Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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here. The manufacturers-recommended operating conditions are compared in Table I. Both processes will deposit silver directly onto copper alloys but both require a strike to promote adhesion to nickel, highly leaded brass, and other metals prone to passivity. Both suppliers recommend using a noncyanide-based copper strike where necessary. Deposits are typically as bright as the substrate onto which they are plated. Brightening systems similar to those available for cyanide processes have not yet been developed. It is recommended that deposits from both these processes be dipped in 10 to 20% sulfuric acid before final rinsing and drying in order to whiten and stabilize the deposit color. It is possible to apply chromate passivation and electrophoretic lacquer coatings in a similar manner to silver deposits obtained from cyanide systems. Careful maintenance of solution pH is very important with both these systems. If the solution pH is allowed to fall below 7.5 the complex degrades, resulting in precipitation of silver metal. This process is irreversible. Early processes lacked suf- ficient buffering and suffered short bath life due to low pH conditions at the anode. With better buffering and careful maintenance both suppliers claim multiple bath turnovers under production conditions. It is interesting to note that the process using agitation at the anode appears to last longer, again emphasizing the importance of pH at the anode. (Compare with high-speed processes below.) Iodide Solutions Several authors report some success with baths that are quite similar. A typical solu- tion might be as follows: Silver iodide Potassium iodide HI or HCl Gelatin (optional) Temperature Current density 20-45 g/L 300-600 g/L 5-15 g/L 1-4 g/L 25-60° C 0.1-15 A/dm2 (2.5-6.0 oz/gal) (40-80 oz/gal) (0.7-2 oz/gal) (0.15-0.55 oz/gal) (80-l40° F) (1.0-150 A/ft2 ) Without exception these authors found iodine in deposits from their particu- lar formula. This fact, and the relatively high price of the iodide salts, has prevented further use of this type of solution. Trimetaphosphate Solution A process was developed for silver plating magnesium and its alloys; its use on oth- er metals is not reported. Silver trimetaphosphate (monobasic), Ag2 Tetrasodium pyrophosphate, Na4 O7 Tetrasodium EDTA Sodium fluoride pHa Temperature Current density HP3 Sodium trimetaphosphate (trimer), Na6 O18 P2 P6 O9 3-45 g/L 100-160 g/L 50-175 g/L 35-45 g/L 3-5 g/L 7.9-9.5 50-60° C 0.5-23 A/dm2 (0.40-0.60 oz/gal) (13.5-21.5 oz/gal) (6.7-23.5 oz/gal) (4.7-6.0 oz/gal) (0.40-0.70 oz/gal) (120-140° (5-25 A/ft) F) aNote: Adjust solution pH with triethanolamine or sodium bicarbonate. Thiosulfate Solutions Thiosulfate-based formulas have proven to be among the most successful of 242

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