A chemical diffusion model for partitioning of transition elements in oxide scales on alloys

A mechanism based on lattice diffusion of cations is proposed to account for segregation of transition elements in scales on oxidized ferrous alloys. From considerations of diffusion paths through close-packed oxide lattices, the mobility of a cation is related to the difference in stabilization energies for octahedral and for tetrahedral sites, as estimated from crystal field theory. The proposed correlation between crystal field preference energy and cation mobility is substantiated by reference to experimental oxidation rates for certain transition elements. The ranking order established for ease of diffusion of cations through close-packed oxide lattices is then shown to agree with the measured distribution of elements in scales formed on iron-chromium alloys by oxidation at 600°C in CO2-based gas. The model also accords with previously published data on partitioning in oxides.