AbstractInterdiffusion coefficients between 1000 and 1200°C have been measured in the Co-Al and Ni-Al solid-solution phases using a modified form of the Boltzmann- Matano method. Interdiffusion in the Co-Al phase is at least half an order of magnitude lower than in the corresponding Ni-Al phase. Established equations using these coefficients were used to determine theoretical critical mole fractions of aluminium necessary to form protective Al2O3scales during oxidation. While these equations are reasonably accurate in predicting the amounts of chromium necessary to form Cr2O3scales, they give inaccurate predictions of the amounts of aluminium necessary to form Al2O3scales because the kinetics of the early stages of oxidation are not taken into account.MST/144