Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 181 of 707

cipitate the sulfate. Sulfate is an impurity in other chemicals such as chromic acid. For this reason, sulfate should be controlled carefully and any sulfate present as impurities should be accounted for. Plating tanks are typically lined with polyvinyl chloride, (PVC, Koroseal). The historical use of lead lined tanks is no longer recommended. Auxiliary equipment should be constructed of PTFE (fluorocarbon resin) or tantalum. Titanium can be used except when a fluoride catalyzed chromium process is used. A ripple filter on the rectifier is used to reduce plating current problems. A non- PFOS surfactant must be used in regions that it is restricted by regulations. Table II contains additional information. TRIVALENT CHROMIUM PROCESSES Decorative trivalent chromium, a safer and more efficient system, was com- mercialized in the mid 1970s as an alternative to hexavalent chromium process- es with its many HES issues. In addition, trivalent chromium eliminated most of the operational problems associated with hexavalent chromium chemistry: high toxicity, low current efficiency, poor metal distribution, lack of coverage around holes, burns in high-current-density areas, and "white-wash." Depending on the design of the part being plated, productivity might be improved because these advantages might permit a higher rack density and lower reject rate compared to hexavalent chromium processes. The literature contains far less information on the chemistry of trivalent chromium processes than for hexavalent. This is the result of the relative newness of the commercially successful processes, the proprietorships of the processes, and the wider differences in the chemistries used. Basically, the significant reactions in all the trivalent chromium processes are very simple. Deposition reaction: Cr+3+3(e)jCr0(metallic chromium) Side reaction: Cr+3jCr+6+3(e) (7) (8) Reaction (7) requires only 3 electrons compared to the six required by hexa- valent chromium processes thus doubling the electron efficiency of the process. The unwanted reaction (8) can take place at the anode under some conditions. The electrolytes for the different trivalent chromium plating processes differ in chemistry, but they all contain a source of trivalent chromium, that is typically added as a sulfate and/or chloride salt. They also contain a stabilizing material (called a catalyst in hexchrome processes) that combines with the chromium to permit it to plate in the desired form. Salts are also added to increase conductivity in the solution. Wetting agents are used to help in the deposition reaction and to reduce the surface tension of the solution. This essentially eliminates the formation of a mist at the anode and cathode. It also helps lower the solution vis- cosity resulting in more solution draining from the part when compared to hexavalent chromium processes. Historically, the two general formulations of trivalent chromium processes obtain their generic names by the method they used to eliminate the side reac- 180

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