Interaction of Cerium Species with Alumina-Filmed Surfaces

Abstract
Cerium compounds have been shown to improve the corrosion resistance of aluminium alloys in aggressive media ; the mechanisms of inhibition are the subject of debate, with the valence state of the Ce species on the aluminium surface reported to be +3 and +4, depending on immersion time and solution oxygen content. In order to provide greater understanding, the effect of cerium species on the corrosion of superpure aluminium, with minimal second phase material, has been examined here, using XANES and analytical TEM of ultramicrotomed sections. It is revealed that cerous species are largely present, initially above a hydrated alumina film and are converted slowly to the more stable Ce⁴⁺ state in the aerated environment. Such data allow the mechanism of film growth to be elucidated and consideration of the improvement in the corrosion behaviour of the superpure aluminium substrate supporting the film. The latter results from the blocking effect of the film over the macroscopic aluminium surface, limiting the anodic process of metal loss, and reduction of the cathodic kinetics through cerium-containing deposits at preferred sites.