Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Fig. 7. Cr(3) passivation meets or surpasses requirements for GM. 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 coat- ings 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 hexa- valent 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 passivation step is recommended. Solution from the drag-out can be returned to the process tank, reducing chemical consumption drastically. Evaporators can be used with the drag-out tank to improve the reuse rate of the chrome solution. When passivation is operated at an elevated temperature, the returned solu- tion volume is balanced by the evaporation in the process tank. Evaporators can also be installed on the chromating tank with no adverse effect on its components. WASTE TREATMENT Since the process is hexavalent chrome-free, the classical sulfite reduction step is eliminated. In principle, simple neutralization will precipitate chromium and oth- 436

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