Metal Finishing Guide Book


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Figure 6: SEM/EDX (10 kV) of a black passivated zinc–nickel (Reflectalloy ZNA, Tridur ZnNi H1) surface with Tridur Finish 300 (20% v/v, pH 5.5, 45¡C) applied. Only elements that are also commonly found in passivate layers are present with the post-dip applied. Gold is present due to sample preparation. Figure 7: SEM/EDX (10 kV) of a black passivated zinc–nickel surface (Reflectalloy ZNA, Tridur ZnNi H1) with a polymer/silicate-based sealer applied (Corrosil Plus 501). Application of the sealer completely changes the surface's composition, which is now clearly dominated by silicon and carbon. and decorative appearance is precluded from some applications. Alternatively, virtually extending the conversion layer by adding chromium from a chromium(III)-based solution to the conversion layer is an option. Consequently, with hexavalent chromium post-dip solutions in mind, the step to post-dip solutions resting on trivalent chromium is obvious. TRIVALENT CHROMIUM APPROACHES IN POST-DIPS Finding the proper coordination environment for Cr(III). Choosing the right coordination sphere for hexavalent chromium was not too difficult. The choice of ligands suitable for industrial use is almost completely limited to the oxygen moiety known from chromium trioxide (CrO3), dichromates (Cr2O72–), or chromates (CrO42–). Upon switching from the 3d0 ion Cr(VI) to the 3d3 ion Cr(III), reactivity and toxicity changes completely. Contrary to chromium(VI), the chromium(III) ion bears exceptionally slow reaction rates with regard to ligand exchange. Also, contrary to chromium(VI), the trivalent ion does not act as an oxidizing agent in conversion coating process baths. The primary criteria for evaluating appropriate ligands are the decorative (aspect) and functional (corrosion protection) properties of the coating. 334

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