Effect of alloy composition on the selective oxidation of ternary FeSiCr, FeMnCr model alloys

Abstract

AbstractIn order to understand the effect of Si and Mn concentration on the selective oxidation of Cr, the model alloys of Fe–1% Si–0.8% Cr, Fe–2% Si–0.8% Cr, Fe–1% Mn–0.8% Cr and Fe–2% Mn–0.8% Cr (all in wt%) were studied. The specimens were annealed at 820 °C in N2–5%H2 gas atmospheres with the different dew points (DPs) of − 80, − 40 and − 29 °C, respectively. Surface analyses were carried out by using the XPS and field emission (FE)‐SEM. Only binary oxides of the alloying elements were observed on the surface of all the ternary model alloys annealed at a low DP of − 80 °C. Increasing the DP to − 40 °C leads to maximum external oxidation of the alloying elements. In the Fe–1% Si–0.8% Cr alloy, annealing at the higher DP of − 29 °C promotes internal oxidation of Cr. On the other hand, increasing the Si concentration in the FeCr alloy (i.e. Fe–2% Si–0.8% Cr), the maximum external oxidation of Cr was observed due to the decrease of surface oxygen potential by Si. Moreover, the oxidation kinetics seems to play a decisive role, i.e. the oxygen competition between Si and Cr leads to partial oxidation of these elements. In both FeMnCr alloys, the effect of Mn concentration on the selective oxidation of Cr is indistinguishable, however, apart from their binary oxides (MnO/Mn3O4, Cr2O3), the MnCr chemical interaction causes the formation of ternary oxide (MnCr2O4) at the DP of − 40 and − 29 °C. Copyright © 2008 John Wiley & Sons, Ltd.