The surface-front oxidation mechanism of iron was investigated by time-resolved, glancing-alngle Fe K-edge fluorescence EXAFS measurements at various oxidation temperatures of 200-700 deg C. The glancing angle was chosen according to the depth of the oxide layer, roughly 1500-2000 A. The oxidation behavior under rapid heating(up to 600 deg C within 10 minutes) was compared with the slowly heated oxidation process using the Quick-EXAFS measurements. In the slowly heated process, Fe_3O_4 was the dominating phase at a relatively low temperature (300-400 deg C) initially. However, at a relatively high temperature (above 600 deg C), the Fe_2O_3 and FeO crystalline phases are gradually enriched as the successive oxidation process involving intrusive oxygen proceeded. Remarkably under a prolonged heat treatment above 600 deg C, the stable FeO phase that exists in a deep-lying interface structure and Fe_2O_3 phase eventually dominates the thick front-surface structure. In a quickly heated process, however, Fe_3O_4 phase is less dominating, which is contradictory to the commonly accepted oxidation models. The EXAFS results are discussed in conjunction with the x-ray diffraction features under the same heat treatment conditions.
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