High-Temperature Oxidation Response of 444 Ferritic Stainless Steel in a Synthetic Automotive Exhaust Gas

The high-temperature oxidation behavior of 444 ferritic stainless steel has been studied in cyclic oxidation experiments using synthetic automotive exhaust gas atmospheres at 950 and 1050 °C. The weight gain per unit area of the 444 ferritic stainless steel following oxidation at 950 °C for 100 h iss 85.7% lower than recorded at 1050 °C. The oxide scale at both temperatures consisted of Fe–Cr and Mn–Cr spinels in the outer layer and Cr₂O₃ in the inner layer. Nodule formation and spallation of the oxide scale are identified as the main causes of breakaway oxidation. The depth of the internal oxides gradually increases with the oxidation time. The nucleation and growth of internal SiO₂ result in the formation of metal protrusions, which are eventually consumed in the formation of a SiO₂ layer. The SiO₂ layer is formed at the interface between the oxide scale and the substrate at 1050 °C. The nucleation and growth of internal SiO₂, in combination with lateral growth of SiO₂ at the Cr₂O₃ layer/substrate interface contributed to the formation of the SiO₂ layer.