Oxidation of copper and mobility of copper ions during anodizing of an Al—1.5 wt.% Cu alloy

Abstract

AbstractThe mechanism of oxidation of copper at the alloy/film interface, and the subsequent migration of copper ions in barrier‐type films, has been examined for anodizing of an Al—1.5 wt.% Cu alloy with a prior chemical polishing treatment. Both chemical polishing and anodizing result in formation of a thin layer of alloy at the alloy/film interface, of ∼2 nm thick, that is highly enriched in copper. The layer is present immediately beneath the different types of film formed by chemical polishing and subsequent anodizing, and contains in both cases ∼6 × 1019 Cu atoms m−2. The amount of copper contained within the enriched layer of alloy is not significantly dependent upon the anodizing voltage. During anodic film growth, both aluminium and copper ions are incorporated into the film at the alloy/film interface, on average in their alloy proportions. However, the film is depleted in copper relative to the alloy because copper ions in the film migrate faster than Al3+ ions and, on reaching the film/electrolyte interface, are ejected directly to solution. The mechanism of oxidation of copper is proposed to depend upon the formation, through prior oxidation of aluminium, of copper‐rich clusters in the enriched layer of alloy at the alloy/film interface. Individual clusters are oxidized only on achieving a critical size. Consequently, copper is incorporated into the film discontinuously both in time and in position along the alloy/film interface. The films contain a high population density of flaws, which affects the film composition, the uniformity of ionic current, the faradaic efficiency of film growth, and the detailed distributions of copper ions within the films. However, the general features of film growth are compatible with the usual growth mechanism of anodic alumina, with transport numbers of Al3+ and O2−/OH− ions of ∼0.4 and ∼0.6, respectively.