Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Tridur Finish 300 Post-dip Solution Base Layer/Passivate Zinc/Tridur Zn H1 Zinc–iron/Tridur ZnFe H1 Concentration Temperature 100 ml/l 200 ml/l Zinc–nickel/Tridur ZnNi H1 200 ml/l 45°C (40–50°C) 45°C (40–50°C) 45°C (40–50°C) pH Corrosion Protection (ISO 9227) pH 4.5 (4–5.5) >72 h to wc pH 5.5 (5–6.5) >240 h to wc pH 5.5 (5–6.5) >300 h to wc Table 1: Application Parameters and Corrosion Results of the Post-dip (Tridur Finish 300) The conversion coating–like Figure 16: Log plot of polarization experiments of an experimental black zinc passivate formulation. Sample Experimental black zinc passivate without final finish Experimental black zinc passivate + Tridur Finish 300 Experimental black passivate + Corrosil Plus 501 Table 2: Corrosion Potential (Ecorr Densities (icorr (3M KCl) Ecorr/mV icorr/µA/cm2 –1,059 –1,055 –1,050 89 40 16 ) and Corrosion Current ) from Tafel Analysis: Potentials vs. Ag/AgCl composition is confirmed by an XPS depth profile, recorded on a sample of Tridur ZnNi H1 with Tridur Finish 300 (20% v/v) applied (Fig. 8). Significant carbon concentra- tions are only found on the sur- face, likely due to adsorption of CO2 from the air or surface cont- aminations. Within about 10 nm, the carbon concentration falls to a very low level, not changing signif- icantly with increasing sputter depth. The composition of the post-dip layer and the passivate's conversion coating appear almost identical. A change in nickel con- centration indicates a diffuse tran- sition between the more post-dip- like and the more passivate-like layers. Therefore, the post-dip con- tributes to an increase in thickness of about 0.2 µm in this applica- tion. Figure 17: Tridur Finish 300 applied to Tridur ZnNi H1 on Zn/Ni (14% Ni, 8 µm) after 1,008 h in neutral salt spray testing according to DIN EN ISO 9227. No voluminous white corrosion products were produced. However, the lack of sharp tran- sitions is also due to the fact that the post-dip penetrates deeply into the passivate layer, effectively filling up micro cracks. Both the passi- vate layer and the post-dip layer bear reactive sites with regard to coordination chemistry. During the deposition at elevated temper- ature, and especially in the subse- quent hot-air drying process, the chromium(III) present in the passivate layer reacts with the post-dip solution's components, finally building up the enhanced conversion coating. Figure 9 295

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