THE OXIDATION OF IRON CHROMIUM MANGANESE ALLOYS AT 900-DEGREES-C

The oxidation of nine ternary iron-chromium-manganese alloys was studied at 900-degrees-C in an oxygen partial pressure of 26.7 kPa. The manganese concentration was set at 2, 6, and 10 wt.%, and chromium at 5,12, and 20 wt.%. The scales formed on the low-chromium alloys consisted of (Mn,Fe)2O3, alpha-Fe2O3, and Fe3O4. These alloys all exhibited internal oxidation and scale detachment upon cooling. The scales formed on the higher-chromium alloys were complicated by nodule formation. Initially, these scales had an outer layer of MnCr2O4 with Cr2O3 underneath, adjacent to the alloy. With the passage of time, however, nodules formed, and the overall reaction rate increased. This tendency was more marked at higher manganese contents. Although these alloys contained a high chromium content, the product chromia scale usually contained manganese. It was concluded that the presence of manganese in iron-chromium alloys had an adverse effect on the oxidation resistance over a wide range of chromium levels.