Chromia protective layers were formed on many industrial alloys to prevent corrosion by oxidation. Their role was to limit the inward diffusion of O and the outward diffusion of cations. A number of chromia-forming alloys contain Ni as a component, such as steels, FeNiCr and NiCr alloys. To ascertain if chromia was a barrier to outward diffusion, nickel diffusion in chromia was studied in both single crystals and polycrystals at 900 to 1100C at an O pressure of 10-4 atm (argon + 100 ppm O2). A nickel film of ∼35nm thick was deposited on the chromia surface and, after diffusing treatment, nickel penetration profiles were established by secondary ion mass spectrometry. Two diffusion domains appear in polycrystals, the first domain was attributed to bulk diffusion and the second was due to diffusion along grain boundaries. For the bulk diffusion domain and diffusion in single crystals, using a solution of Fick's second law for diffusion from a thick film, bulk diffusion coefficients were determined at 900 and 1000C. At the higher temperature, a solution of Fick's second law for diffusion from a thin film could be used. For the second domain in polycrystals, Le Claire's model allowed the grain boundary diffusion parameter to be established. The Ni bulk diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain boundary diffusion was slightly greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Nickel diffusion was compared with cationic self-diffusion and with literature data on Fe and Mn heterodiffusion in the bulk and along grain boundaries. All results were analyzed in relation to the oxidation process of stainless steel.

Diffusion of Nickel in Single- and Polycrystalline Cr2O3. A.C.S.Sabioni, A.M.Huntz, J.N.V.Souza, M.D.Martins, F.Jomard: Philosophical Magazine, 2008, 88[3], 391-405

Table 55

Grain-Boundary Diffusion of Fe in Polycrystalline Cr2O3