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 he terogeneous Al-Ta alloys exposed to aqueous solutions, the Al3Ta compo und was synthesized in bulk form for study by conventional electrochem ical and surface analytical techniques. Under conditions of anodic pol arization in chloride-free buffer solutions of various pH, Al3Ta is pa ssive, however the phase is capable of supporting electron transfer re actions like anodic O-2 evolution and cathodic H-2 evolution at enhanc ed rates compared to pure Al. Correspondingly, Al3Ta is effectively no ble 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 Ta2O 5 which has a low electronic resistivity. Since reduction reactions oc cur 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 anodi c polarization up to the O-2 evolution potential does not induce furth er enrichment however.