Effect of W and Mo on the Hot Corrosion Behavior of Ni-Based Superalloys

The hot corrosion behavior of W-containing and Mo-containing nickel-based superalloys was studied in air (1 atm.) in the presence of a sodium sulfate (Na2SO4) deposit at different temperatures for 24 hours. In-situ high-temperature XRD analyses reveal that the Cr and Co contents are a determining factor for the corrosion resistance of these materials. Indeed, the higher content of Cr and Co in Rene 77 promotes the formation of non-protective spinels of (Ni,Co)Cr2O4, whereas DS200 + Hf primarily leads to the formation of a protective oxide (Cr2O3). The increase of temperature and the chemical effect imparted by Mo and by W in, respectively, Rene 77 and DS200 + Hf alloys, strongly contribute to the acceleration of degradation, beyond the effects related to the microstructure. We demonstrate that the formation of Ni1−xCox(W,Mo)O4 spinels typical of acidic fluxing is favored by the presence of W in DS200 + Hf and of Mo in Rene 77, in addition to oxidation-sulfidation phenomena.