The effect of the Mn addition on the oxidation resistance of high purity Nb containing 19% Cr ferritic stainless steels has been studied using specimens isothermally oxidized in air at temperatures from 1073 to 1273 K. The structures of the scale and the scale/metal interface were investigated in detail by means of FE-SEM, SEM-EBSD and FE-TEM.
The addition of Mn was confirmed to impair the oxidation resistance. Moreover, the scale structure was significantly affected by the addition of Mn. After oxidizing, 1%Mn steel in air for 720 ks at 1123 K, 5-µm thick oxidation scale formed. The Mn rich (Mn, Cr)3O4 spinel formed in the upper most layer following MnCr2O4 just above 3-µm thick Cr2O3 scale. Beneath the Cr2O3 scale, another MnCr2O4 spinel layer formed. The grain size of Cr2O3 of 1%Mn steel was surprisingly fine with approximately 200 nm and it turned out to be much smaller than that of the Mn-free steel where the 2-µm thick Cr2O3 monolayer consisting of the similar size of Cr2O3 formed. Such a difference is postulated to be related to the oxidation resistance. These effects of the Mn addition are inferred to stem from the fast diffusion of Mn through the grain boundaries of the very fine Cr2O3 scale. Furthermore, the scale of 1%Mn steel has a tendency to be bent into folds after long oxidation.
