Use of a sol-gel conversion coating for aluminum corrosion protection

In this study, the behavior of a sol-gel conversion coating alone and in co mbination with a polyurethane unicoat (TT-P-2756, self-priming topcoat) on Al 2024-T3 alloy was investigated under immersion in dilute Harrison's solu tion [3.5 g/l (NH4)(2)SO4, 0.5 g/l NaCl]. The sol-gel coating consisted of SiO2 and ZrO2 with a ratio of 3.4:1. The evolution of the coating system un der immersion was followed by atomic force microscopy (AFM), scanning elect ronic microscopy (SEM), electrochemical impedance spectroscopy (EIS), and X -ray photoelectron spectroscopy (XPS). It was found that though pitting cor rosion and degradation products on the sol-gel single coating surface were observed after 2 days of immersion, further pitting corrosion ceased after a few days of immersion. Under-coating blisters in the sol-gel plus polyure thane topcoat system were found at 4 weeks of immersion, after which no fur ther increase in size of the blisters was observed. It is conjectured that aluminum oxide and silicon oxide may form a stable mixed oxide barrier laye r at the interface after initial corrosion, which prohibits further pitting corrosion development. (C) 2001 Elsevier Science B.V. All rights reserved.