Investigation of the mechanisms of Type-II hot corrosion of superalloy CMSX-4

•Evaluated Na2SO4 hot corrosion of a common single crystal superalloy (CMSX-4).•Observed 2 stages of Type II hot corrosion depending on boundary conditions.•Examined the interdiffusion behavior of Co and Ni in the scale.•Co exhibits “uphill” interdiffusion up its own concentration gradient.•Co diffusion is enhanced down a Ni concentration gradient.

A microstructural investigation of Type-II hot corrosion was carried out on a single crystal Ni-based superalloy (CMSX-4) pre-coated with Na2SO4 at 700 °C in a flowing O2–SO2–SO3 atmosphere. Corrosion occurred by rapid dissolution of the alloy by molten sulfate eutectics, forming an outer layer rich in Co and Ni and an inner layer rich in Cr and Al. The outer layer transformed into mixed oxides of Co and Ni, with concurrent evolution of the continuous S-rich inner layer. A multicomponent analysis provides insight into diffusional interactions among Co, Ni, and S. Crystal orientation and microstructure effects were also studied.