ENRICHMENT-DEPENDENT ANODIC-OXIDATION OF ZINC IN AL-ZN ALLOYS

As part of a systematic study of anodic oxidation of aluminium alloys, focused on the importance of initial enrichment of thin layers of all oy to the oxidation of alloying elements at the alloy/film interface, the behaviour of Al/Zn alloys is considered, which is also of relevanc e to surface treatment of commercial aluminium alloys. The alloys inve stigated, by a combination of transmission electron microscopy and Rut herford 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 A m(-2), both a lloys reveal zinc enrichments immediately beneath the amorphous anodic films, in thin layers of alloy which contain average concentrations o f about 1.8 x 10(19) and 4.6 x 10(19) Zn atoms m(-2), respectively. Fo r 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 Al3+ ions. For the Al-0.23 at% Zn alloy, the enrichment is insufficien t for oxidation of zinc atoms to proceed and, hence, no significant am ount 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 f ilms on Al-Cu alloys following incorporation of copper ions. Further, anodic film material on Al-Zn alloys becomes detached from the substra te 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 A l-0.90 at% Zn alloy. Following detachment of the film, most of the zin c in the zinc-enriched alloy layer is lost from the exposed alloy surf ace and the enrichment reforms during subsequent regrowth of the anodi c film. Copyright (C) 1996 Elsevier Science Ltd