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

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 S 1 alloy, 0.8% Mo + 0.2% W for S 2 alloy, and 0.5% Mo + 2.2% W for S 3 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 FeCr 2 O 4 while the inner layer was mainly composed of Cr 2 O 3 . Peeling of the oxide films formed on the surfaces of S 1 and S 3 alloys was observed but no obvious spalling was observed for the S 2 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.