Nearly all metals form a passivation film due to oxidation in air at ambient temperatures, that acts as a diffusion barrier to protect the materials from further corrosion. Aluminum demonstrates excellent passivation behavior due to the formation of a protective amorphous alumina film during exposure to air at ambient temperatures. However, H. Ebinger and J. Yates discovered that the passivation of aluminum can be significantly improved by artificial oxidation. Both electron-beam induced oxidation in water vapor and oxidation in ozone atmospheres3 showed higher impedance in electrochemical impedance spectroscopy measurements to anion diffusion than the thermally grown oxides. To understand the nature of this beneficial passivation, we probed the microstructure of these amorphous oxide films by transmission electron microscopy (TEM).
The oxide films were grown on a polycrystalline Al substrate. The Al substrate was cleaned with a sputter cleaner inside a UHV (ultra-high vacuum) system.