Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 202 of 707

be sulfur free to prevent dull, red deposits in low current density plating areas. Maintenance and Control Constituents It is recommended that all constituents in the formulation be controlled to within 10% of their nominal values, especially the free cyanide. The copper cyanide con- centration controls the allowable plating current density in combination with agi- tation. The free cyanide concentration controls efficiency, plating range, throwing pow- er, and anode polarization. The hydroxide concentration controls conductivity and throwing power. Carbonates buffer the solution and reduce anode polarization. Although high carbonate concentration of 90.0 to 120.0 g/L (12.0-16.0 oz/gal) decreases the plating range, it is added to new baths to stabilize their initial operation. Rochelle salt enhances anode corrosion and provides some grain refinement. Potassium formulations have a broader plating range than sodium formulations. Temperature Temperature above 71° C (160° F) in the high-efficiency and barrel formulations pro- motes the breakdown of cyanide and the rapid buildup of carbonates. Agitation Mechanical and/or solution agitation is recommended. Use air agitation only when required, as air agitation promotes carbonate buildup. Contamination Organic contamination causes nonuniform, dull, rough, or pitted deposits. In severe cases of organic contamination, the anodes may polarize. Carbon treatment will remove organic contamination. A copper strike should not be considered to be a clean- er and should be carbon treated periodically to prevent organic contamination from spreading to other plating baths. Hexavalent chromium contamination caus- es skip plate in the low current density plating area, blisters, and nonuniform deposits. The best method for eliminating the problem of chromium contamination is to eliminate the source. Hexavalent chromium in the bath can be reduced to trivalent by using proprietary reducing agents. Zinc contamination causes nonuniform or brass-colored deposits and can be removed by dummying the bath at 0.2-0.4 A/dm2 ). Sulfur and its compounds cause dull, red deposits in the low cur- rent density plating areas and usually appear in new baths because of impure cyanides or leaching from tank linings. Small amounts of a zinc salt, such as zinc cyanide, will eliminate sulfur red from the deposits. Most other common types of metallic contamination cause deposit roughness and can usually be removed by dummying and filtration. (2-4 A/ft2 Carbonate Excessive sodium carbonate can be removed by freezing out at a low temperature because of its limited solubility below -3° C (26° bonate can be removed by precipitation with calcium oxide, calcium hydroxide, or calcium sulfate. COPPER PYROPHOSPHATE PLATING BATHS Copper pyrophosphate plating baths require more control and maintenance than 201 F). Both potassium and sodium car-

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