Electrochemical characteristics of the Al₂Cu, Al₃Ta and Al₃Zr intermetallic phases and their relevancy to the localized corrosion of Al alloys

The electrochemical behaviors of θ-Al₂Cu phase, β-Al₃Ta and, to a lesser extent, Al₃Zr were compared to high purity Al in ambient temperature inert buffer solutions and, in certain cases, dilute halide solutions. The aim of this work is to develop a better understanding of electrochemical characteristics of these intermetallic phases. In particular, information was sought concerning (a) their galvanic couple relationship with respect to Al and (b) the relationship between the passive film formed on each phase and its ability to support both cathodic and anodic electron transfer reactions (ETR). The open circuit potentials (OCPs) of all three intermetallic phases were more positive than that of Al in inert solutions ranging from pH 2 to 12. The Al₂Cu phase supports the reduction of water reaction at enhanced rates relative to pure Al due to the presence of metallic Cu^o in an Al₂O₃ rich oxide but supports oxygen evolution due to a combination of this effect as well as formation of more electrically conductive copper oxides. A similar effect is observed for Al₃Ta and is attributed mainly to the formation of a more conductive mixed oxide containing Ta₂O₅.