The lattice and grain-boundary diffusivities under 0.1atm of 18O were determined at 900C in the bulk oxide and in an oxide scale on a Ni70Cr30 alloy. The diffusion profiles were determined by means of secondary ion mass spectrometry, and were analyzed in 2 domains. One of these was related to the effective diffusivity and the other was related to grain boundary diffusion. By allowing for oxidation kinetics, it was found that the bulk O diffusion coefficients which had been determined for oxide scales could be brought into very good agreement with data for the bulk oxide. The present diffusion data also led to the prediction of a parabolic oxidation constant that was equal to the experimentally measured one. Scale growth occurred via the counter-current diffusion of O and Cr; mainly via grain boundary diffusion.

S.C.Tsai, A.M.Huntz, C.Dolin, C.Monty: Radiation Effects and Defects in Solids, 1995, 137[1-4], 285-90