The early stages of anodic oxidation in the potentiostatic condition at the interface between a borate buffer solution at pH 8.4 and Fe(100), (110), and (111) surfaces are investigated by sub-second resolution real-time X-ray reflectometry. The growth process is composed of three stages. The electron density of the inner-oxide layer for Fe(111) after a potential step from-0.8 to +0.7 V versus Ag/AgCl in the steady-state growth, stage III (1.65 s <t, where t denotes the time after the potential step), is 1.52 electrons/Å3, which is close to the density of magnetite. The growth rate is proportional to t-1, which follows the direct logarithmic law or point-defect model. In an earlier stage of the oxide film growth, stage II (0.4<t≤1.65 s), the growth rate is proportional to t-1.5, while the electron density maintains the same value as that in stage III. At the first stage that can be discriminated by our time resolution, stage I (t≤0.4 s), the growth rate is the same as that for stage II, i.e., proportional to t-1.5, while the electron density significantly decreases. The suggested structure is a highly defective spinel oxide, and defects are filled by the end of stage I; there is no signature of the growth of other phases, such as hydroxide, as the inner layer. This behavior indicates that the early stage of the oxide film growth is faster than the formation of a complete spinel layer at the iron-oxide interface. The deviation of the growth rate from the point-defect model in stages I and II is caused by the strong electric field at the iron-oxide interface.
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