cleaning, pretreatment & surface preparation
PICKLING AND ACID DIPPING
BY STEPHEN F. RUDYHUBBARD-HALL, WATERBURY, CONN.
Acid treatment identifies a process whereby the base metal is subjected to mild,moderate, or aggressive etching. It’s reasonable to assume the solution pH isbelow 2.0. What happens chemically can be illustrated by the reaction betweenmetal, oxide, and acid:Metal Oxide+Acid=Metal Salt+WaterMetal+Acid=Metal Salt+Hydrogen (proton)Hydrogen (proton)+Hydrogen (proton)=Hydrogen (gas)The metal, as is, contains an oxide surface layer before immersion in the acidbath. This condition was probably accentuated by a previous reverse electrocleaningstep. The oxide layer hurts the prospective finish two ways: adhesion of
electrodeposits to the base metal will be poor, and the metal surface in an oxidecondition is a poor conductor. The oxide must be totally and cleanly removed.Depending on the degree of surface acid treatment, this can be done in a singleimmersion, double immersion, or cathodically in an electrified acid. Acid formulations,available in liquid or powder, are available in three common formsSingle constituent, mineral, inorganic type.
Combination of two or more acids. May consist of inorganic andorganic acids.Single or multiacid combinations, which also contain surfactants,dispersants, and inhibitors.
The acids dissolve oxides and smuts. They also remove scales and rusts.Sulfuric acid is perhaps the cheapest of the acids used and it’s much less fuming.Hydrochloric acid provides a much better rate of pickling action at lowertemperatures but does generate more fumes. Hydrochloric acid is also widelyused for stripping chrome- and zinc-plated deposits off parts and rack tips.Phosphoric acid must be heated to achieve acceptable pickling activity, andformation of light iron phosphate films on the surface may be detrimental ifelectroplating is part of the cycle. Surfactants and wetting agents lower thesolution surface tension, permitting the bath’s active agents to more readilypenetrate into and attack undesirable surface coatings and films. Other functionsof wetting agents and surfactants include emulsification of residual oilsand grease (not removed in previous alkaline cleaning or carryover) and formationof a thin surface foam blanket to help retard the corrosive effects of fumesand mists. Dispersants prevent redeposition of removed soils. Inhibitors provide
two-fold effects: controlled pickling, which prevents excessive surface action, andprevention of immersion deposits in aged, contaminated baths.A review of the acid dips available and their respective activities should helpto clarify the solution selection process.This issue of the Metal Finishing Guidebook contains additional discussions,references, and suggestions for activation and pickling, as well as more detailed
information regarding rinsing, analysis, testing, and related subjects. The chapter“Surface Preparation of Various Metal Alloys Before Plating and OtherFinishing Applications” describes specific acid treatments that supplement theinformation given in this chapter.
ACID DIP
This is the simplest type because the requirement is not complicated. The partsare conditioned as follows: alkaline cleaner film is neutralized and light oxideand/or rust is attacked and removed. Many parts fall into this category. Thisbath contains an inorganic acid, either hydrochloric or sulfuric. It may alsoinclude a surfactant to improve wetting of parts by lowering surface tension,accelerate the activation process, and emulsify trace oils and grease. Acid dipsgenerally meet the operating criteria shown in Tables I and II. In the case ofprocessing steel, brass, and copper together, the solution may contain a specialinhibitor, preventing immersion copper deposits on steel.For brass activation the acid typically contains a blend of sulfuric and hydrofluoricacids. Most brass parts have been formed, machined, or stamped; therefore,up to 4% lead can be incorporated into the metal to facilitate mechanicalwork. Fluoride is the best additive to dissolve lead smuts, leaving a clean surfacefor plating. (See Table III.)Copper and copper alloys (beryllium, tellurium, etc.) activate well in formulascontaining sulfuric acid and persulfates. This solution provides the preferredlight etch to activate and desmut.Most zinc alloys respond well to activation in sulfamic or sulfuric acid blendscontaining fluorides (preferably ammonium bifluoride). Fluoride effectivelydissolves smuts, while the inorganic/organic acid mixture activates the surface.
ACID PICKLE
This process bath is most readily used to condition steel and stainless steel parts.It can be used as an immersion dip or cathodically. (See Tables IV to VI.) Somemetallic smuts and scales do not respond well to the acid dip. A more aggressivetreatment is supplied by the acid pickle. Some heat-treat scales, weld scales, rusts,and oxides (formed by buffing or polishing operations) are readily attacked andremoved in an appropriate acid pickle bath. The solution will also perform thefunctions of the acid dip. A typical acid pickle contains inorganic acids, such asTable I. Operating Parameters and Applications for Acid DipsAcid Type Concentration Range Temperature Time AgitationLiquid 5-8% v/v 75-110OF 1-3 minutes Solution movementPowder 4-24 oz/gal (30-180 g/L) 75-110OF (24-43OC) 1-3 min Solution movementTable II. Acid Dip Operating Parameters for Steel & Stainless SteelAcid % v/v Wetters, oz/gal Deflocculents, oz/gal Inhibitors, oz/galHydrochloric 5-15 0.01-0.03 0.06-0.09 0.002-0.004Sulfuric 7-20 0.01-0.03 0.06-0.09 0.002-0.004Phosphoric 10-20 0.01-0.03 0.06-0.09 0.002-0.004Table III. Acid Dip Operating Parameters for Nonferrous MetalsSulfuric Acid Fluoride, Persulfate, Wetters,Metal Concentration, % v/v oz/gal oz/gal oz/galBrass 3-5 0.2-0.4a — 0.01-0.03Copper and Alloys 2-4 — 1-3 0.01-0.03Zinc Alloys and Castings 1-2 0.1-0.3 — 0.01-0.03a Fluoride source is typically fluoroboric acid or ammonium bifluoride. Sulfamic acid may also beadded at 1-2 oz/gal.
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hydrochloric, sulfuric, hydrofluoric, and phosphoric; and deflocculents, wettingagents, and inhibitors.In some immersion or electrolytic applications scales are seen to literally peeloff in sheets and rusted surfaces “whiten.” The acid pickle, when used in doublecleaningcycles (immersion or electrolytic), is usually the first acid step, followedby the second electrocleaner and a final acid dip.
DESCALING ACID
This process bath is a step above the acid pickle in regard to chemical strengthand application. The general focus is removal of heavy deposits: rusts, scales (hotforging or rolling, casting), and other heat treatments. The bath constituentsare similar to the acid pickle, but contain higher levels of accelerators such asfluorides and chlorides. Operating parameters: concentration, temperature, andtime, may be increased when compared to the acid pickle. Descaling acids can beused by immersion or electrolytically. These baths are sometimes used to pretreatparts off line before processing in a standard finishing line.
The pickling requirement is a critical factor of time, temperature, and concentrationsof the components. Times may range from 5 to 30 minutes. Thismakes the incorporation of an inhibitor very important. Surface pickling can beaccomplished with minimal attack on the base metal, while also greatly minimizinghydrogen embrittlement.Stainless steels may be passivated as follows:40% v/v of 42OBé nitric acid at 75-145OF (24-63OC).Table IV. Operating Parameters and Applications for Acid Pickling Steel and StainlessSteel by ImmersionAcid % v/v Temperature, OF Fluorides, oz/gal Wetters, oz/gal Inhibitors,oz/galHydrochloric 20-50 80-120 3-5 0.01-0.03 0.002-0.004Sulfuric 20-40 90-150 1.5-3.0 0.01-0.03 0.002-0.004Phosphoric 25-45 90-150 0.5-2.0 0.01-0.03 0.002-0.004Table V. Operating Parameters and Applications for Acid Pickling Steel and StainlessSteel CathodicallySulfuric Acid, Temperature, Fluorides, Chlorides, Wetters, Inhibitors,% v/v OF oz/gal oz/gal oz/gal oz/gal5-10 90-120 1.5-3.0 3.0-4.0 0.01-0.03 0.002-0.0044-6 V. 30-50 A/ft2. Use chemically pure lead anodes or carbon.For any cathodic acid application using carbon anodes in the presence of fluorides, the best resistantgrade of carbon to fluoride attack should be used.Table VI. Operating Parameters and Applications for Descaling Steel and StainlessSteel by ImmersionAcid AcidConcentration, Temperature, Fluorides, Chlorides, Wetters, Inhibitors,% v/v OF oz/gal oz/gal oz/gal oz/gal
1. Sulfuric 7-12 90-160 4.0-6.0 6.0-8.0 0.01-0.03 0.002-0.004
or
2. Hydrochloric 7-12 90-160 4.0-6.0 — 0.01-0.03 0.002-0.004
80
25% v/v of 42OBé nitric acid, 6 oz/gal of molybdic acid at 70-150OF (21-66OC).Aqueous solutions of citric acid at 120 to 140OF (49-60OC) may also be evaluated.Note: Comprehensive information describing electropolishing (the anodicbrightening and descaling of metals) and use of the Wood’s nickel strike are in thechapter titled “Surface Preparation of Various Metal Alloys Before Plating andOther Finishing Applications.”
ACID TREATMENT OF OTHER METALS AND ALLOYS
AluminumBright Dipping
1. Nonetch aluminum soak cleaner.
2. Chemically polish in a special acid solution consisting of: 94% v/v of 85%phosphoric acid and 6% v/v of 42OBé nitric acid at 180-185OF (82-85O), untildesired surface polishing is achieved.
Descaling
1. Nonetch aluminum soak cleaner.
2. 3-5% sulfuric acid or phosphoric acid dip. Neutralizes and conditions thesurface.
3. Chemically polish in 15-20% v/v sulfuric acid and 30-40% by wt. of ferricsulfate at 100-140OF (38-60OC). Solution is wetted and specially inhibited.
4. Tarnish inhibit in dip application using a soap (mechanical tarnish inhibitfilm) or apply on organic topcoat such as a lacquer.Conditioning for Plating
Note: Comprehensive information describing aluminum alloys, castings, andsuggested deoxidizer/desmutting baths is described in the chapter titled,“Surface Preparation of Various Metal Alloys Before Plating and Other FinishingApplications.”Brass and Copper AlloysDescaling—Bright Finishwww.metalfinishing.com/advertisersFilter Presses, Clarifiers, Sludge Dryers,Waste Treatment Systems,Evaporators, Bags, Cloth and Media
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1. Moderately alkaline soak cleaner.
2. 5% v/v sulfuric acid dip. Neutralizes and conditions the surface.
3. Chemically polish in solution consisting of: hydrogen peroxide at 20-25%v/v and 0.3-0.5% v/v sulfuric acid at 90-11OF (32-43OC). Solution is wettedand specially inhibited. A golden-brown surface film must develop withminimal surface gassing to obtain desired surface polishing. 2-5 minuteimmersion should cover most cycle applications.
4. 5% v/v sulfuric acid dip for 10-15 seconds to dissolve the protective brownfilm.
5. Tarnish inhibit in dip application using either a soap (mechanical tarnishinhibit film) or a benzotriazole (active surface antioxidant).
6. Optionally lacquer (dip or electrolytic) or apply electrolytic chromate.or
1. Moderately alkaline soak cleaner.
2. 5% v/v sulfuric acid dip. Neutralizes and conditions the surface.
3. Chemically polish in 15-20% v/v sulfuric acid and 30-40% by wt of ferricsulfate at 100-140OF (38-60OC). Solution is 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.If the scale is very heavy, the following mixed acid may be required.
1. Moderately alkaline soak cleaner.
2. Descale in a solution consisting of: 44% v/v of 66OBé sulfuric acid, 20%v/v of 42OBé nitric acid, balance water, containing 0.25 oz/gal of sodiumchloride, 90-110OF (32-43OC). Monitor solution action to avoid etching, ifa bright, polished finish is required.Descaling—Matte Finish
1. Moderately alkaline soak cleaner.
2. Immersion treatment in solution consisting of 2-3% v/v sulfuric acid and
2-4 oz/gal of sodium or ammonium persulfate, at 80-95OF (27-35OC).or
1. Moderately alkaline soak cleaner.
2. Immersion treatment in solution consisting of: 75% v/v of 85% phosphoricacid and 25% v/v of 42OBé nitric acid, 90-120OF (32-49OC).
Cast IronStandard alkaline soak clean, followed by alternate hot and cold rinsing to pushsolutions out of pores. Anodically electroclean in alkaline descaler. Parts exitingthe electrocleaner should have a uniform light-yellow cast. Dip in 15 to 20% v/vhydrochloric acid or 5 to 10% v/v sulfuric acid to dissolve oxides, desmut, andform an active surface for plating. Control acid dip to avoid overtreatment. Castiron has a low hydrogen overvoltage, which can reduce electroplating efficiency.Extended immersion in the acid will further reduce this efficiency. Mass finishingmay be a better option to remove rust and scale.
Gold and Gold AlloysBrightening
1. Alkaline soak clean.
2. Anodic treatment at 6-12 V in a solution consisting of 20 oz/gal sodiumcitrate and 5 oz/gal Rochelle salts, 80-90OF (27-32OC).
Gold may be recovered by dummying at low current density onto stainlesssteel cathodes, also using stainless steel anodes.High Carbon and Case-Hardened SteelsHigh carbon steel can be heavily scaled. Both metals are susceptible to hydrogenembrittlement. In addition pickling high carbon steels may generate heavysurface smuts. Solutions containing sulfuric acid, fluorides, organic acid complexors,inhibitors, pickle aids, and wetting agents are preferred to process thesemetals. The acid bath surface tension is reduced to promote superior wettingof the substrate. Once the scale or smut is removed an inhibitor film forms to
prevent additional pickling. This prevents hydrogen embrittlement of criticalparts, such as spring steel, and repeated smutting of high carbon steels. In someprocess cycles a follow-up electrocleaner is required to remove the inhibitorfilm following the acid treatment. The final preplate dip should be a dilute,
3-5% v/v sulfuric acid solution to neutralize alkaline films and provide a clean,water-break-free surface. Sometimes the parts can be cleaned in an off-lineeletropolishing or mass finishing step before transfer to the actual process line.
1. Soak clean or preclean as described previously.
2. Anodically clean in an alkaline descaler.
3. Acid dip as described (see appropriate tables for suggested chemical constituentsand operating conditions). In a double cleaning cycle the first acidmay be an anodic electropolishing treatment.Low Carbon Steel
1. Alkaline soak clean.
2. Anodically electroclean (moderate caustic, silicated, complexed, wettedblend). Use at standard operating parameters for rack and barrel.
3. Acid dip in either hydrochloric acid at 5-25% v/v or sulfuric acid at 3-10%v/v. Depending on surface condition, wetters, inhibitors, or deflocculentsmay be required. Use at standard operating parameters.
Magnesium AlloysRemoval of corrosion, heavy oxides, and mill scale:
1. Alkaline soak clean.
2. Acid immersion treatment in either of the following solutions: A.Hydrofluoric acid at 20-25% v/v, 75-90OF (24-32OC) or B. 20 oz/gal of 85%phosphoric acid, 2-3% v/v of 42OBé nitric acid, and 0.2-0.3 oz/gal ammoniumbifluoride, 75-90OF (24-32OC).orChromic acid containing solution consisting of: 44-48 oz/gal chromic acid,
3-4 oz/gal sodium nitrate, 1.5 oz/gal sodium chloride, 0.1 oz/gal sodium sulfate,65-90OF (18-32OC).Nickel and Cobalt Alloys
1. Alkaline soak clean.
2. Cathodically condition in a highly conductive electrocleaner blended forsteel.
3. Acid dip in a solution consisting of 50% v/v of 70% hydrofluoric acid and7% v/v of 42OBé nitric acid at 75-90OF (24-32OC).Silver and Silver Alloys
1. Alkaline soak clean.
2. Acid dip in either of the following solutions: A. 66% v/v of 42OBé nitricacid at 75-160OF (24-71OC). B. 88% v/v of 66OBé sulfuric acid, 75-90OF (24-32OC). C. 22% v/v of 42OBé nitric acid at 75-90OF (24-32OC).
Titanium
1. Alkaline soak clean.
2. Acid treatment in solution consisting of 40% v/v of 42OBé nitric acid and
2-3% v/v of 75% hydrofluoric acid, at 120-135OF (52-57OC).Zinc and Zinc AlloysComprehensive information describing zinc alloys, castings, and suggested acidbaths is described in the chapter titled “SurfacePreparation of Various MetalAlloys Before Plating and Other Finishing Applications.”Activation of Bright Nickel ElectroplateThere are two effective acid solutions for activating passive nickel deposits andto salvage reject parts. Their application is recommended for sulfur bearingelectrolytic nickel deposits.The immersion dip consists of 5 to 10% v/v sulfuric acid and 2 to 4 oz/galof potassium iodide at 75 to 90OF (24-32OC), 1 to 5 minutes or as required. Acathodic activation of nickel consists of 5 to 11% v/v sulfuric acid, 1 to 3 oz/galfluoride, 2 to 4 oz/gal chloride at 5 to 7 V for 1 to 2 minutes (as required) at 20
to 40 A/ft2 and 75 to 120OF (24-49OC).
