Type II hot corrosion of the polycrystalline nickel-based superalloy AD730 in presence of Na2SO4 and Na2SO4-MgSO4 deposits

The type II corrosion resistance of AD730 was studied at 650 °C and 700 °C in air + 400 ppm SO 2 (g), in the presence of Na 2 SO 4 and Na 2 SO 4 -MgSO 4 deposits. At 650°C, the pit-like attacks were of similar depth for both salts and less severe than those observed at 700°C. Although the sulfate mixture (Na 2 SO 4 -MgSO 4 ) was liquid at 700°C from the beginning of the exposure, it proved to be less aggressive than Na 2 SO 4 . Wavelength dispersive spectrometry characterization of the pits revealed the systematic stratification of the oxides from the alloy-corrosion products interface toward the corrosion products-gas interface across the pit corrosion products, i.e. the establishment of a gradient of oxygen ions activity. High resolution transmission electron microscopy highlighted a high level of porosity, sulfur enrichment as well as the presence of the WS 2 sulfide, with an anisotropic bidimensional structure, located at the alloy-oxide interface. Sulfation tests carried out on nickel oxide and cobalt oxide in the presence of Na 2 SO 4 and Na 2 SO 4 -MgSO 4 showed little sulfation with the magnesium-containing salt, thus demonstrating how MgSO 4 contributes to limit the corrosion at 700°C. The results demonstrated that the severe hot corrosion attacks are not only related to the formation of a liquid phase, but also to its chemical composition.