Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table VII. Cathodic Acid Operating Parameters for Steel and Stainless Steel Temperature Anode Pure lead Graphite ° F 85-120 85-120 O C 29-49 29-49 Time (min) 2-7 2-7 Cathode Current Density (A/ft2 20-60 20-60 ) Voltage 3-6 3-6 Use lead anodes in solutions that do not contain any fluorides. Otherwise, graphite anodes are required. The optimum caustic level also dissolves the iron hydroxide surface film that forms, preventing splotchy brown stains and burning due to low conductivity. Current densities are related to the base metal and whether the application is rack or barrel. (See Table III.) Double cleaning cycles are ideally suited to clean- ing and activating welded parts, such as wire goods, or heat-treated parts. Typical operating 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 a critical fac- tor in selecting the optimum acid solution. Sensitive metals (brass, copper alloys, and zinc) require milder acid treatments. (See Tables V and VI.) Steels can be scaled and rusted, needing more aggressive treatment, even cathodic action. The acids used can be grouped into inorganic (hydrochloric or sulfuric) and organic (sul- famic, citric, gluconic, etc.). Accelerators, such as chloride and fluoride, provide extra "bite" to improve pickling. Fluorides activate brass by dissolving lead smuts. Inhibitors prevent over pickling steel that would result in raising excessive surface smuts or detrimental hydrogen embrittlement. Pickle aids help two ways: lower solu- tion surface tension to improve wetting and increase contact action. Wetting agents generate a light foam blanket to minimize corrosive sprays and mist and emulsify residual oils on parts or dragged into the acid bath. Deflocculents prevent the redeposition of soils. Double cleaning cycles may employ an aggressive first acid to meet pickling demands. The second acid should be a milder type sufficient to neutralize the second electrocleaner film while activating the surface as a last step before plating. One note of caution! Hydrochloric acid or chloride salts in the first acid presents a special problem. Insufficient rinsing and draining of parts after this dip can drag chloride, a contaminant, into the anodic second electrocleaner. A sufficient buildup of chloride (measured in part per million levels) in the elec- trocleaner results in corrosive pitting of parts during the reverse anodic clean- ing cycle. Specially inhibited electrocleaners minimize this condition, increas- ing solution tolerance to chloride. Alternatively, a chloride-free acid, if appropriate, should be used before the second electrocleaner. Heavily scaled or 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 rust. Inhibitors are special amines, substituted ureas, and glycol-based organic compounds. Wetters may be anionic or nonionic types. Some wetters and inhibitors provide a filming action to inhibit attack on the base metal. Good 123

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