It was noted that the oxidation behaviour of austenitic stainless steels at 723 to 1173K was strongly influenced by the grain size of the oxidizing alloy. Here, the evolution of the concentration profiles of Cr, Ni and Fe in the substrate below a growing Cr2O3 layer was simulated using a Fisher-type numerical model, which took both volume and grain boundary diffusion into consideration. The model was based upon a two-dimensional control volume-based solution of Fick's second law for multicomponent diffusion and included cross-term diffusion coefficients. The oxide layer was assumed to grow according to a parabolic rate law as a consequence of rate limiting diffusion of Cr cations through the oxide layer; the retraction of the oxide/alloy interface associated with the removal of Cr atoms from the substrate was included in the calculations. Numerically, the movement of the oxide/alloy interface was formulated such that the initial mesh could be used throughout the calculation. The calculated concentration profiles of the alloying elements emphasized the importance of grain boundaries in supplying Cr from the alloy to the growing oxide layer. At 823 and 923K, the simulations predicted a significantly lower concentration of Cr atoms in the alloy at the oxide/alloy interface than that predicted by the conventional one-dimensional analytical Wagner solution, where an effective diffusion coefficient at the interface was assumed.

Modelling Cr Depletion under a Growing Cr2O3 Layer on Austenitic Stainless Steel - the Influence of Grain Boundary Diffusion. A.N.Hansson, J.H.Hattel, K.V.Dahl, M.A.J.Somers: Modelling and Simulation in Materials Science and Engineering, 2009, 17[3], 035009