Metal Finishing Guide Book

2013

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Fig. 8. Cr(3) passivations meets or surpasses requirements for DaimlerChrysler. 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 solution 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 other metal hydroxides. An efficient method to improve total separation of chrome from its complexed form consists of lowering the solution pH with sulfuric acid, followed by lime neutralization and settling. Accurate methods of treatment, however, must be developed for the specific passivating process used to meet local and federal waste disposal requirements. The combination of reduced solution replacement and lower use of treatment chemicals improves the economics of the process. SPECIFICATIONS Several U.S. and European automotive specifications are now available calling for hexavalent chrome-free conversion coatings for zinc and zinc alloy plating. These specifications take into account the need for sealers and topcoats in order to achieve desired performance criteria. Additionally, some specifications require heat-treating components prior to NSS as part of the qualifying testing criteria. Table III lists some of the U.S. automotive specifications published as of this writing. Figs. 6, 7, and 8 illustrate sample performance data of commercially available trivalent chromate passivation in relation to these specifications. The performance shown reflects results obtained from actual production lines and can be exceeded through process modification and optimization. 445

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