Degradation of ferritic stainless steels at 1200 °C in air

Three commercial ferritic stainless steels were investigated at 1200 °C by a thermogravimetric analyser (TGA) in air. The oxidation kinetics of the ferritic stainless steels differed significantly. The adhesion of the Cr2O3 scale, the morphology of the SiO2, with or without (Cr, Mn)3O4 spinel on the top can greatly influence the oxidation and the degradation behaviour of the steels. Although the SUS430 steel had less Cr among the ferritic stainless steels (16.2 wt% Cr), it did not show more degradation behaviour than the B443NT steel (21.0 wt% Cr). The spallation of the protective oxide scale on the B443NT steel was caused by vacancy condensation at the scale/substrate interface where the SiO2 particles were and the developed compressive stresses within the oxide scale during oxidation. (Cr, Mn)3O4 spinel on the top of the Cr2O3 scale on the B445J1M steel influenced its evaporation rate. The thermodynamic aspect of the chemical composition and oxidation atmosphere of the Fe–Cr–O system was also discussed. Three commercial ferritic stainless steels exhibit different degradation behaviours during oxidation in a reheating temperature. Results show that even if Cr content is known to be a key parameter in oxidation resistance, it is not sufficient. The morphology of the SiO2 at the steel/oxide interface and the developed compressive stresses within the Cr2O3 scale influence its adhesion to the steel substrate. The rehealed Cr2O3 scale is stable at 1200 °C in air.