The Electrochemical Behavior of the AI₃Ta Intermetallic Compound and Pitting in Two-Phase Al-Ta Alloys

To identify the role of β-phase Al₃Ta precipitates in pitting of heterogeneous Al-Ta alloys exposed to aqueous solutions, the Al₃Ta compound was synthesized in bulk form for study by conventional electrochemical and surface analytical techniques. Under conditions of anodic polarization in chloride-free buffer solutions of various pH, Al₃Ta is passive, however the phase is capable of supporting electron transfer reactions like anodic O₂ evolution and cathodic H₂evolution at enhanced rates compared to pure Al. Correspondingly, Al₃Ta is effectively noble with respect to its microstructural surroundings. Facile electron transfer appears to be traceable to the fact that the surface oxide is relatively thin and contains a high fraction (—0.4) of Ta₂O₅ which has a low electronic resistivity. Since reduction reactions occur at enhanced rates, Al₃Ta precipitates are expected to act as local cathodes under free corrosion conditions in aggressive environments. Surface analysis shows that significant Ta enrichment of an air-formed oxide film occurs upon initial exposure to solution. Subsequent anodic polarization up to the O₂ evolution potential does not induce further enrichment however.