Metal Finishing Guide Book

2013

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ers achieve the most stringent corrosion protection requirements and also bear excellent decorative properties. Furthermore, by using sealers, a broad range of friction coefficients can be precisely adjusted within narrow tolerances. Despite their assets, polymer-based sealers are specifically restricted from being Figure 2: Structure of CCCs from hexavalent chromium solutions (the solid structure indicates further chromium-olate polymer). used in some applications. To provide excellent corrosion protection as well as advanced decorative properties in those applications, an alternative approach to enhance the conversion layer is required. To date, black finishes, in particular, do not provide reliable and sufficient corrosion protection without the application of a final finish. This final finish is not allowed to introduce any components Figure 3: Surface SEM image of zinc–nickel with a black hexavalent chromate applied. Notice the typical "mud-crack" structure. into the coating that are not already found in trivalent chromium conversion coatings. At the same time, it needs to fulfill the high decorative and functional demands required by the automotive industry. This approach leads to the obvious question: Why not just put more chromium into the layer? This is not a cutting-edge idea. Post-dip solutions based on hexavalent chromium were already used on black chromium(VI)-based chromate conversion coatings, particularly on some black zinc–iron layers. These post-dip solutions mainly consisted of a chromium(VI) source such as sodium dichromate in a dilute acidic solution. With the restrictions on hexavalent chromium use, the logical step from hexavalent chromium post-dips to trivalent chromium–based post-dips became obvious. Just changing from chromium(VI) chemistry to any chromium(III) compounds does not result in sufficient performance as a final finish. Surfaces achieved this way are noticeably inferior with regard to corrosion protection, as well as decorative properties. As with the change from chromates to passivates, R&D had to think about how the more difficult formulation and application 331

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