Studies of the Initial Oxidation of Iron-Silicon Alloys by Aes, XPS, Eels, and Leed.

Thin oxide layers on polycrystalline Fe-8.75at% Si, and single crystalline Fe-6.85at% Si (110) and (111) alloys were investigated by AES (including Ar^+ and Xe^+ depth profiling), EELS, XPS and LEED during the initial oxidation stage at room temperature under very low oxygen pressure. It was observed that a very thin "SiO_2" -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 polycrystalline alloy with about 25% Si-depletion at the alloy/oxide interface. The formation of an Fe silicate -like structure just beneath 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 Fe_2SiO_4 , gradual changes of the oxidation state of Fe from its highest oxidation state ("Fe_2O _3"-like) at the top to the lowest ("FeO"-like) at the bottom were also observed. The rates of oxidation of polycrystalline alloys were determined to be retarded to approximately 40% of the rate of pure Fe after exposure to 200 L of O_2. The best annealing conditions for each single-crystalline alloy were determined to provide the cleanest surface for the following initial oxidation experiments. Annealing at ~550^circ C for 20 min after Ar^+^utter -cleaning was found to be best for Fe-Si (110). The initial oxidation results for the single-crystalline alloys were compared with those of the polycrystal alloys to show a general resemblance in the various initial oxidation features. The retardation in Fe oxidation rate with respect to that of the corresponding pure Fe surface was, however, enhanced by ~33% on (110) compared to the polycrystal case. Comparison between the (110) and (111) planes reveals only a minor crystallographic dependence of the initial oxidation of single-crystalline Fe-6.85at% Si. A general increase in the degree of the retardation of the oxidation of Fe in Fe-Si (110) was observed as the initial surface Si concentration goes up by segregation to a certain critical value, ~13at% Si. Various comparative experiments were performed to support the interpretations. ftn^1DOE Report IS-T-1343. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.