Cross comparison of TCP conversion coating performance on aluminum alloys during neutral salt-spray and thin-layer mist accelerated degradation testing

Abstract

The stand-alone corrosion resistance imparted to aluminum alloy 7075-T6 by different commercial trivalent chromium process (TCP) conversion coatings was evaluated after exposure to 7-day (i) neutral salt-spray (NSS) and (ii) thin-layer mist (TLM) tests. Comparison studies were performed with a non-chromium conversion coating (NCP). Quantitative assessment of the corrosion damage was made using scanning contact profilometry, weight loss and digital microscopy image analysis data. Analysis of the inorganic chemical composition in the coating baths was performed to correlate the Zr, Cr and F levels with the corrosion resistance. All the TCP and NCP conversion coatings provided an equivalent level of corrosion protection to this alloy during the 7-day NSS test. In contrast, greater differences in coating performance were observed after the 7-day TLM test. In terms of average pit depth, surface roughness, weight loss, pit area and pit density, one TCP coating outperformed the other TCP and NCP coatings. This coating bath had the highest Cr concentration (416 ppm) and the highest Zr/Cr ratio (2.1). The results demonstrate that differences in the stand-alone TCP coating performance can be observed but this depends on the nature of the accelerated degradation test used.