THE ELECTROCHEMICAL-BEHAVIOR OF THE AL3TA INTERMETALLIC COMPOUND AND PITTING IN 2-PHASE AL-TA ALLOYS

To identify the role of beta-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 O-2 evolution and cathodic H-2 evolution 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 (similar to 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 O-2 evolution potential does not induce further enrichment however.