Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 242 of 707

Table I. Comparison of Operating Conditions for Proprietary Noncyanide Silver Plating Processes Parameter Silver concentration, rack oz/gal) Silver concentration, barrel Solution pH Temperature range Current density, rack Current density, barrel Anode material Anode to cathode area ratio Agitation Process A 21-24 g/L (2.8-3.2 oz/gal) 21-24 g/L (2.8-3.2 oz/gal) 8.0-9.0 16-29° C (60-85O 0.1-2.2 A/dm2 0.3-0.6 A/dm2 F) (1-20 A/ft2 (3-6 A/ft2 ) Silver, stainless steel, or Pt/Ti 1:1-2:1 Cathode rod + sparger ) Process B 11.25-18.75 g/L (1.5-2.5 15-18.75 g/L (2-2.5 oz/gal) 8.5-9.5 16-24° C (60-75° 0.5-2.2 A/dm2 0.5-1.6 A/dm2 Silver 2:1 F) (5-20 A/ft2 (5-15 A/ft2 ) ) Cathode rod + air at anodes Posttreatments It is essential to employ proper rinsing procedures after silver plating. Inclusion of cyanide in the deposit during the plating process is unavoidable and results in discoloration of the deposit over time if it is not leached out of the deposit immediately after plating is complete. This discoloration of the deposit is referred to as "sweating out." Proper rinsing follows this sequence: silver plate, drag-out rinse, cold water rinse, cold water rinse, hot water soak (minimum tem- perature 90° C/195° F for 2 min), immediate cold water rinse (do not allow parts to dry in air before entering), (apply antitarnish coating if required), (cold water rinse), dry. If desired, antitarnish coatings, such as those based on alkaline chromate, can be applied prior to final rinse and dry steps. Electrophoretic lacquers can also be applied before drying if desired. Solvent-based lacquers are rarely used today but if they are beware of their use on antimony-brightened silver deposits. Some tra- ditional lacquers will react with the antimony in the silver surface, resulting in the formation of black spots. Unfortunately, these black spots usually develop during storage, making quality control difficult. NONCYANIDE SYSTEMS Many compounds of silver have been investigated as potential metal sources for a noncyanide plating process. Several authors have subdivided these studies into three groups by compound type. These groups are (1) simple salts, e.g., nitrate, flu- oborate, and fluosilicate; (2) inorganic complexes, e.g., iodide, thiocyanate thio- sulfate, pyrophosphate, and trimetaphosphate; and (3) organic complexes, e.g., suc- cinimide, lactate, and thiourea. The simple salts all appear to suffer from the same problem: sensitivity of the materials to visible and ultraviolet light. Although some smooth deposits have been obtained from such systems, they are not viable under normal production conditions. Two noncyanide silver plating processes are currently being marketed. These are based on proprietary complexes of silver. Of the inorganic complexes consid- ered, three are worth discussing further; these are the iodide, trimetaphosphate, and thiosulfate solutions. Silver succinimide complexes formed the basis of the first proprietary processes to be offered commercially. These are discussed here also. Commercially Available Systems The two proprietary systems currently being offered commercially are compared 241

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