The test of the validity of the Fromhold-Cook theory of metal oxidation for the ${\mathrm{O}}_2/\mathrm{Fe}$ system in the tunnel regime is impeded by the growth of a passivating ${\mathrm{Fe}}^{2+}/{\mathrm{Fe}}^{3+}$ double layer at $T\le 150°\mathrm{C}$. However, during an intermediate anneal step at $200°\mathrm{C}$, the ${\mathrm{Fe}}^{3+}$ is reduced to ${\mathrm{Fe}}^{2+}$. The oxidation rate of this annealed layer is in agreement with the Fromhold-Cook theory. At room temperature, the ionic current is rate limiting. For $T>\mathrm{}150\mathrm{}°\mathrm{C}$, the thermionic emission of electrons is rate limiting for oxygen coverages larger than $13\times {10}^{15}\mathrm{atoms}/{\mathrm{cm}}^2$.

• Received 8 October 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.3366