INCORPORATION AND MOBILITY OF ZINC IONS IN ANODIC ALUMINA FILMS

The migration rate of zinc ions in growing anodic alumina films has be en determined as part of a systematic study for understanding the ioni c transport processes in anodic films under a high electric field. An Al-0.2 at% Zn alloy, about 35 nm thick, sputter-deposited onto an elec tropolished, high purity aluminium substrate has been anodized at a co nstant current density to various voltages at high current efficiency. During anodizing of the alloy, zinc atoms are accumulated in a layer of alloy, about 2 nm thick, just beneath the anodic film as a conseque nce of prior oxidation of the aluminium atoms. No zinc ions are incorp orated into the anodic alumina film during anodizing of the alloy. Whe n the alloy film is almost totally consumed by anodizing, zinc atoms i n the enriched alloy layer are oxidized and incorporated immediately i nto the anodic film as a result of the presence of an air-modified, el ectropolishing film on the aluminium substrate. During further anodizi ng, zinc ions migrate outwards; the migration rate of the zinc ions is about 2.3 times that of Al3+ ions. The migration rate of the zinc ion s is slower than that of the Cu2+ ions incorporated into the anodic fi lms during anodizing of dilute Al-Cu alloys, although the strengths of the single Zn2+-O and Cu2+-O bonds and the radii and valences of Zn2 and Cu2+ ions are similar.