Surface Preparation for Electroplating

Preparation for plating is one of the most critical of all cleaning operations, because maximum adhesion of the plated coating to the substrate is the major requirement for quality work. Maximum adhesion depends on both the elimination of surface contaminants in order to induce a metallurgical bond whenever possible and the generation of a completely active surface to initiate plating on all areas. In addition to pickling or other descaling operations, adequate cleaning requires multistage cycles, usually comprised of the following steps: (a) precleaning with a solvent to remove most of the soil; (b) intermediate cleaning with alkaline cleaners; (c) electrocleaning to remove the last traces of solids and other contaminants that are especially adherent; (d) acid treatment and surface conditioning to remove light oxide films formed during previous cleaning processes and to microetch the surface; and (e) electrolytic (anodic) desmutting to remove any smut formed during acid pickling of heat treated high-carbon steel parts. Low-carbon steels do not require this desmutting step. Anodic electrocleaning also offers oxidation or conditioning of scale. The oxidized or softened scale is easily removed in subsequent acid pickling. The types of cleaning usually employed in the above steps are:

• Precleaning: cold solvent, vapor degreasing, emulsifiable solvent, solvent emulsion spray, or alkaline spray with or without solvent emulsion

• Intermediate alkaline cleaning: soak cleaning with 30 to 90 g/L (4 to 12 oz/gal) of cleaner at 80 °C (180 °F) to boiling, spray cleaning with 4 to 15 g/L (0.5 to 2 oz/gal) at 65 to 80 °C (150 to 180 °F), and barrel cleaning with 7.5 to 45 g/L (1 to 6 oz/gal) at temperatures below 80 °C (180 °F)

• Electrocleaning: cathodic, anodic, or periodic-reverse

• Acid treatment: practice is highly specific for the metal being processed

• Anodic desmutting: necessary to remove carbon smut

The ASTM recommended practice for cleaning steels with carbon contents less than 0.35% C is ASTM B 183 (Ref 33); ASTM B 242 covers surface preparation of steels with carbon contents of > 0.35% C (Ref 34). Additional information on cleaning carbon and alloy steels prior to electroplating can also be found in the article "Classification and Selection of Cleaning Processes" in this Volume.

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