Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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rier that effectively deceler- ates zinc corrosion. Sealed conversion layers Figure 2: Structure of CCCs from hexavalent chromium solutions (the solid structure indicates further chromium-olate polymer). Figure 3: Surface SEM image of zinc–nickel with a black hexavalent chromate applied. Notice the typical "mud-crack" structure. achieve the most stringent corrosion protection requirements and also bear excellent decorative proper- ties. 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 used in some applications. To provide excellent corrosion protection as well as advanced decorative prop- erties in those applications, an alternative approach to enhance the conversion lay- er is required. To date, black finishes, in particular, do not provide reliable and suf- ficient corrosion protection without the application of a final finish. This final finish is not allowed to introduce any components into the coating that are not already found in trivalent chromi- um 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 chromi- um were already used on black chromium(VI)-based chromate conversion coat- ings, particularly on some black zinc–iron layers. These post-dip solutions main- ly 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 of the new chromium(III)-based post-dip generation could be achieved, circum- 289

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