Enrichment-dependent anodic oxidation of Zinc in Al-Zn Alloys

Abstract

As part of a systematic study of anodic oxidation of aluminium alloys, focused on the importance of initial enrichment of thin layers of alloy to the oxidation of alloying elements at the alloy/film interface, the behaviour of Al/Zn alloys is considered, which is also of relevance to surface treatment of commercial aluminium alloys. The alloys investigated, by a combination of transmission electron microscopy and Rutherford backscattering spectroscopy, are bulk Al-0.90 at% Zn alloy, in the electropolished condition, and Al-0.23 at% Zn alloy, produced by magnetron sputtering. Following anodic oxidation at 50 Am −2, both alloys reveal zinc enrichments immediately beneath the amorphous anodic films, in thin layers of alloy which contain average concentrations of about 1.8 × 10 19 and 4.6 × 10 19 Zn atoms m −2, respectively. For the Al-0.9 at% Zn alloy, the enrichment is sufficient for zinc atoms to be oxidized at the alloy/film interface and, hence, for zinc ions to be incorporated into the mainly alumina film. The incorporated zinc ions migrate outwards in the anodic film about 2.3 times faster than Al 3+ ions. For the Al-0.23 at% Zn alloy, the enrichment is insufficient for oxidation of zinc atoms to proceed and, hence, no significant amount of zinc ions was detected in the film. The oxidation behaviour of zinc in Al-Zn alloys is generally similar to that of copper in dilute Al-Cu alloys, although the incorporation of zinc ions into the anodic films does not create the high population density of flaws found in films on Al-Cu alloys following incorporation of copper ions. Further, anodic film material on Al-Zn alloys becomes detached from the substrate at a particular condition of the alloy/film interface which, in the present work, occurs at an anodizing voltage of about 100 V for the Al-0.90 at% Zn alloy. Following detachment of the film, most of the zinc in the zincenriched alloy layer is lost from the exposed alloy surface and the enrichment reforms during subsequent regrowth of the anodic film.