Effect of microstructure on passive film formation and breakdown of Al–Ta alloys

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Abstract
Two in situ techniques dynamic imaging microellipsometry (DIM) and local electrochemical impedance spectroscopy (LEIS), have been applied to a study of the effect of microstructure on passive film formation and breakdown on Al–Ta alloys. DIM acquires ellipsometric data (thickness and optical constants of the films on surfaces) at a spatial resolution of ca. 20 µm using a radiometric full-field imaging approach. LEIS is an electrochemical impedance technique that generates local a.c. impedance data by measuring a.c. solution current densities very near the microstructural features of an electrode surface. The DIM technique was used to look at passive film formation at (a) the Al3Ta precipitate, (b) the dealloyed zone adjacent to the precipitate and (c) the solid solution phase that is present in Al-Ta alloys. The DIM technique observed different film thickness and optical constants for the films that form on these phases at potentials in the passive regions. The LEIS technique was used to study the precipitate and the region surrounding it under conditions where passive film breakdown occurs. Using these techniques, insight was obtained into the passive film formation and breakdown processes that control localized corrosion