A duplex mechanism-based model for the interaction between chromate ions and the hydrated oxide film on aluminum alloys

In order to understand more clearly the nature of the anodic film formed on aluminum and its alloys, and the mechanism of inhibition of corrosion by chromates, a novel investigation has been made on the anodic film formed on AA1060 using X-ray photoelectron spectroscopy. The goal has been to elucidate whether the interaction of CrO₄^2- on Al alloy surfaces is in certain aspects analogous to that of MoO₄^2- on stainless steel surfaces and investigate whether a bipolar mechanism is applicable to aluminum surfaces. The aluminum hydroxide film was freshly formed on the metal surface and exposed to chromate and chloride solutions. Both chromate and chloride ions were observed to lead to a structural change in the hydrated aluminum anodic film consistent with deprotonation. Although both lead to deprotonation, exposure to chromate resulted in a structural transformation to an oxide layer associated with higher corrosion resistance and exposure to chloride lead to the formation of an oxyhydroxide film that is associated with a lower corrosion resistance. When the alloy was exposed to chromate prior to chloride exposure, the adsorbed chromate acted as a fixed negative charge substantially inhibiting chloride ingress. A reaction scheme, involving the formation of aluminum hydroxychlorides, has been proposed for deprotonation due to chloride. Deprotonation by chromate is explained for the first time in terms of bipolarity. Evidence suggests that two models may be operative on the surface of roughened AA106.