ELECTROCHEMICAL AND TRANSMISSION ELECTRON-MICROSCOPIC CHARACTERIZATION OF METALOXIDE FILMS

Oxide films have been formed electrochemically at several metallic substrates, including base metals such as a new Al-based microcrystalline alloy (Al-Fe, V, Si) and at polycrystalline Pd electrodes. The Al-based materials were oxidized in sulfuric acid media in order to attempt to form adherent and strong anodic oxide films. The cyclic voltammetry and current transients indicated more active electrochemistry of the alloy than of essentially pure Al, consistent with the expected solubility of the Fe and V components of the alloy. ac impedance analysis indicated that the underlying barrier oxide film was either absent altogether or flawed in nature. These results have been tested by examination of cross-sectional specimens by transmission electron microscopy (tem), prepared by ultramicrotomy techniques, which showed that the barrier film was missing at some sites and significantly flawed in others. The overlying porous film contained irregular pore distribution, branching at different angles. It appears that sulfuric acid is too aggressive a medium for the formation of a stable porous oxide film at this alloy. Relatively thin hydrous oxide films were formed at Pd electrodes in alkaline solutions. The electrochemistry suggests a growth mechanism involving the conversion of islands of compact alpha-oxide to columnar hydrous beta-oxide film material, with the remaining patches of alpha-oxide remaining unchanged in thickness. The beta-oxide must be very porous in nature, as the alpha-oxide can be reduced and reformed readily even in the presence of a thick, overlying beta-oxide film. These structural hypothesis have been confirmed by tem analysis of thin hydrous Pd oxide films.