Metal Finishing Guide Book

2013

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Tetrasodium EDTA Sodium fluoride pHa Temperature Current density 35-45 g/L 3-5 g/L 7.9-9.5 50-60ºC 0.5-23 A/dm2 (4.7-6.0 oz/gal) (0.40-0.70 oz/gal) (120-140ºF) (5-25 A/ft) a Note: Adjust solution pH with triethanolamine or sodium bicarbonate. Thiosulfate Solutions Thiosulfate-based formulas have proven to be among the most successful of 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 15-30ºC 0.4-1.0 A/dm2 (4.0 oz/gal) (40-70 oz/gal) (4.0-6.7 oz/gal) (60-85ºF) (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 25ºC 1 A/dm2 (4.0 oz/gal) (1.5-7.4 oz/gal) (6.0 oz/gal) (77ºF) (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) 314 24 g/L (3.3 oz/gal)

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