Studies of the initial oxidation of Fe-Si alloys by AES, XPS and EELS

Abstract

Thin oxide layers on polycrystalline Fe-8.75at%Si alloys were investigated by various surface spectroscopies, i.e., AES (including Ar+ depth profiling), EELS, and XPS during the initial oxidation stage at room temperature under very low oxygen pressure. It was observed that a very thin “SiO2”-rich external layer is formed, as predicted by others, and established preferentially at the first stage of oxidation. A Si-depletion zone ∼5 A deep was found in the alloy with about 25% Si-depletion at the alloy/oxide interface. The formation of an Fe silicate-like structure just benath the Si oxide-rich top layer down to the alloy/oxide interface was also observed. Although this Fe-silicate layer was determined to be predominantly in the form of Fe2SiO4, gradual changes of the oxidation state of Fe from its highest oxidation state (“Fe2O3”-like) at the top to the lowest (“FeO”-like) at the bottom are also observed. The rates of oxydation of these alloys were determined to be retarded to approximately half the rate of pure Fe after exposure to 200 L of O2. The retardation involves the blocking of outward diffusion of Fe, reduction in the inward arrival rate of O by the top two network-forming oxide layers (the Si oxide-rich layer and the underlying mixed layer of trivalent Fe oxide and Fe-silicate), and, work of other, blocking of the alloy grain boundaries by the entire Fe-silicate layer with grains of very small size. Various comparative experiments were performed to support the interpretations.