It was noted that the rate of oxidation of Fe and Ni alloys which contained Al was much faster than the rate calculated from the measured diffusion coefficients of O and Al in alumina. Furthermore the activation energy for oxidation was less than half of those for diffusion. It was shown that the Wagner theory for oxidation rates involving diffusion of ions and electrons or holes did not apply to alumina. To understand these discrepancies, a mechanism of oxidation of these alloys containing Al was proposed that involves diffusion of AlO molecules through the alumina. The actual diffusion coefficients of the AlO molecules were larger than the measured or effective ones because they follow a diffusion-exchange mechanism. Reasonable values of the oxidation rates and their temperature dependence could be calculated from these mechanisms.

Oxidation of Alloys Containing Aluminum and Diffusion in Al2O3. R.H.Doremus: Journal of Applied Physics, 2004, 95[6], 3217-22