The Appearance of Pitting on Thermally Grown Aluminum Oxide caused by Surface Segregation of Fe Impurities from the Bulk

Abstract

Abstract The oxidation of 99 wt.-% purity (low purity – main impurities were Fe and Si) and 99.999 wt.-% purity (high purity) Al foil was studied using a thermo-gravimetric method. The Al oxide was characterized with several techniques such as scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS). The Al foils were oxidized in a 50 % O2 – 50 % Ar mixture between 773 and 843 K. Microscopic examination (SEM) of the oxide revealed that “pitting” occurred on the low purity Al. High Si and Fe concentration were found on the surface or near-surface of the oxide. The presence of Fe, as precipitates, on the surface of the oxide accounts for the pitting of the oxide. XPS inspection revealed the formation of γ-Al2O3 with the presence of some Al hydroxide.