High-temperature corrosion behaviours of nickel–iron-based alloys with different molybdenum and tungsten contents in a coal ash/flue gas environment

Abstract

Abstract High-temperature coal ash/flue gas corrosion behaviours of three nickel–iron based superalloys for boiler tubes with different Mo and W contents (1.8% Mo + 1.2% W for S1 alloy, 0.8% Mo + 0.2% W for S2 alloy, and 0.5% Mo + 2.2% W for S3 alloy) at 650 and 700°C were studied. The microstructure, phase compositions, and morphologies together with element distribution for corrosion products were investigated with a metallographic microscope, a scanning electron microscope equipped with an energy-dispersive spectroscope, and X-ray diffractometer. The results show that the corrosion resistance of the Ni–Fe-based alloy decreases with the increasing total content of Mo and W in the alloy at both 650 and 700°C; the outer layer of the corrosion products was mainly composed of FeCr2O4 while the inner layer was mainly composed of Cr2O3. Peeling of the oxide films formed on the surfaces of S1 and S3 alloys was observed but no obvious spalling was observed for the S2 alloy. The reactions among Mo, W, and S in the coal ash/flue gas increased the internal stress in the oxide scale, which would cause the failure of the oxide scale.