Mobilities of foreign metallic species in anodic alumina

Using a novel approach for determining the mobilities of foreign metallic species in growing anodic alumina, the migration rates of outwardly mobile Cr3+, Cu2+, Zn2+ species have been determined to be 0.8, 3.2 and 2.3 times that of A13+ ions respectively. The approach consists of sputter deposition of a thin dilute aluminium alloy on to electropolished high purity aluminium and subsequent anodizing at a constant current density in ammonium pentaborate electrolyte. During anodizing of the thin alloy layer, an alumina film free from alloying element species is formed, with the alloying element enriched in the alloy immediately beneath the anodic film. When the alloy layer is almost totally consumed by anodizing, the alloying element in the enriched alloy layer is abruptly oxidized and incorporated into the anodic film. With further anodizing, the incorporated alloying element species migrate outward. From the position of the alloying-element-rich band in the anodic film at a particular voltage, the migration rates of the alloying element species can be determined precisely. However, when nickel species are incorporated in a similar manner, oxygen generation on the nickel-rich band occurs. The mechanism of the oxygen evolution is also discussed, based on the energy band structure of amorphous alumina modified with alloying element species.