High-Temperature Corrosion Behavior of Heat-Resistant Alloys in Atmosphere Containing Water Vapor and Ammonia

Abstract Various chromia-forming alloys were exposed to atmospheres of Ar-10%H₂O and Ar-10%H₂O-2.5%NH₃ at 800°C. Protective Cr₂O₃ scale formed on SUS310S and Inconel 718 in both atmospheres, and no nitride formation was observed. However, the high-temperature corrosion resistance of Alloy 800H substantially decreased in Ar-10%H₂O-2.5%NH₃ compared with in Ar-10%H₂O, and internal oxide and thick internal nitride layers formed, despite the initial formation of Cr₂O₃ scale. These results suggest that the nitrogen permeability through Cr₂O₃ scale may depend on the alloy substrate. In Inconel 718, CrNbO₄ layers formed beneath the Cr₂O₃ scale, indicating that this layer may also act as a nitrogen diffusion barrier. In Alloy 800H, because the initially formed Cr₂O₃ scale did not suppress the nitrogen diffusion, an internal nitride layer formed beneath the Cr₂O₃ scale, resulting in the breakaway of the Cr₂O₃ scale and severe internal oxidation and nitridation.