Comparison of a chromic acid and a BTSE final rinse applied to phosphated 2024-T3 aluminum alloy

Dilute chromic acid and the organosilane bis-1,2-(triethoxysilyl)ethane (BTSE) were compared by their abilities to act as post-treatment rinses for samples of 2024-T3 aluminum alloy, which had previously been coated by zinc phosphate. The first approach is conventionally used to improve corrosion protection, but alternatives are needed because of environmental concerns with Cr(VI). Variations in conditions for application of the final rinse, especially through changes in solution concentration, were compared by measuring polarization curves, which provide an indication of corrosion stability. These comparisons were related to coating morphology, which was studied using scanning electron microscopy (SEM), and to studies of chemical composition with X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX) spectroscopy. Immersion tests in NaCl solution were applied to post-treated zinc phosphate coatings that had been formed on mirror-polished alloy surfaces. Overall, the post-treatment in 4% BTSE at 25 °C appears to provide a comparable corrosion resistance to that found when the final rinse is done in a conventional CrO3 solution (concentration 0.5 g/l) at 70 °C. At the microstructural level, initial observations were also made for comparing the effects of the post-treatments at protecting alloy matrix areas adjacent to Al–Cu–Mg second-phase particles.