Abstract
The migration rate of zinc ions in growing anodic alumina films has been determined as part of a systematic study for understanding the ionic 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 electropolished, high purity aluminium substrate has been anodized at a constant 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 consequence of prior oxidation of the aluminium atoms. No zinc ions are incorporated into the anodic alumina film during anodizing of the alloy. When the alloy film is almost totally consumed by anodizing, zinc atoms in the enriched alloylayer are oxidized and incorporated immediately into the anodic film as a result of the presence of an air-modified, electropolishing film on the aluminium substrate. During further anodizing, zinc ions migrate outwards; the migration rate of the zinc ions is about 2.3 times that of Al 3+ ions. The migration rate of the zinc ions is slower than that of the Cu 2+ ions incorporated into the anodic films during anodizing of dilute Al Cu alloys, although the strengths of the single Zn 2+ O and Cu 2+ O bonds and the radiiand valences of Zn 2+ and Cu 2+ ions are similar.
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