The observation of stress effects during the high temperature oxidation of iron

Electron microscopy has been used to characterize the stress effects which occur during the oxidation of iron in the temperature range 400–700° C. Spalling and de-cohesion of the outer hematite (-Fe₂O₃) layer is often observed, and analysis of the resulting scrolled oxide indicates a strong compressive stress gradient. In contrast, tensile cracks are frequently seen in the magnetite (Fe₃O₄) layer, while the underlying wustite (Fe_1–xO) and the iron substrate are apparently able to accommodate the stresses to some extent by plastic deformation. The Pilling-Bedworth model can adequately be applied at the -Fe₂O₃-Fe₃O₄ interface since anion diffusion occurs in the hematite. However, since cation diffusion is dominant in the other oxides, it is suggested that the anion volume ratio can be applied to the Fe₃O₄-Fe_1–xO interface where the anion sublattice remains unchanged, in order to predict the stress state.