INTERACTION OF CERIUM SPECIES WITH ALUMINA-FILMED SURFACES

Cerium compounds have been shown to improve the corrosion resistance o f aluminium alloys in aggressive media; the mechanisms of inhibition a re 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 underst anding, the effect of cerium species on the corrosion of superpure alu minium, with minimal second phase material, has been examined here, us ing XANES and analytical TEM of ultramicrotomed sections. It is reveal ed that cerous species are largely present, initially above a hydrated alumina film and are converted slowly to the more stable Ce4+ state i n the aerated environment. Such data allow the mechanism of film growt h to be elucidated and consideration of the improvement in the corrosi on behaviour of the superpure aluminium substrate supporting the film. The latter results from the blocking effect of the film over the macr oscopic aluminium surface, limiting the anodic process of metal loss, and reduction of the cathodic kinetics through cerium-containing depos its at preferred sites.