Metal Finishing Guide Book

2012-2013

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inorganic complex-based silver-plating processes investigated. Early attempts to plate silver from such a solution resulted in rapid oxidation of the complex and precipitation of insoluble silver compounds. Additions of sodium metabisulfite were found to minimize this tendency and all thiosulfate-based processes now contain this ingredient. Solution composition can be expressed as follows: Silver as thiosulfate Sodium thiosulfate Sodium metabisulfite pHa Temperature Current density 30 g/L 300-500 g/L 30-50 g/L 8-10 15-30��C 0.4-1.0 A/dm2 (4.0 oz/gal) (40-70 oz/gal) (4.0-6.7 oz/gal) (60-85��F) (4-10 A/ft2) a Note: Adjust solution pH with sodium bisulfite or sodium hydroxide. These electrolytes can be operated with stainless steel or silver anodes; however, the latter should be bagged. Problems of poor adhesion can be overcome by using a conventional silver strike or one in which there is no free cyanide. In either case, rinsing before entry into the thiosulfate solution is a good practice. A small amount of cyanide drag-in will react with thiosulfate in the solution to form thiocyanate: CN��� + S2O32��� CNS��� + SO32��� One reported advantage of thiosulfate over cyanide systems is that thickness distribution is better on complex-shaped objects; however, deposits seem to tarnish in air much quicker than cyanide-produced ones. Postplating passivation is recommended. Succinimide Solutions Several electrolytes based on this organic complex of silver have been patented, two of which are described below: Silver (as potassium silver disuccinimide) Succinimide Potassium sulfate pH Temperature Current density 30 g/L 11.5-55 g/L 45 g/L 8.5 25��C 1 A/dm2 (4.0 oz/gal) (1.5-7.4 oz/gal) (6.0 oz/gal) (77��F) (10 A/ft2) Potassium nitrite or nitrate can be substituted for the sulfate. The addition of amines, such as ethylene diamine or diethylenetriamine, and wetting agents produce bright, stress-free deposits. Silver (as potassium silver disuccinimide) Succinimide Potassium citrate pH Temperature Current density 24 g/L 25 g/L 50 g/L 7.5-9.0 20-70��C 0.54 A/dm2 (3.3 oz/gal) (3.4 oz/gal) (6.7 oz/gal) (70-160��F) (5.5 A/ft2) Potassium borate may be used in place of potassium citrate. Tarnish resistance of deposits obtained from these processes is inferior to that of deposits produced from cyanide-based electrolytes unless they are dipped in dilute sulfuric acid immediately after plating is complete. 356

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