CONTRIBUTION TO UNDERSTANDING PARABOLIC OXIDATION-KINETICS OF DILUTE ALLOYS .2. OXIDES WITH METAL DEFICIT OR OXYGEN EXCESS

Concerning the parabolic oxidation of alloys with a low content of a d ifferent valence element it is shown that the Wagner-Hauffe valence ap proach contains some inaccuracies. This paper is devoted to the growth of an oxide MO with metal deficit or oxygen excess. The case is devel oped where the doping is high enough to modify the defect concentratio ns over the entire oxide scale. It is shown that lower-valence metal a dditions decrease the scaling constant and modify the influence of oxy gen pressure. For additions of higher-valence metals, as long as elect ronic conductivity remains over the whole scale, the limiting step mig ht be the diffusion of ionic defects under only an electric, with an i ncrease of the scaling constant. When the conductivity becomes essenti ally ionic over the whole scale the limiting step might be the diffusi on of electron holes, and the scaling constant will decrease with an i ncrease in the doping level. Globally the parabolic constant might the refore increase and pass through a maximum before decreasing.