Barrier-type anodic film formation on an Al-3.5 wt% Cu alloy

Barrier-type anodic film formation on a bulk A1-3.5wt% Cu alloy has been examined by analytical transmission electron microscopy. Anodic alumina Nm formation proceeds in the usual way, with Al3+ egress and O2- ingress across the pre-existing film under the high electric field. Copper species are incorporated into the alumina Nm at the alloy/film interface and have a greater mobility under the held than outwardly mobile aluminium cations; the absence of any enrichment of copper species at the film/electrolyte interface also suggests their direct ejection into the electrolyte. The incorporation of copper species into the alumina film is related to clustering processes at the alloy/film interface which lead to a significant interfacial enrichment of copper and its local oxidation in the form of relatively short-lived copper oxide fingers. The generation of fine clusters and their preferential oxidation, with subsequent reformation as a result of the anodic oxidation of the adjacent aluminium matrix, explain the relatively narrow region of copper enrichment at the alloy/film interface.