ELECTRON-MICROSCOPIC STUDY OF ANODIC OXIDE-FILMS FORMED ON ALUMINUM WITH THERMAL-OXIDATION

The effects of thermal oxidation on the formation of barrier-type oxide films on aluminum were investigated by electron microscopy, gravimetry, and chemical analysis. Highly pure aluminum specimens were heated in air at 773 - 873 K for 1 - 24 hr, and then anodized in a neutral borate solution at 353 K with a constant current density of 50 A/m2. Thermal oxide films were found to have a terminal thickness of ca. 30 nm after long heating periods, and to have a crystalline structure of gamma-alumina. During anodizing, the rate of increase in the anode potential, E(a), for heat-treated specimens was twice as high as that for unheated specimens. The formation efficiency of anodic oxide films was 0.80 for heated specimens and 0.72 for unheated specimens. Anodic oxide film formed on heated specimens was a single layer including gamma-alumina, and had a low thickness/E(a) ratio of 0.77 nm/V. At high E(a), amorphous oxide islands were formed in the anodic oxide layer through the local breakdown of the crystalline oxide.