Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Final Finish Applied None/passivate only Hours to White Corrosion 24–48 h Tridur Finish 300 (20% v/v) 312 h Corrosil Plus 501 BG* 432 h *Organic polymer/silicate-based sealer. Table 3: Minimum Corrosion Resistance of Different Finishes Applied to Black Passivated Zinc-Nickel (Tridur ZnNi H1): Neutral Salt Spray Testing (ISO 9227). Conversion Coating Tridur Zn H1 F/Nm T/kN KM10 shows a structural proposal for this layer's composition. Polynuclear chromium(III) complexes bearing µ-phosphato bridges are described in litera- ture10–12 and they most likely contribute to the post-dip layer's composition. Due to the very similar composition of Tridur Finish 300 layers and passivate layers, it is very difficult to find µthread µhead µtot 120.3 ± 11.3 36.1 ± 0.01 0.33 ± 0.03 Tridur ZnNi H1 150.9 ± 13.7 36.1 ± 0.02 0.42 ± 0.04 0.32 ± 0.04 0.33 ± 0.07 0.22 ± 0.04 0.27 ± 0.03 0.35 ± 0.03 0.34 ± 0.03 Table 4: Friction Properties Determined on M10×50 Bolts (measurements ± standard deviation) some contrast between both layers by means of SEM imaging. However, Figure 10 shows an SEM image of a FIB cross section through a sample with Tridur Finish 300 applied to black passivated zinc–nickel (14% nickel). The image reveals a thickness of 100–200 nm for the passivate and the post-dip layer. Layer morphology. The morphology of the post-dip layer was investigated using different concentrations of Tridur Finish 300 applied to a black passivated (Tridur ZnNi H1) zinc–nickel alloy surface. The morphology of the deposit in dependence of the concentration of the post-dip bath was studied by means of SEM micrographs on samples of black passivated zinc–nickel (Figs. 11–14). The post-dip caulks the micro cracks of the black passivated zinc–nickel sur- face. The post-dip layer's appearance itself resembles that observed with a hexa- valent black chromate on zinc–nickel with regard to the mud-crack-like sur- face observed. Above 200 ml/l the post-dip layer's cracks become larger in size (Fig. 15). This means that with excessive concentrations a lesser extent of the surface may be covered by the post-dip layer. No significant advantage concerning nei- ther the aspect nor the corrosion protection could be determined with higher con- centrations. Corrosion-protection properties. Corrosion-protection properties were investigated with different concentrations of the post-dip solutions (Tridur Finish 300) applied to black passivated zinc–nickel. It was found that a high level of corrosion protection was already established with 50 ml/l of Tridur Finish 300, not increasing significantly with higher concentrations (100–300 ml/l). However, the aspect of the parts finished was found to be best at 200 ml/l (20% v/v). Evaluation on black passivated zinc–iron (Tridur ZnFe H1) produced similar results. On black passivated zinc (Tridur Zn H1), 100 ml/l was found to be a suitable concentration. With regard to the decorative aspect of the finished surfaces as well as their cor- rosion-protection properties by means of neutral salt spray testing, the applica- tion parameters shown in Table 1 have been proven in practice for application on 296

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