Metal Finishing Guide Book

2013

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plating processes, procedures & solutions TRIVALENT PASSIVATES NEED TRIVALENT POST-DIPS BY BJÖRN DINGWERTH, ATOTECH DEUTSCHLAND GMBH, BERLIN, GERMANY The application of hexavalent chromium for conversion coatings reflects on a long and successful history in corrosion-resistant coatings starting in the early 1930s.1 Although chromium(III)-based conversion coatings date back as early as 1951,2 their industrial application was forced into practice by the European End-Of-Live-Vehicle (ELV) directive.3 In addition, the RoHS4 and WEEE5 directives preclude chromium(VI) from being used in metal coatings in electrical and electronic equipment. Therefore, the use of hexavalent chromium is actually limited to a rather small number of remaining applications. In the years preceding the legal ban of chromium(VI), publications concerning trivalent alternatives grew dramatically. Most papers published during this pre-regulatory era discussed trivalent chromium–based systems, and rarely were they innovative in nature. These systems were quite often said to be at least as good as the good old hexavalent chromium systems but based on the environmentally sound chromium(III) compound. Trivalent chromates, often referred to as passivates, are less easily applied than chromates. Keeping the application parameters, such as concentration and particularly the solution's pH, within narrow upper and lower limits became much more important when switching from hexavalent to trivalent formulations. Simultaneous with the introduction of the new chromium(VI)-free passivate technology, the needs of decorative and particularly corrosion protection properties have been revised in many specifications, leading to more stringent requirements for trivalent passivates. The use of sealers is often mandatory to achieve these elevated demands, especially on black passivated surfaces. Sealers, in general, mean polymer solutions or dispersions or silicate solutions that are dried on the surface, resulting in a film of either an organic or inorganic polymer on top of the conversion layer (Fig. 1). It is characteristic for a sealer layer to bear a completely different composition with regard to the underlying passivate layer. Together with the chromium(III)-based conversion layer, the sealer layer acts as a highly efficient barrier that effectively Figure 1: FIB cross section, SEM image of a sealer layer on top of decelerates zinc corrosion. a black passivated zinc layer (Protolux 3000 with Tridur Zn H1, Corrosil Plus 501). Sealed conversion lay330

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