Single crystal superalloys are used in the hottest sections of gas turbines and jet engines because of their superior resistance against high temperature loading. Accelerated oxidation attack is expected in environments containing a critical amount of sulfur. This problem is normally avoided by controlling the sulfur content of the fuel, but may come back via sulfate containing dust in the cooling air. In this study, sodium sulfate was deposited on the surface of three different commercial single crystal superalloys in order to monitor hot corrosion behavior in a broad temperature range.
It was found that Na2SO4 promotes the formation of thick NiO layers, and, within distinct temperature intervals, leads to the recrystallization of the single crystal. The underlying damage mechanism is a cascading process which is self-sustaining, as short diffusion paths are available in the form of grain boundaries, and selective oxidation takes place of the oxide forming elements in the alloy.