Evaluation of supercritical oxidation resistance of boiler tube materials

To improve the efficiency of fossil fuel power plants the operating temperatures and pressures have a need to be increased. However, at high temperatures the steam side oxidation resistance becomes a critical issue for the steels used especially at the final stages of superheaters and reheaters. Apart from the chemical composition of the material, surface condition is a major factor affecting the oxidation resistance in steam and supercritical water. In this paper, stainless boiler steels (UNS S34710, S31035, S31042, S30942) are investigated in elevated steam oxidation conditions. Tests were conducted in a supercritical water autoclave environment (250 bar, with 125 ppb dissolved oxygen and a pH of 7) at 625°C, 650°C and 675°C for up to 1000h. Materials were tested with different surface finishes shot peened, milled and spark eroded. The results show an influence of surface finish on the early stages of oxidation. Oxides formed on more rough surfaces were more adherent, and thinner than on a ground surface. Tested materials with a chromium over 20 wt-% showed similar oxidation behavior at the lowest temperature, though oxides formed were dependent on the small alloying agents (Cu, Mn). More variation could be seen with increasing temperature.