cleaning, pretreatment & surface preparation
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/ft2for 5 minutes or at 25 A/ft2for 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/ft2in the Woods’ strike, or at 40 to 60 A/ft2in 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
