On the Composition of the Passivating Films Formed on Aluminum in Chromate Solutions

Abstract

The chemical constitution of the passive oxide films formed on a luminum in chromate and chromate/chloride solutions under simple immersion (at open-circuit potential) and under anodic polarization has been investigated by means of x-ray photoelectron spectroscopy. It has been shown that these films, whose thickness is comparable with the mean escape depth of photoelectrons, are composed of a mixture of hydrated chromium (lII) oxide, a luminum (III) oxide, and chromium (VI) specms. A considerable amount of chloride species has been detected in the films formed in chromate/chloride solutions, inde-pendently whether chlorides were initially present in the solution or were added only after 18 hr immersion of the sample in chromates. The extent of chloride incorporation within the oxide film from CRO42-/C1- solution was not modified by the anodic polarization of a luminum from open-circuit po-tential up to critical pitting potential. This situation is quite different from that observed in chloride solutions, in the absence of chromates, where the