ability might be less if used in soak cleaning. In fact, it is sometimes possible to gain all the advantages inherent in nonsilicated sequestering type cleaners in spite of poor oil removal properties.

The spray washer is usually located after the soak cleaning operation and before the electro cleaner. In addition to the influence of cleaner formulation (light duty, mildly alkaline with low foam wetting agents) the following design details should be considered:

a. Duration of time and the length of spraying washer. This is dictated by production requirements, optimum cleaning time and line speed.

b. Spray-jet angles, spacing and patterns for uniform coverage of work.

c. Size (caliber) of nozzle selected according to distance from nozzle to work surface and pump specification, selected on delivering the required volume of sprayed solution.

d. Temperature and concentration of the cleaner.

e. Filtration for preventing clogging of the nozzles.

f. Solution reservoir of sufficient volume to fill the supply lines to the jets, without starving the pump.

g. Control of overspray from the ricochet from the walls and from the parts.

On the down side, spraying liquid is relatively inefficient way to deliver mechanical energy to the cleaning object, since only a portion of energy is directly impinging the parts that needs cleaning. In addition, atomized liquid evaporates rapidly, requiring the frequent water additions as well the considerable amount of heat is needed to maintain the required cleaning temperature. Spray cleaners are used at about 4-16 gr/l at 68-740 C with spray pressure from 0.7-3.5 kg/cm².Formulations are similar to soak cleaners, except that wetting agents are of a low foaming type.

 E.  ULTRASONIC CLEANING

Sound to nearly all of us, is anything we hear, while to the physicist is a form of vibrational energy. Most of us also can remember the fascinating phenomenon of the great singer Caruso shattering the wine glass with his voice. The range of sounds audible to the human ear is from 20 to about 20000 vibrations /sec. This is sonic sound. Around and above these frequencies, it is called ultrasound. Ultrasonic is a branch of acoustics that deals with mechanical sound waves at all frequencies above the audible range.

Ultrasonic cleaning functions because of changes in pressure and temperature that occur within vapor bubbles that implode 47. There are constantly changing negative and positive pressures, which cause the simultaneous formation and implosion of thousands of minute vapor bubbles. This is termed cavitation. It is the formation and bursting of these vapor pockets with their fantastic pressures and temperatures (approximately 10,000 PSI and 20,000oF) that does the cleaning. The sound waves are simply the mechanical means to achieve cavitation. It is important to have sufficient power to generate cavitation. Transducers must be located properly and close enough to the parts to be cleaned which minimize energy loss through the solution.

Ultrasonic cleaning generates a number of specific advantages for the electroplater. Since it is based on sound, it is omni directional.

Ultrasonic cleaning can be of alkaline, solvent, acidic or detergent base (neutral) nature. Effective cleaning action occurs anywhere that cleaning chemistry and ultrasound penetrates e.g., crack, pores, blind holes, etc. Maximizing the ultrasonic cleaning process was expertly treated by Fuchs 48-49 .

The basic components of an ultrasonic cleaning system consists of a tank for the cleaner, a transducer which converts electrical energy into mechanical (sound) energy, and a source of high frequency alternating current (a generator that changes 60 cycles/second AC into high frequency, A-C) 44,48-53.

Maximum cleaning efficiency is obtainable for the different temperatures for different cleaning media. Another interesting fact is that dilute detergent based solutions are more effective than concentrated ones under identical conditions when sonic energy is utilized. The cavitations produced by ultrasound are well documented in the practice by their ability to drive the packed dirt from deep or blind holes. This sound approach attracted a number of progressive finishers. The cost of the necessary equipment will limit adoption of this method in many cases, especially when large parts are being processed. Nevertheless, to our knowledge, it is the best approach so far suggested.

Surface Preparation of Metals Prior to Plating  (part 2: introduction to ELECTROLYTIC CLEANING)

B. ELECTROLYTIC CLEANING

i .General. In preparation of metal for plating or finishing, it is seldom that electrocleaning stage is not used. Exceptions are hard chromium plating, electroless plating, plating over aluminum, anodizing phosphatizing and chromating. Electro cleaners are usually considered as final cleaners. They are basically heavy-duty alkaline types, but are always employed with DC current – either cathodicdirect), anodic (reverse) or periodic reverse (alternating anodic and cathodic currents), PRC. Even though they usually follow precleaning step, in some cases final electro cleaning alone will suffice. The objective of final cleaning is to remove completely all soil and to activate the metal surface. Activation is usually obtained by using reverse current electrocleaning. The gas scrubbing of the generated oxygen assists soil removal, whiles the reverse current aids in its removal and prevents the deposition of any metallic film or non-adherent-metallic particles. A dilute mineral acid dip almost always follows the final cleaner, to neutralize the alkaline film on the metal surface, and to remove light oxide layer.

Parts containing heat-treat, welding, or other oxides may require a double cleaning cycle, depending on the degree of oxidation. In such cases, the part is usually anodically electro cleaned, acid dipped to remove the oxide, final electro cleaned, and acid dipped to neutralize the alkaline film. The second electro cleaner (final cleaner) is used to remove any smut developed from the scale removal in the first acid dip. The first cleaner removes any oil or other soil, which would reduce the effectiveness of the scale removal properties of the first acid dip. 

ii. Electrocleaning Formulations. Alkaline cleaning blends used for electrolytic cleaning, typically contain mixture of alkaline material, to provide high conductivity at established pH and to have enough reserve alkalinity. High alkalinity is needed for steel, lower for zinc, copper and brass. Alkali metal hydroxide, carbonate, silicate and phosphate are used as a principal source of alkalinity. Sequestering organic additives, such as gluconates, EDTA or trisodium nitriloacetate are also commonly present Formulation of electrocleaners in addition to basic fundamentals, all attention on foam control and optimal current densities. A problem with excessive foam in electrocleaning is that hydrogen and oxygen gas accumulated in the foam can explode at a sparking electrode.   

While this is more of a nuisance than a menace in most cases, it can make for an annoying situation and is taken to mean as poor plant practice. Another difficulty is that foam dried onto the work may be difficult to remove by rinsing, and shows up as a pattern in the final electroplated product. Water spray with fog nozzles may be necessary to wet down the work to prevent drying. Due to the lower cost sodium salts are more frequently used, although potassium based electrocleaners have better solubility, lower electrical resistance and better throwing power. Potassium salts are used in special occasions, e.g. when electrocleaning very large parts where otherwise very large voltages will be required, to obtain high current densities required.   

While this is more of a nuisance than a menace in most cases, it can make for an annoying situation and is taken to mean as poor plant practice. Another difficulty is that foam dried onto the work may be difficult to remove by rinsing, and shows up as a pattern in the final electroplated product. Water spray with fog nozzles may be necessary to wet down the work to prevent drying. Due to the lower cost sodium salts are more frequently used, although potassium based electrocleaners have better solubility, lower electrical resistance and better throwing power. Potassium salts are used in special occasions, e.g. when electrocleaning very large parts where otherwise very large voltages will be required, to obtain high current densities required.   

On the other hand, zinc and zinc die-castings are cleaned anodically for another reason. Because of the sensitivity of the metal to attack by alkaline electro cleaners, it is desirable to use inhibited cleaners. When silicate is used as an inhibitor, an insoluble film seems to develop when cleaning is cathodic. Hence, cleaning is anodic, with specially formulated cleaners, under mild conditions, and for short times. Steel, which is relatively insensitive to oxidation-reduction effects, can be cleaned anodically or cathodically. At high current densities, there is a tendency toward browning of the steel unless an inhibitor, silicate, is present in the cleaner. Stainless steel can also be cleaned either way, although more drastic pickling action is required when anodic cleaning is employed. The trend, in recent years, has been strongly towards anodic cleaning of steel. 

iii. Anodic, (reverse) electrocleaning:  The work is made anodic (positive) in an alkaline electro cleaner using low voltage (3 to 12 V) DC current. Current densities vary from about 1 to 15 A/dm2 depending on the metal being cleaned and the cleaning time. Cleaning times of ½ to 2 minutes generally suffice for most applications. Higher current densities are possible when shorter cleaning times are used. 

Anodic electrocleaning is desirable for cleaning whenever possible, because of the fact that the metal is actually being slightly dissolved as well as cleaned. This action removes metallic smuts and oxidation products and prevents the deposition of non-adherent metallic films.  Anodic current generates thin, fresh oxides that can oxidize organics, and also be easily removed in subsequent acid dipping step. Without this oxidation, some stubborn organics may remain insoluble. The oxygen generated at the surface creates a scrubbing action that assists soil removal. In addition, hydrogen embrittlement is avoided by using anodic cleaning. 

It is important to control the current density, temperature, and concentration, particularly on nonferrous metals to avoid etching and tarnishing. Prolonged reverse current cleaning, high current densities, high temperature, and low concentrations are to be avoided, particularly on zinc based die Castings and brass and to prevent dezincification and over etching. Reverse current alkaline cleaning is not recommended for aluminum, chromium, tin, lead or other metals that are soluble in alkaline electro cleaners. 

iv. Cathodic, (direct) electrocleaning:  The parts are made cathodic (negative) and the same equipment, voltage, and current densities are generally used, as described under anodic leaning. Hydrogen is liberated at the surface of the work. The volume of hydrogen liberated at the cathode is twice that of oxygen liberated at the anode for a given current density. Therefore, more gas scrubbing is achieved at the cathode than at the anode. For this reason cathodic cleaning is sometimes employed as a precleaner followed by anodic cleaning. Soil removal is accomplished by the mechanism described under alkaline precleaning and assisted by gas scrubbing. 

The work is actually being “plated” in a direct current cleaner. Any positive charged material is attracted to, and may be reduced and deposited on the surface. Any film (metallic) deposited is usually non-adherent, but difficult to detect and remove. Such films can cause poor adhesion roughness and/or staining of electroplated metal.

Parts critical to hydrogen embrittlement (e.g. spring steel) should not be cleaned cathodically unless adequate steps are taken after processing to remove the hydrogen. Generally, heat treatment for 1 hour a 200 o C (~400 o F), immediately after processing will remove the embrittling effect of hydrogen. Parts with hardness exceeding Rockwell40 C can be embritlled and should be baked after plating. Chromium contamination of cleaners is sometimes unavoidable, due to the use of the same rack for chromium as well as other plating. Direct current cleaning is more susceptible to staining from chromium-contaminated cleaners than reverse current cleaning. 

Direct current cleaning is used for the following applications:

- To clean metals such as chromium, tin, lead, brass, magnesium, and aluminum, which are dissolved or etched by anodic cleaning.

- To clean and activate high nickel steels, buffed nickel or high nickel alloys, and when plating nickel over nickel or prior to chromium plate. Anodic cleaning would produce a passive film on the nickel, due to oxidation, which would prevent the deposition of bright chromium.

 v. Periodic reverse electrocleaning (PRC): Since direct and reverse electrocleaning have individual advantages and disadvantages, the best answer is to use both, either in separate tanks, or in one tank, by periodically reversing the current. PRC electrolytic cleaning is used generally to remove smut, oxide and scale from ferrous metals. It is by far the most effective way of electrocleaning. Alkaline compounded materials containing sequestering or chelating agents are usually used.  The work is made alternately cathodic and anodic; using DC current at 6 to 12 V. Work may be cleaned on racks, in a barrel or as continuous metal strip. Cleaning and scale removal are accomplished by incorporating the mechanism of alkaline cleaning and the use of reducing and oxidizing conditions, coupled with strong metal chelating. 

One of the advantages of PRC cleaning is to eliminate the acid treatments on certain types of parts where entrapment of acid aggravates bleed-out after alkaline plating (brass, copper, zinc, cadmium,tin). Oxides and light rust may also be removed without the danger of etching or the development of smut usually encountered from acid pickling. When PRC electrocleaning, sometimes the charge on the parts as they are exiting can be critical. In other words the parts leaving should be all cathodic or anodic polarity. The most effective, correct polarity will be the one that shows no discoloration on the metal, may enhance the luster, or gives the most rapid flash gassing in the acid dip. If no acid is used, excellent adhesion and no skip plating evidence the correct polarity. 

vi. Bipolar electrocleaning: This particular kind of electrocleaning is used mainly in continuous strip lines where high amperages and/or voltages are required. In order to prevent excessive currents and potentials being applied on contact rolls and metal strips as well to minimize detrimental stray currents and electrochemical corrosion of equipment, bi-polarity is utilized as means of electrocleaning. Anodes and cathodes are separated and by such arrangement opposite polarity are induced on the strip. It is important that strip finish anodic at the end of the cycle to prevent any possible, unwanted cathodic deposition. High current density design  cells are utilizing any means of enhancing cleaning via close spacing forced flow, grid arrangements and similar. 

vii. Physico-chemical aspects   of electrocleaning.  Electrocleaning action is, in part provided by scrubbing action of hydrogen and oxygen gases that are generated on the parts or counter anodes, according to simple reactions:

4OH^- - 4e          2H₂O + O₂  and,

 4H₂O + 4e         2H₂ +4OH^-. Cathodic and anodic processes, in electroplating as well in electrocleaning steps are directly dependent on the applied current density (CD). By convention, this is taken as amperes per of surface area, (A/dm2 or A/ft 2), of both sides of the electro cleaned part. However, the primary current distribution, which is dependent on geometry of the parts, and, consequently working CD is not uniform, tending to be higher at edges and lower in recesses. In accordance with Faradays Law, at given time, the higher the CD, the greater is the evolution of gaseous hydrogen or oxygen, metal dissolution, and cleaning. There is point diminishing return, as at very high current densities, damaging dissolution (etching) and oxidation (tarnishing) effects become more prominent.

Often treated as empirical and non-scientific art, in many plating plants, effort is not made to calculate the actual electro cleaning current density. These instances are especially true where parts are of irregular shape or vary from batch to batch. It should be, however kept in mind that resistance depends on anode/cathode distance, type of the  electrocleaner, its concentration, temperature and the type and the condition (e.g., passivity) of counter electrode. Electrocleaning is usually carried out at four to nine volts. If only low voltages are available, resistance of the electrocleaner can be too great for the DC power supply output to provide adequate CD, and electrocleaning can be marginal. A cleaner of higher conductivity should be used (e.g. potassium based) and the electrocleaning system examined for poor contacts or other electrical problems. 

Often treated as empirical and non-scientific art, in many plating plants, effort is not made to calculate the actual electro cleaning current density. These instances are especially true where parts are of irregular shape or vary from batch to batch. It should be, however kept in mind that resistance depends on anode/cathode distance, type of the  electrocleaner, its concentration, temperature and the type and the condition (e.g., passivity) of counter electrode. Electrocleaning is usually carried out at four to nine volts. If only low voltages are available, resistance of the electrocleaner can be too great for the DC power supply output to provide adequate CD, and electrocleaning can be marginal. A cleaner of higher conductivity should be used (e.g. potassium based) and the electrocleaning system examined for poor contacts or other electrical problems. 

The relative effects of conductivity of the electro cleaner on cleaning have been assessed only qualitatively. The difference between a "high conductivity" and "low conductivity" electrocleaner might be 10 to 25% of the total resistance. Other factors such as condition of the contacts on the rack or on the bus bars, variations in total surface area, shape of the parts as they influence current distribution or surface film on the opposing electrode may be equally significant.  

                                              viii. Chromium Contaminations After chromium plating, the chromium metal deposited on the racks is normally stripped, in the separate tank, before the same racks are loaded with the parts and send again through the line. If this is not done, or if done incompletely, the chromium is dissolved in the cleaner, in the hexavalent state (Cr⁺⁶).Moreover, the anodic cleaner is often used as a mean of stripping chromium from the racks, and loading the electrocleaner with unwanted Cr⁺⁶. Another source of chromium contamination of the electrocleaneris the chromic acid, from the chromium plating tank that is trapped in cracks or holes of improperly maintained plating racks. 

The effects of "chromium contamination” is often argued. It is quite likely that effects are minimal in the  anodic cleaning of steel, but quite significant in  cathodic cleaning of steel, where surface films may form which prevents good adhesion of electroplate. It is considered that the greatest effect of Cr 6+ contamination is the possibility of drag-over to the copper or nickel   plating bath, where chromium is very detrimental. Obviously, this is dependent on effective rinsing and can be avoided. Hexavalent chromium can be reduced to the trivalent state, in which form it appears to be less harmful. This is done by the addition of small amounts of reducing agents e.g., 0.2 to 0.4 % (~ 0.5-1 oz/gal), of sodium bisulfite (NaHSO₃), hydrosulfite (Na₂S₂O₄), or metabisulfite (Na₂S₂O₅). For emergencies, sugar can be used, just like in old times. This effect is a transient one, as the reducing power of the sulfites is eventually lost and the chromium reoxidizes at the anode. Eventually, excessive quantities of reducing agent are required and it is cheaper to replace the bath. 

ix. Maintenance and Operation of Electrocleaning Equipment. Electrocleaner tank configuration is probably least considered when designing of any plating line, because of the large operating window. Corrugated or mesh steel can be used as anodes or cathodes to provide optimum surface area and solution circulation. They should be positioned in such way as to be easily accessed for periodic inspection and cleaning. Periodic cleaning of the anode/cathode is necessary to remove plated-on smut, oxides, and other charged particles. Using the tank as the anode or cathode is not recommended, because the current distribution is fixed and little control is obtained. This leads to uneven current distribution and a source for stray currents.  Many electrocleaning problems, such as under- and over-cleaning, have been traced to such a practice.

Surface Preparation of Metals Prior to Plating (part 1: introduction to surface cleaning)

I. DEFINITION OF CLEAN SURFACE

In the preparation of almost all metals for decorative plating, one of the most, if not the most important consideration, is the preplating sequence: cleaning process. This is so, because the appearance, adhesion and acceptance of the finished article depend primarily on a sound foundation for the final finish, which is achieved only with an active and clean substrate.

Clearly, only a properly designed preplating sequence will result in quality parts. It is not question of whether or not cleaning is required, but what type of cleaning should be used. Clean can mean many different conditions to many people.

Cleaning is loosely defined as the process of removing unwanted contaminants or dirt from a surface. A practical definition of word clean is “containing no contaminants that would interfere with satisfactory deposition of one adhering finish”. It is differentiated from other finishing processes in that the cleaning process does not alter the surface physically or chemically.

A properly cleaned surface is just the same as it was prior to cleaning, except for the missing soil For example, to the spray painter, “clean” can be simple freedom from oil or grease. A plater will need to go further, in that his work must also be free of rust, scale, oxide and smut.

A nickel-chromium decorative plater would be especially more critical, since minor rust and scale would appear on finished parts as white frosty spots, pits, roughness or even black spots

A cyanide zinc plater may not be as critical, since less than thoroughly clean parts would come acceptable, mostly because cyanides, inherently, are good cleaners because of their high alkalinity and ability to complex many metal ions.

Can conditions of clean and active surface be achieved ion a reasonable, uncomplicated and cost effective manner? For the plater, we can contentedly state that such a condition can be attained.

Today, platers are taking parts, as they receive them, and are placing these parts through a cleaning process cycles in hand lines, hoist lines, automatic lines and strip lines with even more successful than ever before. If we want to be successful in cleaning today, we must handle cleaning processes with the same care and control as we do with plating processes.

The day of the single cleaner tank is gone forever. Contemporary cleaning systems must remove oil, grease, scale, rust and inert particles. In other words, they must degrease,  saponify, emulsify, acid dip (pickle), neutralize, activate, etc. in order to obtain a “clean” part for plating. To do all this, several tanks and solutions are required, depending upon the type of soil, available time and temperature, and the basis metal being processed. To measure the degree of cleaning more sophisticated methods for cleanliness evaluation are now available: Radio Isotopes, UV Fluorescence, Evaporative Rate Analysis, Atomizer test, X-ray fluorescence, Water Spray, Modified Contact Angle Method, Conductivity Method, Solubility Parameter Technology, Surface Potential Difference, Optical Stimulated Electron Emission, Electrochemical Meassurments, Kinetic parameters (overpotential (among others.  Kuhn elaborated in detail a number of methods for measuring surface cleaningless, ranging from simple to more sophisticated instrumental methods

The final test, of course, is the end result: an acceptable finish. Perhaps the most universal practical measure of the “clean surface” which is usually reliable, but not necessarily foolproof, is the “water break free” surface. This means that the part will be enveloped with a film of water, which does not form droplets or water beads. If a part is processed by alkaline type of cleaning, a quick dip in a mild acid, followed by a clean rinse is usually more indicative. On iron and steel parts, a uniform continuous cooper coating is fair indication of a clean surface. indication of a clean surface.

. II. THE BASIS METAL

The composition, physical properties and chemistry of the basis metal influence the selection of the cleaning procedure. The condition of the basis metal is equally important. For example, a piece of metal with heat or welding scale requires much more processing than non-oxidized cold rolled steel. High carbon steels require a different cleaning process than low carbon types, etc. The cleaning medium must be designed to be compatible with the metal being processed. A cleaning process that does an excellent job of soil removal but severely attacks or even slightly etches the metal surface is usually unacceptable. Therefore, it is important to select a medium, which either does not attack the metal, or one, which the attack is controllable to produce a desire effect.

III. CHOICE OF CLEANING METHODS

Durney eloquently describes the definition of soil  where he compared the soil with the weed: “A weed is a plant that is out of place. A rose bush in a wheat field is a weed. A wheat stalk in a rose garden is a weed. A rust proofing oil on a part in storage is not a soil. Only when a part moves to the finishing room  does it become a soil”. Poet John Milton observed in a bit more lyrical way: “…  that soil may best/ Deserve the precious bane ”.

Obviously the type of soil, as well as both the amount of it present and degree of adhesion to the metal surface are factors that must be considered. Other important factors will include the cleaning cycle and the proper method of soil removal. In addition to the composition of the cleaner, the selection of a proper cleaning cycle will depend on a number of variables. Some of them are the design and fragility of component parts, the contemplated methods of parts handling, and the production requirement.

With the ever-increasing government regulations of effluents, it is now universally accepted that surfactants be biodegradable, meaning that they can be destroyed by the bacteria present in sewage and waste treatments plants.There are many, many different types of soils that can contaminate parts to be plated. The examples  of some of the common soils that platers are encountering in their daily practice are

1. Mill oil.

2. Forming lubricants: a)  Sulfonated or chlorinated types as applied to metals such as brass. b) Lard oil – as used in forming aluminum and as a protective coating.

3. Drawing Compounds– lubricants containing molybdenum disulfide or powdered graphite and chlorinated oils.

4. Rust preventative oils– high viscosity oils containing  sulfonated soaps or organic corrosion inhibitors.

5. Shop soils– dirt, dust, metal chips, cutting oils, marking inks, and fingerprints.

6. Polishing and buffing residues– can and do contain metallic particles.

7. Metallic smuts– powdering of the basis metal mixed with the oils on the surface.
8. . Carbon smuts.
9. Oxides and scale, and weld spots.

It is often assumed that all soils can be easily removed. Often neglected is that soils can change character on standing, unfortunately in the direction of becoming more difficult to remove. Sometimes they can attack the base metal, leading to additional troubles latter. In addition to all other variables, time of standing has to be accounted for. However, the cleaning operations can be much less troublesome, and reject rates lower if the origin and the nature of the individual soils are more widely understood. Clearly, the types of cleaning methods for removing the above soils is closely related and directly dependent on the composition, condition and the amount of the soils present. It can be deliberated as follows:

A.  SOAK CLEANING

The workhorse of the industry, soak cleaners are intended to remove the major portion of heavy, oily soils, quickly, effectively, safely and economically. In addition, they should also meet P006 sludge reduction mandate, OSHA regulations, facilitate analytical control and simplify waste management.

The parts are submerged in tanks containing hot alkaline cleaning solution. The concentration and temperature should be as high as it is safe for the particular metal. In this way the cycle time is minimized. Agitation is required, although not universally practiced, in order to in addition to chemical and thermal energy, speed up solubilization, wetting, emulsification and saponification of the soils. The soak cleaners are generally classified as light and heavy duty.

light Duty Soak Cleaners. Numerous metal finishers use these of type cleaners, especially those who process buffed metals. These cleaners are composed of inorganic builders, wetting agents, buffering salts, sequestering agents, dispersants, inhibitors, and sometimes solvents and coupling agents. Powder blend or liquid concentrates are formulated to accommodate specific applications. They function by wetting, emulsifying, dispersing and solubilizing the soil. They are rarely used at room temperature, but rather at temperatures ranging from 66⁰C (150 0F) to boiling, at concentrations from 4 to 10% by volume or 4 to 8 g/l (6 -12 oz/gl). They are frequently used for light type of soils and non-ferrous metals.

The builders used in light-duty alkaline cleaners are those yielding a lower pH range of 11.2–12.4, for a concentration range of 3 to 4 g/l (4–6 oz/gl). They differ from the heavy-duty alkaline cleaners, in that they contain little or no caustic soda (NaOH) or caustic potash (KOH), thus they are of little value where saponification of soils and oils containing the fatty acids is required. By virtue of having the lower pH, they are also less stable to acidic contamination from acid drag-in than heavy-duty cleaners.

The popularity of light-duty alkaline cleaners is due to their effectiveness in removing contamination from aluminum, steel, copper and zinc base die-castings without etching or tarnishing the base metal. This is made possible with silicates, which effectively inhibit or protect zinc and aluminum from etching at the otherwise etching region pH: 11.2–12.4.

The light duty alkaline cleaners are preferred for the removal of light petroleum oil residues, buffing compounds, marking inks on aluminum sheet and residual inorganic salts. The best combination, usually depends on type of the soil.

ii. Heavy-duty Soak Alkaline Cleaners:

The major consumer in the cleaning industry, these enjoy the most widespread use of any type of cleaning. They are composed of balanced blend of highly alkaline builders, such as sodium or potassium hydroxide, carbonates and  trisodium phosphate (TSP). In addition they usually contain silicates as dispersants and corrosion inhibitors, along with sequestering agents, like sodium tripolyphosphate (STPP), tetrapotassium pyrophosphate (TKPP), and tetrasodium pyrophosphate (TSPP). Various surface-active agents* are always present, with occasionally coupling agents, like sodium xylene sulfonate (SXS). They are developed to remove a specific type of soil.

Cleaning is usually performed under conditions that promote the speed and completion of the cleaning reactions, such as elevated temperatures [60–700C (~140- 200 o F)], highconcentrations [80–240 g/l (½ - 2 lb/gl)], and high pH (~ 12.4 –13.8).

Those formulations are usefully used as general cleaning mediums for the removal of gross contaminants such as animal and vegetable fats and oils, mineral oil, fatty oils, grease, and rust preventatives and drawing compounds. These compounds are chiefly used for cleaning ferrous metals, and alkali-resistant glass; less commonly for copper and copper alloys because of possible “caustic burn” (tarnishing), and least commonly, for zinc and aluminum because of their reactivity in highly alkaline media.Choice of pH for cleaning different metals and soils is given in Table 3.

The main action as saponification, where  NaOHor KOH chemically reacts with these fatty acids to form water-soluble soaps. Unfortunately, sometimes these saponified fats or oils form metallic, sodium or potassium “soaps” that are often much more difficult to remove than the original fats or oils.

If there is other dissolved metallic salts in the solution, insoluble metallic salt will be formed, which cannot be rinsed of or dissolved in acid dip solutions. The role of the silicates in heavy-duty silicated cleaners is primarily to emulsify and disperse soil and to act as corrosion inhibitors. Consequently they are used for the removal of petroleum oils and greases. For mixed fatty acid-petroleum oil soils, saponification plus emulsification and dispersion are required that calls for the use of caustic-silicate formulations.

A secondary, but still important use for heavy-duty alkaline cleaners, are the removal of obnoxious, carbonized or partially coked contaminants formed by the breakdown of petroleum, vegetable or animal fats and oils.

Carbon smuts or other lightly adhering particles on the surface of the parts can sometimes be removed by proprietary smut removing compounds. Some specially formulated electro cleaners can be equally effective. Lightly adhering can be defined as being moved light finger pressure across the surface.

 Also included in the category of heavy-duty alkaline cleaners are alkaline descaling and rust-removing compounds. These contain caustic soda or potash plus, a  chelating or  complexing agent, e.g. sodium gluconate capable of removing rust and hard water scale from metal parts by dissolution and undercutting. They are often fortified with surfactants that are stable at high pH. Since only the iron oxides are removed, there is no attack of the base metal and, therefore, no danger of hydrogen embrittlement.  Caustic-gluconate compounds are also used for etch-type aluminum cleaners. However, since the reaction of NaOH/KOH with aluminum is rapid and is accomplished by a copious evaluation of hydrogen gas, the time should be carefully controlled. For difficult smuts, periodicreverse current is used, at 5-15 A/dm²

Rhodium plating from Metal Finishing (2002)

RHODIUM PLATING

Although several different electrolytic baths for rhodium plating have been proposed the only baths to achieve commercial significance are (1) phosphate for very white and reflective deposits; (2) sulfate for general jewelry and industrial deposits; and (3) mixed phosphate sulfate for general decorative deposits.

DECORATIVE PLATING

The jewelry and silverware industries were the primary users of rhodium electroplates until quite recently. Although both the phosphate and sulfate baths gave bright white deposits the phosphate bath was preferred for soft-soldered jewelry, especially before the general adoption of bright nickel plating. Cold nickel did not always cover the soft solder, and the acid electrolyte attacked and dissolved some of the solder. Lead in a rhodium bath gave dull, dark deposits and destroyed its decorative white finish. Phosphoric acid attacked the solder less than sulfuric acid did, so phosphate rhodium was preferred. After the introduction of bright nickel most of the industry changed to sulfate because it could operate at a slightly lower rhodium concentration. The phosphate-sulfate solution was used because some considered the color to be a bit whiter or brighter. The typical rhodium electroplate on costume or precious jewelry is 0.000002to 0.000005 in. and is produced in 20 sec to 1 min at about 6 V in the following baths.

Phosphate Rhodium Bath

Rhodium as phosphate concentrate, 2 g/L

Phosphoric acid [85% chemically pure (CP) grade], 40-80 ml/L

Anodes, platinum/platinum clad

Temperature, 40-50°C

Agitation, none to moderate

Current density, 2-10 A/&*

Sulfate Rhodium Bath

Rhodium as sulfate concentrate, 1.3-2 g/L

Sulfuric acid (95% CP grade), 25-80 ml/L

Anodes, platinum/platinum clad

Temperature, 4&50"C

Agitation, none to moderate

Current density, 2-10 A/dm²

Phosphate-Sulfate Rhodium Bath

Rhodium as phosphate concentrate, 2 g/L

Sulfuric acid (95% CP grade), 25-80 g/L

Anodes, platinum/platinum clad

Temperature, 40-50°C

Agitation, none to moderate

Current density, 2-10 A/dm2

Tanks for these baths should all be made of glass, Pyrex, plastic, or plastic-lined steel. If plastic is used it should be leeched once or twice with 5% sulfuric or phosphoric acid for 24 hr before the rhodium is added. In mixing a new solution distilled or deionized water should be used, and the acid should be added to the water carefully and mixed thoroughly before the rhodium concentrate is added. This will prevent precipitation of the rhodium. Rhodium is, of course, plated out and also lost through drag-out. Because of the expense of Rhodium the first rinse after plating should be a stagnant drag-out rinse, also contained in a glass or plastic tank. As water is lost from the plating solution it should be replaced with this drag-out rinse so that some of the “lost” rhodium is returned for reuse. Even with two drag-out tanks the actual amount of rhodium lost will be about 25 to 30% of the rhodium plated; therefore, rhodium should be replenished at the rate of 5 g/18 to 20 ampere-hours (A-hr) of flash plating. Because the drag-out is so high in jewelry plating sulfuric (or phosphoric) acid should also be replenished at the rate of 5 mu18 to 20 A-hr. This recommended replenishment is only an average value. If possible, it should be checked by analysis. Bright nickel is the preferred base for decorative rhodium electroplates. It provides a bright base for rhodium and also prevents the rhodium solution from attacking a brass, copper, steel, lead, or tin base. All of these metals adversely affect the color and tarnish and corrosion resistance as well as the covering power of the rhodium solution. Nickel, of all the metals, has the least adverse effect on a rhodium solution. Baths can tolerate as much as 1 to 2 g/L and still give a satisfactory deposit. There are no truly satisfactory methods to purify a contaminated rhodium plating solution.

DECORATIVE BARREL PLATING

The usual decorative barrel finish is also 0.000003 to 0.000005 in. A variation of the sulfate-rhodium bath is always used. It is necessary, however, to reduce the metal concentration and to raise the acid concentration to get economical and satisfactory deposits. With many parts in the barrel it is necessary to plate quite slowly so that the parts have time to mix and be evenly exposed to the plating solution. This ensures that they are all plated to a similar thickness before more than 0.000005 in. is deposited. It is not advisable to slow the rate of plating by decreasing the current density (and voltage) because this may lead to nonadhering deposits over a bright nickel base. Therefore, the plating rate is best slowed by decreasing the cathode current efficiency by raising the acid and lowering the rhodium. A typical formulation for decorative barrel plating would be the following:

Rhodium as sulfate concentrate, 1 g L

Sulfuric acid (95% CP grade), 80 g/L

Anodes, platinum/platinum clad

Temperature, 45-50°C

Current density, 0.5-2 /cm2

ELECTRONIC/INDUSTRIAL PLATED RHODIUM

The emerging electrical/electronics industry in the 1950s and 1960s made considerable use of rhodium electrodeposits for many diverse uses, but it was particularly used on sliding and rotating contacts, printed circuit switches and commutators, and high-frequency switches and components. There are many requirements for rhodium deposits of 0.000020 to 0.0002 in. over nickel or, occasionally, silver. These may be plated from the following solution:

Rhodium metal as sulfate concentrate, 5 g/L

Sulfuric acid (95% CP grade), 25-50 mlL

Anodes, platinudplatinum clad

Temperature, 45-50°C

Current density, 1-3 A/dm2

Current efficiency, 70-90% with agitation; 50-60% without agitation

See the previous section under Decorative Plating for instructions on leeching the plating tank before use. It is preferable to use water jacket heating of the solution to prevent local overheating by an immersion heater or steam coil. Even a short exposure to temperatures over 160°F will result in chemical changes to the solution that will result in a permanent increase in stress of the deposit. The stress will be present even if the bath is later operated within the correct temperature range.

Because of the expense of the solution it is advisable to plate with as low a rhodium concentration as possible to achieve the desired plating thickness and finish. If some of the plating is to be 0.0002 in. and over it will be necessary to raise the rhodium concentration to 7^1/2 or 10 g L . Replenishment is based on ampere-hours plated and the cathode current efficiency. It is best determined by analytical control; however, an approximation would be to replenish 5 g of rhodium for every 5 to 10 A-hr of plating. The actual value will depend on the average thickness plated the current density used. The cathode current efficiency is quite low, even with agitation, and hydrogen gas bubbles will tend to cling to the work and leave imperfections. This effect may be minimized by adding a 1% solution of sodium lauryl sulfate to the bath. The rate of addition should be 1 to 5 ml of a 1% solution per gallon of the plating bath.

INDUSTRIAL BARREL PLATING

Not only the expense of rhodium but the high drag-out of barrel plating recommends the use of a low metal concentration. Coatings in the millionth inch range can be produced with as little as 1 g rhodium/L . Thicker deposits must use proportionally higher concentrations. Deposits of 0.000020 in. may be achieved with 2^1/2 g/L; 0.000050 in. with as little as 3^1/2 g/ L ; 0.0001 in. with as little as 4 g/L; and deposits of 0.0002 in. and over with 5 g/L. If the holes in the barrel are very small, and the parts have a high surface area, it will be necessary to use higher concentrations to compensate for poor solution transfer.Otherwise, the formulations for barrel plating are the same:

Rhodium metal as sulfate concentrate, 2.5-5 g/L

Sulfuric acid (CP grade), 20 m/L

Anodes, platinum/platinum clad

Barrels, horizontal, submerged

Temperature, 45-50°C

Current density, 0.5-2 A/cm2

CARE OF RHODIUM SOLUTION

Contamination of the rhodium solution is the cause of most rhodium plating problems. The major contaminants are (1) organics, (2) rhodium basic salts, (3) rhodium complexes and (4) inorganics such as iron, lead tin, copper, gold silver, and nickel. The most common contaminants are organics such as dust, dirt, adhesives from masking tape, stop-off paints and printed circuit board material, and organics from improperly leached plastic tanks. They are usually easily removed by hatch-type carbon treatment. It is imperative that the carbon used be very low in acid-soluble residues. It is also important not to use a diatomaceous earth filter aid. If a single carbon treatment does not clean the solution a second treatment or a treatment with a carbon designed for the removal of very short chain organic molecules may be necessary. Carbon treatment will frequently eliminate stress brittleness and flaking of the deposit. It will also often cure finger staining or apparent tarnishing of the deposit. Basic rhodium salts will precipitate from a rhodium solution and act as a contaminant if the pH of the bath rises above 2. The acidity of the solution should be controlled and never be allowed to fall below 25 ml/L. If plating is normally done at higher current densities of over 25 A/ft2 the acidity should be kept even higher. Levels of sulfuric acid of at least 50 ml/L are generally satisfactory. Phosphoric acid is not recommended for industrial plating baths. Contamination and increased stress by unwanted rhodium complexes, as has been mentioned, can occur if the solution is overheated. Rhodium solutions should be indirectly heated and be thermostatically controlled.

Inorganic contaminants are usually introduced by the basis metal or base plates. The warm sulfuric acid electrolyte is extremely corrosive, and work should never be allowed to hang in the tank without current. Preferably, work should be connected to the negative power source before it is introduced into the rhodium tank. This may occasionally require a flying cathode bar or, in the case of barrel plating, a cathodic battery clamp and wire to be attached to the barrel before it is lowered into the tank. Of course, dropped parts should immediately be removed from the bottom of the tank. Copper, iron, tin, and lead, even after exhibiting a brief brightening effect in the parts per million range, will cause highly stressed heavy rhodium deposits. They will also cause dark and stained deposits and skip plating. Most metallic impurities, theoretically, can be precipitated from a rhodium solution by potassium ferrocyanide; however, in practice the procedure is very difficult, time-consuming, and not very successful, especially with solutions used for heavy rhodium deposits. The best practice is to prevent metallic contamination. The parameters that will tend to decrease the stress and brittleness of a rhodium deposit are the following:

1. Increased rhodium metal concentration

2. Increased sulfuric acid concentration

3. Increased temperature

4. Carbon treatment of the bath

5. Decreased inorganic contaminants.

Low-stress rhodium proprietary baths are available that contain trace amounts of selenium and indium. Although the stress and attendant stress cracking are almost totally eliminated, the baths operate like conventional sulfate baths.

ADVANCED SURFACE COATINGS A HANDBOOK