Ab initio density functional theory + U predictions of the shear response of iron oxides

The products of steel corrosion in moist air are α-Fe ₂O₃, FeO, and Fe₃O₄. Here we employ an ab initio density functional theory + U method to predict the shear properties of these oxides, to gain insight into failure mechanisms of oxide scales that form on steel. A renormalization model proposed by Mosey and Carter is employed to extrapolate atomic scale shear strengths to those of macroscopic sample sizes. The extrapolated macroscopic predictions are consistent with experimental measurements. The shear strengths of FeO and Fe₃O ₄ are predicted to be similar, ∼4.5 MPa, while the shear strength of α-Fe₂O₃ is predicted to be the largest, ∼7 MPa, for samples 5 mm thick. The preferred slip systems are predicted to be {0 0 0 1}(1 0 1 0) for α-Fe₂O₃ and {1 1 0} (1 1 0) for both FeO and Fe₃O₄. These shear strengths are significantly lower than the corresponding tensile strengths, suggesting that these oxide scales are likely to slip before fracturing.