High temperature oxidation behavior of type 444 stainless steel in synthetic automotive exhaust gas

The oxidation behaviors of type 444 ferritic stainless steel (FSS) were studied in air and synthetic exhaust gas at 1000, 1050 and 1100 °C, respectively, by means of mass gain, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The oxidation rate of 444 FSS enlarges in both environments with the increase of temperature, but is faster in synthetic exhaust gas than in air under isothermal conditions. The oxidation rate becomes very fast since the occurrence of breakaway transition. Both the higher temperature and the synthetic exhaust gas facilitate the breakaway oxidation. The breakaway oxidation results mainly from the growth of oxide nodules and the spallation of oxide scale. The synthetic exhaust gas isn't conducive to the formation of SiO2 layer in comparison with the air. The oxidation mechanisms are discussed on the basis of the oxide scale characteristics.