When clean iron is exposed to oxygen at temperatures of 200°–400 °C, a film of forms rapidly on the surface. Soon after this a second phase, , begins to form in the outer surface of the layer. The rates of nucleation and lateral growth of the second phase depend on the oxygen pressure and on the crystallographic orientation of the matrix which is determined by the crystallographic orientation of the iron substrate. When the oxygen pressure is high and/or when the close‐packed oxygen‐ion planes in the are parallel to the surface, the rate of formation of the second oxide phase is high, and rapid coverage of the phase occurs. Conversely, when the oxygen pressure is low and/or when the close‐packed oxygen‐ion planes in the are not parallel to the surface, the rate of formation of the phase is lower and the coverage of the by a layer of proceeds more slowly. Coverage of the by reduces the iron‐oxidation rate by reducing the flux of iron ions outward through the layer. Thus, the more rapidly the layer forms, the more quickly the rate of oxidation is reduced. The order of oxidation rate with oxygen pressure, , and with iron crystallographic orientation, , can be explained by this mechanism.
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Boggs et al. (1967) studied this question.