The oxidation behavior of the new nickel-based superalloy Inconel 740 with and without Na2SO4 deposit

The new nickel-based superalloy, Inconel 740, with and without Na2SO4 deposit were oxidized in still air between 850 and 1000 °C to investigate the oxidation and hot corrosion behavior by means of XRD, SEM and EDS. The kinetic curves of the alloy oxidized at 950 °C without Na2SO4 deposit and at 850 °C with Na2SO4 deposit obeyed the parabolic law, whereas, the uniform parabolic growth behavior of oxide was not followed at 1000 °C without Na2SO4 deposit and at 950 °C with Na2SO4 deposit due to oxide spallation at 1000 °C and the evaporation of Na2CrO4 melt at 950 °C, respectively. The oxide scales consisted of Cr2O3, (Ni,Co)Cr2O4, TiO2, Al2O3 and internal oxides at all temperatures. The internal sulfidation took place due to the existence of Na2SO4 deposit. The complex layered structure of oxide scales was in favor of the resistance to oxidation. The deposition of Na2SO4 accelerated the oxidation of the alloy and the degradation of corrosion resistance of alloy with Na2SO4 deposit was attributed to the dissolution of Cr2O3 induced by basic fluxing in molten Na2SO4.