Roles of Manganese in the High-temperature Oxidation Resistance of Alumina-forming Austenitic Steels at above 800 °C

A new family of alumina-forming austenitic (AFA) stainless steels is under development for uses in aggressive oxidizing conditions. This paper investigates the effect of manganese additions on the oxidation kinetics and alumina scale formation in two series of AFA steels, i.e., Fe-20Ni-14Cr-2.5Al and Fe-18Cr-25Ni-3Al base. At 800 degrees C in dry air, the oxidation resistance was moderately degraded with additions of larger than 1 wt% Mn in the AFA steels based on Fe-14Cr-20Ni-2.5Al. At 900 degrees C in air with 10 % water vapor, however, additions of Mn in these AFA steels based on Fe-18Cr-25Ni-3Al would significantly destabilize the alumina scale formation and degrade the oxidation resistance. Our analysis revealed that additions of Mn stimulated formation of the coarse spinel CrMn1.5O4 and Cr2O3 oxide and destroyed the continuity of the protective alumina scales, thus worsening the oxidation performance. In addition, it was found that there exists an upper limit for the Mn additions which is decreased with the increase of the service temperatures and presence of aggressive oxidizing agents.