Metal Finishing Guide Book


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Fig. 7. Cr(3) passivation meets or surpasses requirements for GM. lent chrome tanks, with little or no modification to the line. Provisions for heating may be required along with proper ventilation. Postpassivation may be used inline if it is compatible with the operation and space for extra stations is available. Organic topcoats, especially for barrel plating processes, are best applied off-line. The critical operating parameters are the chrome content, pH, and temperature. Analytical methods for trivalent chrome using spectrophotometric or simple titration techniques are commonly used and readily available. Chrome content affects the ultimate film thickness. Temperature and pH control the reaction kinetics, film strength, and adhesion. EFFECT OF CONTAMINANTS The most common contaminants are zinc and iron resulting from processed parts. Excessive amounts of these metals result in the formation of uneven thin coatings and yellowing of the deposit with possible reduction in corrosion resistance. Removal methods of these metals by precipitation and selective ion exchange are available. Drag-in of alkalinity affects the bath pH and can be corrected with mineral acids as recommended for the specific process. BATH LIFE Trivalent chrome passivating solutions have much longer bath life than their hexavalent counterparts. There is no composition imbalance resulting from gradual depletion of Cr(6) and buildup of reduced Cr(3). The corrosive effect is lower, and dissolution and buildup of zinc and iron are reduced. As a result, the solution composition is more stable over time. Unless there is gross contamination, these baths will perform satisfactorily for long periods of time. Under proper steady state conditions, the solution can last indefinitely. The use of a drag-out tank and multiple counter-current flow rinsing after the 444

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