Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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psi) compressive. To control the internal stress and other properties during operation, the solu- tion is electrolyzed continuously at low current density by circulating through a small, separate conditioning tank. The conditioning tank should have 10 to 20% of the capacity of the main tank and the total solution should be circulated through it two to five times per hour. For this to work, the anodes in the condi- tioning tank must be nonactive, whereas the anode materials in the main tank must be fully active (containing sulfur). This is a means of controlling the anode potential in the conditioning tank so that only a stress reducer that does not increase the sulfur content of the nickel is produced. The use of an active anode material in the main tank prevents formation of sulfamate oxidation products in that part of the system. Zero-stress conditions can be obtained at the temperature and current densi- ty values given in Table VI. The plating rate is also indicated in the table. For exam- ple, at 50° C, the stress is zero at approximately 8 A/dm2 pressive below and tensile above that value. To deposit nickel at 32 A/dm2 stress, the temperature must be raised to 70° , and will become com- at zero C. Despite its seeming complexity, this process is being used successfully to electroform stampers for compact disc man- ufacture where flatness of the stamper is critical and to electroform ultrathin nick- el foil continuously on rotating drums. The internal stress in deposits from sulfamate solutions is influenced by reac- tions at the nickel anode. When a nickel anode dissolves at relatively high poten- tials, stress reducers are produced by anodic oxidation of the sulfamate anion. The use of pure nickel in the conditioning tank and active nickel in the main tank is designed to control the nature and amount of the stress reducer formed in this high-speed bath. QUALITY CONTROL Improvement in total quality is required by all industrial activity, including nick- el plating. Quality assurance involves maintaining the purity of the nickel-plating solution and controlling the properties of the deposits. Some of the control pro- cedures are summarized here. Purification of Solutions Nickel-plating baths freshly prepared from technical salts contain organic and inor- ganic impurities that must be removed before the bath is operated. Older baths gradually become contaminated from drag-over from preceding treatments, from components that are allowed to fall off the rack and allowed to remain in the tank, from corrosion products from auxiliary equipment, from tools dropped into the tank, and from other sources. It is more effective to keep impurities out of the plating bath than to deal with rejects and production interruptions resulting from the use of impure solutions. The maximum concentrations of impurities normally permissible in nick- el plating solutions and recommended treatments for their removal are shown in Table VII. The electrolytic treatment referred to in the table, known as "dummying," involves placing a large corrugated cathode in the solution and plating at low current densities, 2 and 5 A/ft2 bearing organic addition agents are best removed at 2 A/ft2 zinc are more effectively removed at 5 A/ft2 because it gives a wider current density range. At 2 A/ft2 . Copper, lead, and certain sulfur- , whereas iron and . A corrugated cathode is preferred , impurities should be 231

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