Effect of Mn content on the high-temperature oxidation behaviors of Mn-substituted-for-Ni alumina-forming austenitic stainless steel

We studied high Mn-substituted-for-Ni AFA steel (Fe-14Cr-(6–12) Mn-10Ni-2.5Al-3Cu-Nb-C AFA steel) oxidation behaviors at 800°C in air. (6–12) Mn AFA steel formed a three-layered scale: the outer layer is (Mn, Fe)-rich oxide, such as (Mn0.983Fe0.017)2O3, the intermediate is Cr-rich oxide, and the inner layer is Al2O3. (6-8) Mn-AFA steels formed a protective Al2O3 layer with stronger oxidation resistance than (10-12) Mn-AFA steels. The oxidation rate constants of 6Mn- and 8Mn-AFA steels are 1.7 x 10-3 and 2.4 x 10-3 mg2/(cm4 h), respectively, approximately 10 times higher oxidation resistance than that of 10Mn- and 12Mn-AFA steels. (10-12) Mn-AFA steels cannot form a continuous Al2O3 layer. The discontinuous Al2O3 layer allows the elements to diffuse outward, eventually forming Mn-Fe-Nb-rich oxide nodules on the surface of the steel. Mn concentrations with the potential to form protective alumina at 800°C were identified.