Studying the effect of the elements Ce and Cr on the high-temperature oxidation behavior of low-Cr steels

The oxidation behavior of steels containing low-Cr concentrations (0.5-2.25 wt.%) has been studied in laboratory air in the temperature range of 400-550oC. The oxidation rate of the steels were lower than that of pure iron, but higher than that of pure iron when a small amount of rare earth element cerium (0.03 wt.%) is added to the steel 2.25Cr1Mo. The mass change follows a nearly parabolic law for the case of pure iron and the steel without Ce addition, while linear behavior describes the oxygen uptake for the case of the steel 2.25Cr1Mo+0.03Ce. SEM cross section observations and thermodynamic calculations confirm that there is no wustite (FeO) formation during oxidation of pure iron and low-Cr steels at 550oC, whereas FeO might be formed in the oxide scale of 2.25Cr1Mo+0.03Ce at the same oxidation conditions (temperature, atmosphere and exposure time). By investigating the temperature for FeO stability, this study reveals that the temperature for FeO formation on pure iron is 568oC, for the steel 2.25Cr1Mo 589oC, and 471oC for the steel 2.25Cr1Mo+0.03Ce. This low value for the FeO stability temperature found for the steel 2.25Cr1Mo+0.03Ce explains why this steel oxidizes very fast at 550oC