Steam oxidation of ferritic heat-resistant steels for ultra supercritical boilers

Steam oxidation of heat-resistant steels is expected to be one of the major concerns for degradation of ultra supercritical (USC) boilers for an advanced power generation plant. This paper focuses on steam oxidation kinetics of newly developed heat resistant ferritic steels, which contain either 2.2%, 9%, or 11% Cr and about 2% W (HCM 2 S, NF 616, HCM 12 A). Oxidation rates of those steels were measured in a pressurized superheated steam as functions of temperature and pressure. Oxidation rate always showed parabolic nature. Effects of chemical composition and pressure showed interesting contrast depending on testing temperature ranges. In low temperature ranges, 570°C-600°C, clear negative effects of steam pressure and no compositional effect were found on the oxidation rate. In contrast, steam pressure effect was minor and chemical composition (Cr content) became dominant in higher temperature range, 620°C-700°C. Structure and chemical composition of oxides scales were analyzed using an analytical electron microscope and X-ray diffraction technique. Although oxide scales were always formed in a duplex layered structure for all the steels, oxide type, structure, and chemical composition of the oxide varied depending on material and testing condition. Formation of a Cr-enriched thin layer in the inner scales and oriented growth of the inner scale oxides were correlated with the oxidation kinetics.