Metal Finishing Guide Book

2012-2013

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a boric acid concentration of 30 to 40 g/L. It is operated at a pH of 4.0, a temperature of 140 to 160��F (60 to 71��C) and at current densities as high as 800 A/ft2. The high rates of plating are made possible by the high nickel concentration. When the bath is properly conditioned and operated, it is possible to control internal stress at or close to zero because of the interrelations of stress, current density, and solution temperature (Table VI). After purification with carbon to remove all organic contaminants, the concentrated solution is given a preliminary electrolytic conditioning treatment consisting of (1) electrolysis at 0.5 A/dm2 on both anode and cathode for up to 10 A-hr/L; (2) electrolysis at 0.5 A/dm2 on the anode and at 4.0 A/dm2 on the cathode for up to 30 A-hr/L of solution. For this conditioning treatment, the anode must be nonactivated (sulfur-free). A corrugated steel sheet may be used as the cathode. When the solution has been conditioned, a deposit at a current density of 5 A/dm2 and at 60OC should be lustrous and the internal stress as determined with a spiral contractometer or other device should be 48 �� 14 MPa (7,000 �� 2,000 psi) compressive. To control the internal stress and other properties during operation, the solution 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 conditioning 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 density values given in Table VI. The plating rate is also indicated in the table. For example, at 50��C, the stress is zero at approximately 8 A/dm2, and will become compressive below and tensile above that value. To deposit nickel at 32 A/dm2 at zero stress, the temperature must be raised to 70��C. Despite its seeming complexity, this process is being used successfully to electroform stampers for compact disc manufacture where flatness of the stamper is critical and to electroform ultrathin nickel foil continuously on rotating drums. The internal stress in deposits from sulfamate solutions is influenced by reactions at the nickel anode. When a nickel anode dissolves at relatively high potentials, 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 nickel plating. Quality assurance involves maintaining the purity of the nickel-plating solution and controlling the properties of the deposits. Some of the control procedures are summarized here. Purification of Solutions Nickel-plating baths freshly prepared from technical salts contain organic and inor344

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