Surface effects in chromate conversion coatings on 2024-T3 aluminum alloy

Chromate conversion coatings formed on samples of 2024-T3 aluminum alloy, w hich had been given different pre-treatments, were examined by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), scannin g electron microscopy (SEM) and corrosion tests. Two pre-treatments were co nsidered, namely a simple mechanical polish, and polishing followed by an e tch in a HF-H2SO4 solution. The latter treatment leads to significant Cu en richment at the oxide-alloy interface, and this in turn can lead to a delet erious effect on the corrosion protection afforded by a subsequently applie d chromate coating. Discussions are given of the behaviour of Cu in the coa ting formed on the sample that received an acid etch in the pre-treatment. This involves both migration through the coating and a non-uniform redeposi tion of Cu on to the coating surface. By contrast, the sample that initiall y was given just the mechanical polish in the pre-treatment does not show a Cu enrichment in the surface region, and the subsequently applied coating appeared stable after a 24 h immersion in a NaCl test solution. (C) 2001 Kl uwer Academic Publishers.