Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 432 of 707

Fig. 1. Passivation film thickness. First-generation trivalent chrome conversion processes were based on fairly sta- ble Cr(3) complexes, which slowed their reactivity rates even at high temperatures. They produced film thicknesses of 20 to 30 nm with limited corrosion resistance. To produce thicker passivation layers, a second-generation trivalent passiva- tion process was developed. It incorporates accelerators, modified complexors, and is operated at higher concentration and temperature to drive the reaction kinetics at a faster rate. When applied as recommended, film thicknesses of 300 to 380 nm, equivalent to those produced from yellow Cr(6) passivating solutions, were obtained. The film in this case consists of an insoluble barrier lay- er free of hexavalent chrome. Table II and Fig. 1 compare operating conditions and resulting conversion coat- ing films produced by various passivating solutions. Fig. 2. Comparison of neutral salt spray results for Cr(6) versus Cr(3) passivation on zinc (WC [minus] white corrosion). 431

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