Metal Finishing Guide Book

2012-2013

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Fig. 3. Neutral salt spray results for Cr(3) thick layer passivation of zinc and zinc alloys. TRIVALENT CHROMATE PASSIVATION PROPERTIES Trivalent chrome passivation films are similar to hexavalent types as far as the barrier portion of the coating is concerned in many respects; however, the absence of soluble Cr(6) compounds contributes to a range of new properties. Corrosion Resistance NSS corrosion testing data per ASTM B 117 of zinc and zinc alloy plated surfaces with Cr(6) and Cr(3) passivation are illustrated in Figs. 2 and 3. Results support the concept of thick layer trivalent chrome passivation being a viable alternative to conventional hexavalent chromate. In all cases, except for barrel plated work, its performance matches or exceeds that of hexavalent types. This is due to mechanical damage to the passivation film during bulk processing and the absence of self-healing properties. This shortcoming is overcome by the use of sealers or topcoats to protect the film. Postpassivation treatment increases corrosion resistance further, and provides the flexibility to add a range of properties such as lubricity, torque-tension modification for fasteners, and developing various colored finishes. Fig. 4. Scanning electron micrograph of Cr(6) at left and Cr(3) at right. Passivation films dried at room temperature prior to heat treatment at 1,000[times]. 494

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