Characterization of surface oxide layers formed on Fe-Al alloys by annealing under different atmospheres

XPS and SIMS were used for characterizing the surface oxide layers formed on Fe-Al alloys during annealing under atmospheres with different partial pressures of oxygen, which were controlled by H₂O/H₂ ratios in the gas. The XPS results showed that an aluminum oxide (Al ₂O₃) layer was formed on the surfaces of samples annealed at a high temperature under a low partial pressure of oxygen, while such a layer was not formed on the surfaces of samples annealed under a high partial pressure of oxygen. SIMS depth profiles showed that the surfaces of samples annealed at high temperatures under a low partial pressure of oxygen were covered with a thin Al₂O₃ layer of approximately 50 nm thickness. It was also shown that oxygen penetrated the samples annealed under a high partial pressure of oxygen, and the depth profile of oxygen was correlated with that of aluminum. This indicates that internal oxidation of aluminum occurs in the samples annealed under a high partial pressure of oxygen. Grazing-incidence X-ray diffraction (GIXD) was also employed for analyzing the structure of the Al₂O₃ layer formed on the surface of samples annealed under a low partial pressure of oxygen.