Metal Finishing Guide Book


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Final Finish Applied Hours to White Corrosion None/passivate only 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 enhanced conversion coating. Figure 9 shows a structural proposal for this layer's composition. Polynuclear chromium(III) complexes bearing μ-phosphato bridges are described in literature10–12 and they most likely contribute to the post-dip layer's composition. Due to the very similar composition of F/Nm T/kN KM10 μthread μhead μtot Tridur Zn H1 120.3 ± 11.3 36.1 ± 0.01 0.33 ± 0.03 0.32 ± 0.04 0.22 ± 0.04 0.27 ± 0.03 Tridur ZnNi H1 150.9 ± 13.7 36.1 ± 0.02 0.42 ± 0.04 0.33 ± 0.07 0.35 ± 0.03 0.34 ± 0.03 Table 4: Friction Properties Determined on M10×50 Bolts (measurements ± standard deviation) Tridur Finish 300 layers and passivate layers, it is very difficult to find 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 surface. The post-dip layer's appearance itself resembles that observed with a hexavalent black chromate on zinc–nickel with regard to the mud-crack-like surface 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 neither the aspect nor the corrosion protection could be determined with higher concentrations. 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 corrosion-protection properties by means of neutral salt spray testing, the appli338

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