An X-ray Study on Breakaway Oxidation of Mild Steels in High-Pressure CO₂

Abstract

Strain generation behavior during the oxidation of mild steels at 400°C in pressurized CO₂, 1.45MN.m^-2, was studied by X-ray diffraction technique. The observed strains in both the steel and the surface oxide are explained qualitatively with a model derived from the combined effects of local elastic stress and lattice parameter change due to compositional change. The material with low silicon content, SS 41 A (0.003wt%), showed breakaway oxidation with high post-breakaway rate, while the other with high silicon content, SS 41 D (0.27wt%), showed very low post-breakaway rate. The onset of the breakaway in the material, SS 41 A, correlated well with the occurence of the lattice expansion. In contrast, slight contraction of the lattice in the material, SS 41 D, was observed. It is concluded that the change in lattice parameteres results from local compositional change in carbon and/or silicon at the oxide-metal interface. Metallographic observation suggests that the oxidation reaction proceeds at the inner side interface of the porous scale, and carbon deposition occurs due to the Boudouard reaction. AES analysis reveals that the carbon deposit in the breakaway scale is in a graphite state. Critical discussion is given to the previous model for the scale stress, which is based on the observed creep of mild steel foil. The obtained results indicate rather negative possibility of the strain measurement in predicting the onset of breakaway.