Metal Finishing Guide Book


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performance. A common, simple example being part of oxidant mixtures in passivates is nitrate. Zinc reduces nitrate according to the formulas given (Dikinis V, et al. Trans Inst Metal Finishing 2004;82[3–4]:98): NO3– + Zn + 2H3O+ ␱ NO2– + Zn2+ + 3H2O 2NO2– + 4H3O+ + Zn ␱ 2NO⌫ + 6H2O + Zn2+ 2NO + 5Zn +12H3O+ ␱ 2NH4+ + 5Zn2+ + 14H2O NO3– + 4Zn + 10H3O+ ␱ NH4+ + 4Zn2+ + 13H2O All these processes consume oxonium ions and, therefore, contribute to higher pH on the zinc surface compared with the bulk solution. Except for the kinetic properties of the individual reduction reactions, ligand exchange on the chromium(III) ion plays a crucial role in passivate as well as in chromate layer buildup. Lastly, the soluble chromium(III) compounds in passivates hydrolyze and build up a similar but usually thinner conversion layer on the zinc surface. These passivate layers, not bearing adsorbed chromium(VI) on the surface, sometimes lead to inferior corrosion protection results compared to chromates. Thick-film passivates can be used to generate transparent, iridescent layers with thicknesses of about 300 nm.9 The corrosion protection provided by these layers is similar to that of yellow chromates, although currently not achieved with black passivates. Generating black finishes using trivalent chromium passivates is similar, in general, to black hexavalent chromates. On zinc, the black pigment may be generated from other transition metals Figure 4: Model of chromium conversion layer from solutions from the process solution. Zinc– of trivalent chromium. No hexavalent chromium is adsorbed nickel or zinc–iron surfaces are to the surface. usually rendered black by etching the surface, dissolving zinc, and leaving an iron- or nickel-based black pigment within the layer. Although a chromium(III)-conversion coating like the one preFigure 5: Effect of fine tuning the solution's composition. viously described is also generated Basic composition (left); advanced composition (middle); and composition fine-adjusted to final composition of Tridur simultaneously with these black Finish 300 (right). pigments, the layer's growth is commonly found to be limited, resulting in inferior corrosion protection results compared with black chromates. Without any further treatment, black trivalent passivates generally protect the respective zinc and zinc–alloy layers only for 24–48 h in neutral salt spray testing (ISO 9227). New developments in pigmenting a chromium conversion coating without using noble metals (with respect to zinc) at Atotech actually enable more stable results of 48 h to white corrosion. However, the current specifications of the automotive industry for unsealed black passivates require higher performance, which, to date, cannot be met without additional post-treatment. Applying sealers that add an efficient layer to enhance corrosion protection 333

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