Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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et al. Trans Inst Metal Finishing 2004;82[3–4]:98): NO3 – + Zn + 2H3 oNO2– O+ O+ NO3 O+ + Zn2+ + 3H2 2NO2– + 4H3O+ + Zn o2NOG + 6H2 2NO + 5Zn +12H3 o2NH4 –+ 4Zn + 10H3 oNH4 + + 5Zn2+ + + 4Zn2+ O O + Zn2+ + 14H2 + 13H2 O O All these processes consume oxonium ions and, therefore, contribute to high- er pH on the zinc surface compared with the bulk solution. Except for the kinet- ic 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, Figure 4: Model of chromium conversion layer from solutions of trivalent chromium. No hexavalent chromium is adsorbed to the surface. Figure 5: Effect of fine tuning the solution's composition. Basic composition (left); advanced composition (middle); and composition fine-adjusted to final composition of Tridur Finish 300 (right). 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 yel- low chromates, although currently not achieved with black passivates. Generating black finishes using trivalent chromium passivates is similar, in general, to black hexa- valent chromates. On zinc, the black pigment may be generated from other transition metals from the process solution. Zinc–nickel or zinc–iron surfaces are usually rendered black by etching the sur- face, dissolving zinc, and leaving an iron- or nickel-based black pig- ment within the layer. Although a chromium(III)-conversion coat- ing like the one previously described is also generated simul- taneously with these black pig- ments, the layer's growth is com- monly found to be limited, resulting in inferior corrosion pro- tection 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 neu- tral salt spray testing (ISO 9227). New developments in pigmenting a chromi- um 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 passi- vates require higher performance, which, to date, cannot be met without addi- tional post-treatment. Applying sealers that add an efficient layer to enhance corrosion protection and 291

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