Microstructure of oxide scales formed at early stage of oxidation of 17%Cr-Mn-Ni austenitic stainless steel was examined using SEM and TEM. Samples were oxidized at 973 to 1373K for 1 to 15 min. Nodules were observed on the surfaces of specimens oxidized at 1173K. The nodules were composed of two layers which were Fe oxides and Fe-Cr-Mn oxides rich in Cr, respectively. The other scales were composed of two layers of Fe-Cr-Mn oxides with different compositions. In the specimens oxidized at 973 and 1373K, no noduls were observed on the surfaces. The scales were composed of three layers after oxidation at 1373K for 1 min. The first layer consisted of Fe-Mn oxides with spinel structure, the second one consisted mainly of Cr oxides with corundum structure, and the last one consisted of Cr-Mn oxides. It is considered that Cr-Mn oxides with spinel structure, which result in the loss of resistance to oxidation, are formed under Cr2O3 layer which is formed at earlier stage of oxidation. Moreover, the oxide scale was characterized by uneven interface of the second layer and the last one.
