The lattice and grain-boundary diffusivities of 18O, under an O partial pressure of 0.1atm at 900C, were determined in bulk samples, and in a scale which grew on a Ni-30Cr alloy. The diffusion profiles were determined by means of secondary ion mass spectrometry. Two domains were analyzed in the case of polycrystalline oxide (bulk or scale). The first one was related to the apparent diffusion, and the second was related to grain-boundary diffusion. A ridge model was proposed, in the case of oxide scale samples, in order to modify the fraction of sites that was associated with the grain boundary. According to this model, the fraction was equal to 0.0006 and 0.0005 for scales which formed over 15h and 165h, respectively. The O lattice diffusion coefficients which were determined in scale samples were in very good agreement with those found for the bulk oxide. It was concluded that scale growth occurred via the counter-current diffusion of O and Cr, and involved mainly grain-boundary diffusion.

S.C.Tsai, A.M.Huntz, C.Dolin: Oxidation of Metals, 1995, 43[5-6], 581-96