Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 344 of 843

Tridur Finish 300 Post-dip Solution Corrosion Protection Base Layer/Passivate Concentration Temperature pH Zinc/Tridur Zn H1 100 ml/l 45¡C (40–50¡C) pH 4.5 (4–5.5) >72 h to wc Zinc–iron/Tridur ZnFe H1 200 ml/l 45¡C (40–50¡C) pH 5.5 (5–6.5) >240 h to wc Zinc–nickel/Tridur ZnNi H1 200 ml/l 45¡C (40–50¡C) 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 passivate or the passivate with post-dip system. The conversion coating–like 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 concentrations are only found on the surface, likely due to adsorption of CO2 from the air or surface contaminaFigure 16: Log plot of polarization experiments of an tions. Within about 10 nm, the experimental black zinc passivate formulation. carbon concentration falls to a very low level, not changing significantSample Ecorr /mV i /μA/cm2 ly with increasing sputter depth. corr The composition of the post-dip Experimental black zinc layer and the passivate's conver–1,059 89 passivate without final finish sion coating appear almost identical. A change in nickel concentraExperimental black zinc –1,055 40 tion indicates a diffuse transition passivate + Tridur Finish 300 between the more post-dip-like Experimental black passivate and the more passivate-like layers. –1,050 16 + Corrosil Plus 501 Therefore, the post-dip contributes to an increase in thickness of about Table 2: Corrosion Potential (E ) and Corrosion Current 0.2 μm in this application. Densities (i ) from Tafel Analysis: Potentials vs. Ag/AgCl (3M KCl) However, the lack of sharp transitions is also due to the fact that the post-dip penetrates deeply into the passivate layer, effectively filling up micro cracks. Both the passivate layer and the postFigure 17: Tridur Finish 300 applied to Tridur ZnNi H1 on Zn/ dip layer bear reactive sites with Ni (14% Ni, 8 μm) after 1,008 h in neutral salt spray testing regard to coordination chemistry. according to DIN EN ISO 9227. No voluminous white corroDuring the deposition at elevated sion products were produced. temperature, and especially in the subsequent hot-air drying process, the chromium(III) present in the passivate layer reacts with the post-dip solution's components, finally building up the corr corr 337

Articles in this issue

view archives of Metal Finishing Guide Book - 2013