Oxidation of alloys containing aluminum and diffusion in Al2O3

The rate of oxidation of iron and nickel alloys containing aluminum is much faster than the rate calculated from measured diffusion coefficients of oxygen and aluminum in alumina. Furthermore the activation energy for oxidation is less than half those for the diffusion coefficients. It is shown that the Wagner theory for oxidation rates involving diffusion of ions and electrons or holes does not apply to alumina. To understand these discrepancies a mechanism of oxidation of these alloys containing aluminum is proposed that involves diffusion of AlO molecules through the alumina. The actual diffusion coefficients of the AlO molecules are larger than the measured or effective ones because they follow a diffusion-exchange mechanism. Reasonable values of the oxidation rates and their temperature dependence can be calculated from these mechanisms. (C) 2004 American Institute of Physics.