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Pickling and acid dipping
chemical surface preparation PICKLING AND ACID DIPPING BY STEPHEN F. RUDY ENEQUIST CHEMICAL CO., BROOKLYN, N.Y.
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 is below 2.0. What happens chemically can be illustrated by the reaction between metal, oxide, and acid: Metal Oxide+Acid=Metal Salt+Water Metal+Acid=Metal Salt+Hydrogen (proton) Hydrogen (proton)+Hydrogen (proton)=Hydrogen (gas) The metal, as is, contains an oxide surface layer before immersion in the acid bath. This condition was probably accentuated by a previous reverse electrocleaning step. 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 oxide condition 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 single immersion, double immersion, or cathodically in an electrified acid. Acid formulations, available in liquid or powder, are available in three common forms: • Single constituent, mineral, inorganic type. * Combination of two or more acids. May consist of inorganic and organic 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 lower temperatures but does generate more fumes. Hydrochloric acid is also widely used for stripping chrome- and zinc-plated deposits off parts and rack tips. Phosphoric acid must be heated to achieve acceptable pickling activity, and formation of light iron phosphate films on the surface may be detrimental ifelectroplating is part of the cycle. Surfactants and wetting agents lower the solution surface tension, permitting the bath's active agents to more readily penetrate into and attack undesirable surface coatings and films. Other functions of wetting agents and surfactants include emulsification of residual oils and grease (not removed in previous alkaline cleaning or carryover) and formation of a thin surface foam blanket to help retard the corrosive effects of fumes and mists. Dispersants prevent redeposition of removed soils. Inhibitors provide two-fold effects: controlled pickling, which prevents excessive surface action, and prevention of immersion deposits in aged, contaminated baths. A review of the acid dips available and their respective activities should help to 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 Other Finishing 139
Table I. Operating Parameters and Applications for Acid Dips Acid Type Concentration Range
Temperature
Time
Agitation
Liquid
5-8% v/v
75-110°F
1-3 minutes
Solution movement
Powder
4-24 oz/gal (30-180 g/L)
75-II0°F (24-43°C)
1-3 min
Solution movement
Applications" describes specific acid treatments that supplement the information given in this chapter.
ACID DIP This is the simplest type because the requirement is not complicated. The parts are conditioned as follows: alkaline cleaner film is neutralized and light oxide and/or rust is attacked and removed. Many parts fall into this category. This bath contains an inorganic acid, either hydrochloric or sulfuric. It may also include a surfactant to improve wetting of parts by lowering surface tension, accelerate the activation process, and emulsify trace oils and grease. Acid dips generally meet the operating criteria shown in Tables I and II. In the case of processing steel, brass, and copper together, the solution may contain a special inhibitor, preventing immersion copper deposits on steel. For brass activation the acid typically contains a blend of sulfuric and hydrofluoric acids. Most brass parts have been formed, machined, or stamped; therefore, up to 4% lead can be incorporated into the metal to facilitate mechanical work. Fluoride is the best additive to dissolve lead smuts, leaving a clean surface for plating. (See Table I l L ) Copper and copper alloys (beryllium, tellurium, etc.) activate well in formulas containing sulfuric acid and persulfates. This solution provides the preferred light etch to activate and desmut. Most zinc alloys respond well to activation in sulfamic or sulfuric acid blends containing fluorides (preferably ammonium bifluoride). Fluoride effectively dissolves 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.) Some metalTable II. Acid Dip Operating Parameters for Steel & Stainless Stee| Acid
96 v/v
Wetters, oz/gal
Deflocculents, oz/gal
Inhibitors, oz/gal
Hydrochloric
5-15
0.01-0.03
0.06-0.09
0.002-0.004
Sulfuric
7-20
0.01-0.03
0.06-0.09
0.002-0.004
Phosphoric
10-20
0.01-0.03
0.06-0.09
0.002-0.004
Table III. Acid Dip Operating Parameters for Nonferrous MetaLs Metal
Sulfuric Acid Concentration, 96v/v
Fluoride, oz/gal
Persulfate, oz/gal
Wetters, oz/gal
Brass
3-5
Copper and Alloys
2-4
0.2-0.4 a
-
0.01-0.03
-
1-3
Zinc Alloys and Castings
1-2
0.01-0.03
0.1-0.3
-
0.01-0.03
Fluoride source is typically fluoroboric acid or ammonium bifluoride. Sulfamic acid may also be added at 1-2 oz/gal. 140
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Table IV. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel by Immersion Acid
% v/v
Temperature,°F
Fluorides,oz/gal
Wetters,oz,/gal
Inbibitors, oz/gal
Hydrochloric
20-50
80-120
3-5
0.01-0.03
0.002-0.004
Sulfuric
20-40
90-150
1.5-3.0
0.01-0.03
0.002-0.004
Phosphoric
25-45
90-150
0.5-2.0
0.01-0.03
0.002-0.004
lic smuts and scales do not respond well to the acid dip. A more aggressive treatment is supplied by the acid pickle. Some heat-treat scales, weld scales, rusts, and oxides (formed by buffing or polishing operations) are readily attacked and removed in an appropriate acid pickle bath. The solution will also perform the functions of the acid dip. A typical acid pickle contains inorganic acids, such as hydrochloric, sulfuric, hydrofluoric, and phosphoric; and deflocculents, wetting agents, and inhibitors. In some immersion or electrolytic applications scales are seen to literally peel off in sheets and rusted surfaces "whiten." The acid pickle, when used in doublecleaning cycles (immersion or electrolytic), is usually the first acid step, followed by the second electrocleaner and a final acid dip.
DESCALING ACID This process bath is a step above the acid pickle in regard to chemical strength and application. The general focus is removal of heavy deposits: rusts, scales (hot forging or rolling, casting), and other heat treatments. The bath constituents are similar to the acid pickle, but contain higher levels of accelerators such as fluorides and chlorides. Operating parameters: concentration, temperature, and time, may be increased when compared to the acid pickle. Descaling acids can be used by immersion or electrolytically. These baths are sometimes used to pretreat parts offline before processing in a standard finishing line. The pickling requirement is a critical factor of time, temperature, and concentrations of the components. Times may range from 5 to 30 minutes. This makes the incorporation of an inhibitor very important. Surface pickling can be accomplished with minimal attack on the base metal, while also greatly mini-
Table V. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel Cathodicaliy Sulfuric Acid, % v/v 5-10
Temperature, °F
Fluorides, oz/gal
Chlorides, oz/gal
90-120
1.5-3.0
3.0-4.0
Wetters, oz/gal 0.01-0.03
Inbibitors, oz/gal 0.002-0.004
4-6 V. 30-50 A/ft 2. Use chemically pure lead anodes or carbon. For any cathodic acid application using carbon anodes in the presence of fluorides, the best resistant grade of carbon to fluoride attack should be used.
Table VI. Operating Parameters and Applications for Descaling Steel and Stainless Steel by Immersion Acid
1. Sulfuric or 2. Hydrochloric
142
Acid Concentration, % v/v
Temperature, OF
Fluorides, oz./gal
7-12
90-160
4.0-6.0
7-12
90-160
4.0-6.0
Chlorides, oz/gal 6.0-8.0 -
Wetters, oz/gal
Inbibitors, oz/gal
0.01-0.03
0.002-0.004
0.01-0.03
0.002-0.004
mizing hydrogen embrittlement. Stainless steels may be passivated as follows: • 40% v/v of 42°B~ nitric acid at 75-145°F (24-63°C). • 25% v/v of 42°B~ nitric acid, 6 oz/gal of molybdic acid at 70-150°F (21-66°C). Aqueous solutions of citric acid at 120 to 140°F (49-60°C) may also be evaluated. Note: Comprehensive information describing electropolishing (the anodic brightening and descaling of metals) and use of the Wood's nickel strike are in the chapter titled "Surface Preparation of Various Metal Alloys Before Plating and Other Finishing Applications." ACID TREATMENT OF OTHER METALS AND ALLOYS
Aluminum
BrightDipping 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 42°B~ nitric acid at 180-185°F (82-85°), until desired surface polishing is achieved.
Descaling 1. Nonetch aluminum soak cleaner. 2.3-5% sulfuric acid or phosphoric acid dip. Neutralizes and conditions the surface. 3. Chemically polish in 15-20% v/v sulfuric acid and 30-40% by wt. offerric sulfate at 100-140°F (38-60°C). Solution is wetted and specially inhibited. 4. Tarnish inhibit in dip application using a soap (mechanical tarnish inhibit film) or apply on organic topcoat such as a lacquer.
Conditioningfor Plating Note: Comprehensive information describing aluminum alloys, castings, and suggested deoxidizer/desmutting baths is described in the chapter titled, "Surface Preparation of Various Metal Alloys Before Plating and Other Finishing Applications."
Brass and CopperAlloys
Descaling--BrightFinish 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 2025% v/v and 0.3-0.5% v/v sulfuric acid at 90-1 I°F (32-43°C). Solution is wetted and specially inhibited. A golden-brown surface film must develop with minimal surface gassing to obtain desired surface polishing. 25 minute immersion should cover most cycle applications. 4. 5% v/v sulfuric acid dip for 10-15 seconds to dissolve the protective 143
brown film. 5. Tarnish inhibit in dip application using either a soap (mechanical tarnish inhibit 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 offerric sulfate at 100-140°F (38-60°C). Solution is wetted and specially inhibited. 4. Tarnish inhibit in dip application using either a soap (mechanical tarnish inhibit film) or a henzotriazole (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 66°B~ sulfuric acid, 20% v/v of 42°B~ nitric acid, balance water, containing 0.25 oz/gal of sodium chloride, 90-110°F (32-43°C). Monitor solution action to avoid etching, ira bright, polished finish is required.
Descaling--MatteFinish 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 80-95°F (27-35°C).or 1. Moderately alkaline soak cleaner. 2. Immersion treatment in solution consisting of: 75% v/v of 85% phosphoric acid and 25% v/v of 42°B~ nitric acid, 90-120°F (32-49°C). Cast Iron
Standard alkaline soak clean, followed by alternate hot and cold rinsing to push solutions out of pores. Anodically electroclean in alkaline clescaler. 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. Control acid dip to avoid overtreatment. Cast iron has a low hydrogen overvoltage, which can reduce electroplating efficiency. Extended immersion in the acid will further reduce this efficiency. Mass finishing may be a better option to remove rust and scale.
Gold and Cold Alloys
Brightening 1. Alkaline soak clean. 2. Anodic treatment at 6-12 V in a solution consisting of 20 oz/gal sodium citrate and 5 oz/gal Rochelle salts, 80-90°F (27-32°C). Gold may be recovered by dummying at low current density onto stainless steel cathodes, also using stainless steel anodes. lo.4
High Carbon and CaseHardened Steels High carbon steel can be heavily scaled. Both metals are susceptible to hydrogen embrittlement. In addition pickling high carbon steels may generate heavy surface smuts. Solutions containing sulfuric acid, fluorides, organic acid complexors, inhibitors, pickle aids, and wetting agents are preferred to process these metals. The acid bath surface tension is reduced to promote superior wetting of the substrate. Once the scale or smut is removed an inhibitor film forms to prevent additional pickling. This prevents hydrogen embrittlement of critical parts, such as spring steel, and repeated smutting of high carbon steels. In some process cycles a follow-up electrocleaner is required to remove the inhibitor film 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, waterbreak-free surface. Sometimes the parts can be cleaned in an off-line eletropolishing 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 constituents and operating conditions). In a double cleaning cycle the first acid may be an anodic electropolishing treatment.
Low Carbon Steel 1. Alkaline soak clean. 2. Anodically electroclean (moderate caustic, silicated, complexed, wetted blend). 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 deflocculents may be required. Use at standard operating parameters.
Ma~aesium Alloys Removal 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, 75-90°F (24-32°C) or B. 20 oz/gal of 85% phosphoric acid, 2-3% v/v of 42°B6 nitric acid, and 0.2-0.3 oz/gal ammonium bifluoride, 75-90°F (24-32°C). or
Chromic acid containing solution consisting of.' 44-48 oz/gal chromic acid, 34 oz/gal sodium nitrate, 1.5 oz/gal sodium chloride, 0.1 oz/gal sodium sulfate, 65-90°F (18-32°C).
Nic~l and CobaltAlloys 1. Alkaline soak clean. 2. Cathodically condition in a highly conductive electrocleaner blended for steel. 3. Acid dip in a solution consisting of 50% v/v of 70% hydrofluoric acid and 145
7% v/v of 42°B~ nitric acid at 7$-90°F (24-32°C).
Si/ver and S/lver Agoys 1. Alkaline soak clean. 2. Acid dip in either of the following solutions: A. 66% v/v of 42°B~ nitric acid at 75-160°F (24-71°C). B. 88% v/v of 66°B~ sulfuric acid, 75-90°F (24-32°C). C. 22% v/v of 42°B~ nitric acid at 75-90°F (24-32°C).
Titanium 1. Alkaline soak clean. 2. Acid treatment in solution consisting of 40% v/v of 42°B~ nitric acid and 2-3% v/v of 75% hydrofluoric acid, at 120-135°F (52-57°C).
Zinc and Zinc Alloys Comprehensive information describing zinc alloys, castings, and suggested acid baths is described in the chapter titled "Surface Preparation of Various Metal Alloys Before Plating and Other Finishing Applications."
Activation ofBrightNick.elElectroplate There are two effective acid solutions for activating passive nickel deposits and to salvage reject parts. Their application is recommended for sulfur bearing electrolytic nickel deposits. The immersion dip consists of 5 to 10% v/v sulfuric acid and 2 to 4 oz/gal of potassium iodide at 75 to 90°F (24-32°C), 1 to 5 minutes or as required. A cathodic activation of nickel consists of 5 to 11% v/v sulfuric acid, 1 to 3 oz/gal fluoride, 2 to 4 oz/gal chloride at 5 to 7 V for 1 to 2 minutes (as required) at 20 to 40 A/ft 2 and 75 to 120°F (24-49°C).
146
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 is below 2.0. What happens chemically can be illustrated by the reaction between metal, oxide, and acid: Metal Oxide+Acid=Metal Salt+Water Metal+Acid=Metal Salt+Hydrogen (proton) Hydrogen (proton)+Hydrogen (proton)=Hydrogen (gas) The metal, as is, contains an oxide surface layer before immersion in the acid bath. This condition was probably accentuated by a previous reverse electrocleaning step. 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 oxide condition 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 single immersion, double immersion, or cathodically in an electrified acid. Acid formulations, available in liquid or powder, are available in three common forms: • Single constituent, mineral, inorganic type. * Combination of two or more acids. May consist of inorganic and organic 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 lower temperatures but does generate more fumes. Hydrochloric acid is also widely used for stripping chrome- and zinc-plated deposits off parts and rack tips. Phosphoric acid must be heated to achieve acceptable pickling activity, and formation of light iron phosphate films on the surface may be detrimental ifelectroplating is part of the cycle. Surfactants and wetting agents lower the solution surface tension, permitting the bath's active agents to more readily penetrate into and attack undesirable surface coatings and films. Other functions of wetting agents and surfactants include emulsification of residual oils and grease (not removed in previous alkaline cleaning or carryover) and formation of a thin surface foam blanket to help retard the corrosive effects of fumes and mists. Dispersants prevent redeposition of removed soils. Inhibitors provide two-fold effects: controlled pickling, which prevents excessive surface action, and prevention of immersion deposits in aged, contaminated baths. A review of the acid dips available and their respective activities should help to 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 Other Finishing 139
Table I. Operating Parameters and Applications for Acid Dips Acid Type Concentration Range
Temperature
Time
Agitation
Liquid
5-8% v/v
75-110°F
1-3 minutes
Solution movement
Powder
4-24 oz/gal (30-180 g/L)
75-II0°F (24-43°C)
1-3 min
Solution movement
Applications" describes specific acid treatments that supplement the information given in this chapter.
ACID DIP This is the simplest type because the requirement is not complicated. The parts are conditioned as follows: alkaline cleaner film is neutralized and light oxide and/or rust is attacked and removed. Many parts fall into this category. This bath contains an inorganic acid, either hydrochloric or sulfuric. It may also include a surfactant to improve wetting of parts by lowering surface tension, accelerate the activation process, and emulsify trace oils and grease. Acid dips generally meet the operating criteria shown in Tables I and II. In the case of processing steel, brass, and copper together, the solution may contain a special inhibitor, preventing immersion copper deposits on steel. For brass activation the acid typically contains a blend of sulfuric and hydrofluoric acids. Most brass parts have been formed, machined, or stamped; therefore, up to 4% lead can be incorporated into the metal to facilitate mechanical work. Fluoride is the best additive to dissolve lead smuts, leaving a clean surface for plating. (See Table I l L ) Copper and copper alloys (beryllium, tellurium, etc.) activate well in formulas containing sulfuric acid and persulfates. This solution provides the preferred light etch to activate and desmut. Most zinc alloys respond well to activation in sulfamic or sulfuric acid blends containing fluorides (preferably ammonium bifluoride). Fluoride effectively dissolves 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.) Some metalTable II. Acid Dip Operating Parameters for Steel & Stainless Stee| Acid
96 v/v
Wetters, oz/gal
Deflocculents, oz/gal
Inhibitors, oz/gal
Hydrochloric
5-15
0.01-0.03
0.06-0.09
0.002-0.004
Sulfuric
7-20
0.01-0.03
0.06-0.09
0.002-0.004
Phosphoric
10-20
0.01-0.03
0.06-0.09
0.002-0.004
Table III. Acid Dip Operating Parameters for Nonferrous MetaLs Metal
Sulfuric Acid Concentration, 96v/v
Fluoride, oz/gal
Persulfate, oz/gal
Wetters, oz/gal
Brass
3-5
Copper and Alloys
2-4
0.2-0.4 a
-
0.01-0.03
-
1-3
Zinc Alloys and Castings
1-2
0.01-0.03
0.1-0.3
-
0.01-0.03
Fluoride source is typically fluoroboric acid or ammonium bifluoride. Sulfamic acid may also be added at 1-2 oz/gal. 140
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Table IV. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel by Immersion Acid
% v/v
Temperature,°F
Fluorides,oz/gal
Wetters,oz,/gal
Inbibitors, oz/gal
Hydrochloric
20-50
80-120
3-5
0.01-0.03
0.002-0.004
Sulfuric
20-40
90-150
1.5-3.0
0.01-0.03
0.002-0.004
Phosphoric
25-45
90-150
0.5-2.0
0.01-0.03
0.002-0.004
lic smuts and scales do not respond well to the acid dip. A more aggressive treatment is supplied by the acid pickle. Some heat-treat scales, weld scales, rusts, and oxides (formed by buffing or polishing operations) are readily attacked and removed in an appropriate acid pickle bath. The solution will also perform the functions of the acid dip. A typical acid pickle contains inorganic acids, such as hydrochloric, sulfuric, hydrofluoric, and phosphoric; and deflocculents, wetting agents, and inhibitors. In some immersion or electrolytic applications scales are seen to literally peel off in sheets and rusted surfaces "whiten." The acid pickle, when used in doublecleaning cycles (immersion or electrolytic), is usually the first acid step, followed by the second electrocleaner and a final acid dip.
DESCALING ACID This process bath is a step above the acid pickle in regard to chemical strength and application. The general focus is removal of heavy deposits: rusts, scales (hot forging or rolling, casting), and other heat treatments. The bath constituents are similar to the acid pickle, but contain higher levels of accelerators such as fluorides and chlorides. Operating parameters: concentration, temperature, and time, may be increased when compared to the acid pickle. Descaling acids can be used by immersion or electrolytically. These baths are sometimes used to pretreat parts offline before processing in a standard finishing line. The pickling requirement is a critical factor of time, temperature, and concentrations of the components. Times may range from 5 to 30 minutes. This makes the incorporation of an inhibitor very important. Surface pickling can be accomplished with minimal attack on the base metal, while also greatly mini-
Table V. Operating Parameters and Applications for Acid Pickling Steel and Stainless Steel Cathodicaliy Sulfuric Acid, % v/v 5-10
Temperature, °F
Fluorides, oz/gal
Chlorides, oz/gal
90-120
1.5-3.0
3.0-4.0
Wetters, oz/gal 0.01-0.03
Inbibitors, oz/gal 0.002-0.004
4-6 V. 30-50 A/ft 2. Use chemically pure lead anodes or carbon. For any cathodic acid application using carbon anodes in the presence of fluorides, the best resistant grade of carbon to fluoride attack should be used.
Table VI. Operating Parameters and Applications for Descaling Steel and Stainless Steel by Immersion Acid
1. Sulfuric or 2. Hydrochloric
142
Acid Concentration, % v/v
Temperature, OF
Fluorides, oz./gal
7-12
90-160
4.0-6.0
7-12
90-160
4.0-6.0
Chlorides, oz/gal 6.0-8.0 -
Wetters, oz/gal
Inbibitors, oz/gal
0.01-0.03
0.002-0.004
0.01-0.03
0.002-0.004
mizing hydrogen embrittlement. Stainless steels may be passivated as follows: • 40% v/v of 42°B~ nitric acid at 75-145°F (24-63°C). • 25% v/v of 42°B~ nitric acid, 6 oz/gal of molybdic acid at 70-150°F (21-66°C). Aqueous solutions of citric acid at 120 to 140°F (49-60°C) may also be evaluated. Note: Comprehensive information describing electropolishing (the anodic brightening and descaling of metals) and use of the Wood's nickel strike are in the chapter titled "Surface Preparation of Various Metal Alloys Before Plating and Other Finishing Applications." ACID TREATMENT OF OTHER METALS AND ALLOYS
Aluminum
BrightDipping 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 42°B~ nitric acid at 180-185°F (82-85°), until desired surface polishing is achieved.
Descaling 1. Nonetch aluminum soak cleaner. 2.3-5% sulfuric acid or phosphoric acid dip. Neutralizes and conditions the surface. 3. Chemically polish in 15-20% v/v sulfuric acid and 30-40% by wt. offerric sulfate at 100-140°F (38-60°C). Solution is wetted and specially inhibited. 4. Tarnish inhibit in dip application using a soap (mechanical tarnish inhibit film) or apply on organic topcoat such as a lacquer.
Conditioningfor Plating Note: Comprehensive information describing aluminum alloys, castings, and suggested deoxidizer/desmutting baths is described in the chapter titled, "Surface Preparation of Various Metal Alloys Before Plating and Other Finishing Applications."
Brass and CopperAlloys
Descaling--BrightFinish 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 2025% v/v and 0.3-0.5% v/v sulfuric acid at 90-1 I°F (32-43°C). Solution is wetted and specially inhibited. A golden-brown surface film must develop with minimal surface gassing to obtain desired surface polishing. 25 minute immersion should cover most cycle applications. 4. 5% v/v sulfuric acid dip for 10-15 seconds to dissolve the protective 143
brown film. 5. Tarnish inhibit in dip application using either a soap (mechanical tarnish inhibit 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 offerric sulfate at 100-140°F (38-60°C). Solution is wetted and specially inhibited. 4. Tarnish inhibit in dip application using either a soap (mechanical tarnish inhibit film) or a henzotriazole (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 66°B~ sulfuric acid, 20% v/v of 42°B~ nitric acid, balance water, containing 0.25 oz/gal of sodium chloride, 90-110°F (32-43°C). Monitor solution action to avoid etching, ira bright, polished finish is required.
Descaling--MatteFinish 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 80-95°F (27-35°C).or 1. Moderately alkaline soak cleaner. 2. Immersion treatment in solution consisting of: 75% v/v of 85% phosphoric acid and 25% v/v of 42°B~ nitric acid, 90-120°F (32-49°C). Cast Iron
Standard alkaline soak clean, followed by alternate hot and cold rinsing to push solutions out of pores. Anodically electroclean in alkaline clescaler. 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. Control acid dip to avoid overtreatment. Cast iron has a low hydrogen overvoltage, which can reduce electroplating efficiency. Extended immersion in the acid will further reduce this efficiency. Mass finishing may be a better option to remove rust and scale.
Gold and Cold Alloys
Brightening 1. Alkaline soak clean. 2. Anodic treatment at 6-12 V in a solution consisting of 20 oz/gal sodium citrate and 5 oz/gal Rochelle salts, 80-90°F (27-32°C). Gold may be recovered by dummying at low current density onto stainless steel cathodes, also using stainless steel anodes. lo.4
High Carbon and CaseHardened Steels High carbon steel can be heavily scaled. Both metals are susceptible to hydrogen embrittlement. In addition pickling high carbon steels may generate heavy surface smuts. Solutions containing sulfuric acid, fluorides, organic acid complexors, inhibitors, pickle aids, and wetting agents are preferred to process these metals. The acid bath surface tension is reduced to promote superior wetting of the substrate. Once the scale or smut is removed an inhibitor film forms to prevent additional pickling. This prevents hydrogen embrittlement of critical parts, such as spring steel, and repeated smutting of high carbon steels. In some process cycles a follow-up electrocleaner is required to remove the inhibitor film 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, waterbreak-free surface. Sometimes the parts can be cleaned in an off-line eletropolishing 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 constituents and operating conditions). In a double cleaning cycle the first acid may be an anodic electropolishing treatment.
Low Carbon Steel 1. Alkaline soak clean. 2. Anodically electroclean (moderate caustic, silicated, complexed, wetted blend). 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 deflocculents may be required. Use at standard operating parameters.
Ma~aesium Alloys Removal 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, 75-90°F (24-32°C) or B. 20 oz/gal of 85% phosphoric acid, 2-3% v/v of 42°B6 nitric acid, and 0.2-0.3 oz/gal ammonium bifluoride, 75-90°F (24-32°C). or
Chromic acid containing solution consisting of.' 44-48 oz/gal chromic acid, 34 oz/gal sodium nitrate, 1.5 oz/gal sodium chloride, 0.1 oz/gal sodium sulfate, 65-90°F (18-32°C).
Nic~l and CobaltAlloys 1. Alkaline soak clean. 2. Cathodically condition in a highly conductive electrocleaner blended for steel. 3. Acid dip in a solution consisting of 50% v/v of 70% hydrofluoric acid and 145
7% v/v of 42°B~ nitric acid at 7$-90°F (24-32°C).
Si/ver and S/lver Agoys 1. Alkaline soak clean. 2. Acid dip in either of the following solutions: A. 66% v/v of 42°B~ nitric acid at 75-160°F (24-71°C). B. 88% v/v of 66°B~ sulfuric acid, 75-90°F (24-32°C). C. 22% v/v of 42°B~ nitric acid at 75-90°F (24-32°C).
Titanium 1. Alkaline soak clean. 2. Acid treatment in solution consisting of 40% v/v of 42°B~ nitric acid and 2-3% v/v of 75% hydrofluoric acid, at 120-135°F (52-57°C).
Zinc and Zinc Alloys Comprehensive information describing zinc alloys, castings, and suggested acid baths is described in the chapter titled "Surface Preparation of Various Metal Alloys Before Plating and Other Finishing Applications."
Activation ofBrightNick.elElectroplate There are two effective acid solutions for activating passive nickel deposits and to salvage reject parts. Their application is recommended for sulfur bearing electrolytic nickel deposits. The immersion dip consists of 5 to 10% v/v sulfuric acid and 2 to 4 oz/gal of potassium iodide at 75 to 90°F (24-32°C), 1 to 5 minutes or as required. A cathodic activation of nickel consists of 5 to 11% v/v sulfuric acid, 1 to 3 oz/gal fluoride, 2 to 4 oz/gal chloride at 5 to 7 V for 1 to 2 minutes (as required) at 20 to 40 A/ft 2 and 75 to 120°F (24-49°C).
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