Improvement in High Temperature Oxidation Resistance of 9 %Cr Ferritic-Martensitic Steel by Enhanced Diffusion of Mn

The high-temperature oxidation behavior of coarse-grained (CG) and ultrafine-grained (UFG) 9 %Cr ferritic-martensitic steel in air at 923 K up to 500 h was investigated. The UFG sample showed considerably greater oxidation resistance than the CG sample due to the fact that the outward diffusion of Mn was enhanced and the formation of Mn-rich oxide favored in the former. A duplex-layered scale structure consisting of an outer Fe-rich (Fe, Cr)(2)O-3 layer and an inner Cr-rich (Fe, Cr)(2)O-3 layer was identified on the CG sample, while a thin compact scale with a mixture of (Fe, Cr)(2)O-3, MnCr2O4 and Mn2O3 oxides developed on the UFG sample. A continuous and stable scale composed of Cr-rich (Fe, Cr)(2)O-3 and MnCr2O4 on the UFG sample in the early stage served as a protective barrier between the matrix and environment. With increased oxidation time, formation of Mn2O3 with low growth rate improved the compactness of oxide scale.