The diffusion of 60Co, 63Ni and 45Ca at 953C, and of 51Cr at 953 and 1109C, was measured in polycrystalline CoO scale by using tracer and sectioning techniques. High-density scales were obtained by oxidizing high-purity Co plates at 1000 or 1200C in air. It was found that the diffusion penetration profiles exhibited 2 well-defined slopes which corresponded to different diffusion regimes. The bulk diffusion coefficient which was determined from the diffusion profiles with the higher slope was in good agreement with reported bulk diffusion coefficients. The product of the grain-boundary thickness, diffusivity and segregation coefficient was deduced from the diffusion profiles with the lower slope. It was found that the grain boundary diffusion enhancement factors had similar values for 60Co and 63Ni, and were an order of magnitude higher than those for 45Ca and 51Cr. The presence of Ca in the grain boundaries was confirmed by auto-radiography and secondary ion mass spectrometry. It was noted that the diffusion coefficients for Ni and Ca at 953C had the same values for specimens which had been prepared at 1000 or 1200C. This was so in spite of their differing porosities and grain sizes.

K.Kowalski, E.G.Moya, J.Nowotny: Journal of the Physics and Chemistry of Solids, 1996, 57[2], 153-63