Steam Effects on the Oxidation Behaviour of Al2O3-Scale Forming Ni-Based Alloys

The effects of steam on the 1,000 °C oxidation behavior of Al2O3-scale forming Ni-based alloys were studied by conducting tests in dry air and air + 30 % steam (wet air) gas environments. It was found that the critical concentration of Al (N Al * ) to form a continuous alumina scale is increased when the environment is wet air. From the morphology of and the depth at which the internal oxides formed in the two oxidizing conditions, it was determined that the enrichment factor α remained the same in dry and wet air, though the internal-oxide precipitates were coarser and their average spacing greater in the wet air. Based on an assessment of the parameters contained in Wagner’s criterion for transition from internal oxidation to external scale formation, it was deduced that the only factor that can change the critical concentration N Al * to the extent measured is the critical volume fraction f v * .