Metal Finishing Guide Book

2013

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Keeping the objective in mind that the final composition of the post-dip's layer should only bear components that are commonly found in passivate layers limits the range of eligible compounds. The formulation of the post-dip solution was adjusted until the best combination was found that achieved satisfactory appearance and corrosion protection performance. By fine adjusting an integrated additive system, the deposition of the post-dip could Figure 8: XPS profile of Tridur Finish 300 (20% v/v, pH 5.5, 45¡C) on Tridur ZnNi H1. The vertical line indicates the sputter be attuned towards a highly unidepth at which the change in composition, mainly nickel form dispersal of the deposit while concentration, indicates the transition from post-dip imparting a homogenous gloss to passivate at about 180 nm. with a minimized tendency to form drop marks (right panel in Fig. 5). With the major component of the final composition being chromium(III) in a carefully adjusted coordination moiety, the surface layer from post-dip treatment does not bear any non-passivate like components. Variations on the application parameters within reasonable limits around a set of initial figures were conducted in order to establish the final application Figure 9: Structural proposal for the chromium(III) parameters on black passivated phosphate–based post-dip layer deposited on the trivalent zinc–nickel (Tridur ZnNi H1). chromium based passivate. R represents either hydrogen Temperature, pH, make-up con(H) or some organic rest, such as an alkyl or aryl-rest. centration, and dwell time, as well as drying temperature, have been changed individually, keeping the remaining parameters constant. Application at pH 5.5 (45°C) in a 20% v/v solution has been elaborated to be the optimum set of parameters for the application on Tridur ZnNi H1. Dwell-time variations showed no visible difference within 10–40 s. Drying temperature has been shown to be best between 70–90°C, for 10 min. The results were evaluated for both their decorative aspects and corrosion protection properties. A semi-quantitative EDX analysis of the surface of black passivated zinc–nickel shows only elements, which are typically also found on zinciferous surfaces with trivalent chromium conversion coatings applied (Fig. 6). The deposit from the final formulation adds to the passivate layer. Upon application, the newly developed post-dip acts like a second conversion coating on the conversion layer. For comparison, the EDX spectrum from black passivated zinc-nickel with an organic polymer/silicate-based sealer (Corrosil Plus 501) applied is shown in Figure 7. Obviously, the layer's composition is completely different from that of 335

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