The behaviour of chromium during anodizing of Al-Cr alloys

The anodic oxidation of dilute Al-Cr alloys, containing 0.8 and 1.7 at% Cr, has been investigated in order to understand the oxidation behaviour of th e alloying element and its influences on the film composition and morpholog y. The alloys reveal two stages of oxidation: an initial stage, in which on ly aluminium atoms are oxidized to form a chromium-free anodic alumina film , and a subsequent stage, in which both aluminium and chromium are oxidized , in their approximate alloy proportions, with generation of a chromium-con taminated anodic alumina film. In the first stage, chromium is enriched in a thin layer of alloy, immediately beneath the anodic film, to an amount co rresponding to a layer of average thickness 1.5 nm and of average compositi on, Al-20 at% Cr. Following the oxidation of chromium, oxygen is produced e lectrochemically within the film at or near the alloy/film interface, proba bly associated with the development of chromium-rich clusters in the enrich ed alloy layer and, subsequently, formation of semiconducting chromium-rich oxide. Thus, the film material formed at the alloy/film interface by inwar d migration of O2- ions contains many oxygen-filled bubbles with associated high pressures. The chromium species present in the film migrate outward m ore slowly than Al3+ ions. Hence, a layer of chromium-free anodic alumina, which thickens as the film grows, is maintained adjacent to the film/electr olyte interface. (C) 1999 Elsevier Science Ltd. All rights reserved.