Oxidation of iron: the relation between oxidation kinetics and oxide electronic structure

The test of the validity of the Fromhold-Cook theory of metal oxidation for the O2/Fe system in the tunnel regime is impeded by the growth of a passivating Fe2+/Fe3+ double layer at T</=150 degrees C. However, during an intermediate anneal step at 200 degrees C, the Fe3+ is reduced to Fe2+. 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>150 degrees C, the thermionic emission of electrons is rate limiting for oxygen coverages larger than 13x10(15) atoms/cm(2).