Corrosion fatigue and damage tolerance in the nickel-based superalloy RR1000 subjected to SO2 environments

When exposed to a high-temperature corrosive environment, nickel-based superalloys may experience surface pitting and sulphide diffusion, which will influence concurrent or subsequent fatigue behavior. Sulphur, pre-existing in the environment or as a bi-product of burning fossil fuels, reacts with sodium (as an atmospheric pollutant), creating molten sodium sulphate deposits on the metal surface. Combined with sodium chloride, these deposits attack the protective oxide layer allowing sulphides to migrate along grain boundaries. Continued sulphide diffusion promotes a weakened subsurface layer, inducing grain dropout and fatigue crack initiation. The present investigation focussed upon the subsequent effects of exposure to SO2 containing atmospheres on low cycle fatigue performance, together with the impact of an intermediate cleaning process. Damage tolerance data suggest that exposure to a SO2 environment fails to affect fatigue crack growth threshold or Stage II growth behaviors when compared with standard laboratory air.