Early stages of iron anodic oxidation: Defective growth and density increase of oxide layer

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/${\AA }^3$, 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 $(0.4<t\le 1.65$ 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 $(t\le 0.4$ 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.

Figure 1

Time evolution of x-ray reflectivity for Fe(111). The Fe potential was changed from $-0.8$ to $+0.7$ V at $t=0$. The wide range of $Q$-space was measured with the time resolution of 25 ms.

Figure 2

(a) X-ray reflectivity and (b) electron density distribution for Fe(111) obtained 0.25 s after the potential step from $-0.8$ to $+0.7$ V. Closed symbols in (a) show experimental results, and the blue curves show the 2000 samples obtained by Monte Carlo sampling. Each curve is so thin that they appear to be a thick blurred curve. Corresponding electron densities are presented in (b). The plane at $z=0$ corresponds to the Fe–inner-layer interface and that at $z\sim 30\AA$ corresponds to the inner-layer–outer-layer interface. A large uncertainty is observed for the outer layer.

Figure 3

Thickness $L$ (black) and electron density $\rho$ (red) of the inner-layer grown on Fe(111) at $+0.7$ V as a function of time $t$. Three distinct stages are observed. In stage I, the electron density is significantly lower than that in stages IIand III, and monotonically increases until the end of stage I. Direct logarithmic law $L=\alpha \ln \left(t\right)+\beta$ is followed only in stage III. The pink solid line is a guide for the eye. The error bars for $L$ are smaller than the symbol size.

Figure 4

Time evolution of the film thickness measured at (a) different potentials for Fe(111), and (b) different surface orientations at $V=+0.7$ V.

Figure 5

Growth rate as a function of time for (a) Fe(111) at the potentials of $+0.3,+0.5$, and $+0.7$ V and for (b) Fe(100), (110), and (111) at $+0.7$ V. All data sets exhibit the same trend in which growth rates in the early stages follow $dL/dt\propto t^{-1.5}$ and in the later stage follow $dL/dt\propto t^{-1}$. The time evolutions following $t^{-1.5}$ and $t^{-1}$ are shown by the dashed lines.

Figure 6

Growth rate vs thickness for (a) Fe(111) under $+0.3,+0.5$, and $+0.7$ V, and for (b) Fe(100), (110), and (111) under $+0.7$ V. Three stages are observed for each set of data.

Figure 7

Schematic of the iron potential profile around the metal-oxide interface. The thin curves show the profiles for the continuum approximation or ordinary PDM, and the thick curves show the profiles for the atomistic model. Black and red profiles show the situations for thick oxide film and thin oxide film cases, respectively. ${\varphi }_{m/f}$ denotes the potential difference between the metal and the oxide regions (see the text). Inset: The potential curve for a wider $z$ range used for ordinary PDM.