ELECTROCHEMICAL CHARACTERISTICS OF THE AL2CU, AL3TA AND AL3ZR INTERMETALLIC PHASES AND THEIR RELEVANCY TO THE LOCALIZED CORROSION OF AL-ALLOYS

The electrochemical behaviors of theta-Al2Cu phase, beta-Al3Ta and, to a lesser extent, Al3Zr were compared to high purity Al in ambient tem perature inert buffer solutions and, in certain cases, dilute halide s olutions. The aim of this work is to develop a better understanding of electrochemical characteristics of these intermetallic phases. In par ticular, information was sought concerning (a) their galvanic couple r elationship with respect to Al and (b) the relationship between the pa ssive film formed on each phase and its ability to support both cathod ic and anodic electron transfer reactions (ETR). The open circuit pote ntials (OCPs) of all three intermetallic phases were more positive tha n that of Al in inert solutions ranging from pH 2 to 12. The Al2Cu pha se 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 oxi de but supports oxygen evolution due to a combination of this effect a s well as formation of more electrically conductive copper oxides. A s imilar effect is observed for Al3Ta and is attributed mainly to the fo rmation of a more conductive mixed oxide containing Ta2O5.