A new solid-state mode of hot corrosion at temperatures below 700°C

Preliminary results on a single-crystal nickel-based superalloy indicated that hot corrosion can occur at temperatures as low as 550 degrees C, where liquid formation, generally believed to be responsible for Type II hot corrosion, is not predicted. Additional tests were conducted on pure-nickel samples at 650 degrees C and below to more clearly elucidate the mechanism of this very low-temperature hot corrosion. Environments in dry air and O-2-(2.5, 10, 100, and 1000) ppm SO2 were studied. Based on the results obtained, a solid-state corrosion mechanism was inferred. The mechanism relies on the formation of a previously unreported compound phase, which was identified using transmission electron microscope analysis that indicated the stoichiometry of Na2Ni2SO5. Furthermore, it was nanocrystalline in structure and metastable. It was deduced that the Na2Ni2SO5 formation was responsible for the rapid nickel transport required for the observed accelerated corrosion process. Moreover, its eventual decomposition resulted in a mixed product of porous NiO with embedded particles of Na2SO4. Application of the proposed mechanism to nickel-based alloys is discussed.