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

Various chromia-forming alloys were exposed to atmospheres of Ar-10%H ₂ O and Ar-10%H ₂ O-2.5%NH ₃ at 800°C. Protective Cr ₂ O ₃ 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%H ₂ O-2.5%NH ₃ compared with in Ar-10%H ₂ O, and internal oxide and thick internal nitride layers formed, despite the initial formation of Cr ₂ O ₃ scale. These results suggest that the nitrogen permeability through Cr ₂ O ₃ scale may depend on the alloy substrate. In Inconel 718, CrNbO ₄ layers formed beneath the Cr ₂ O ₃ scale, indicating that this layer may also act as a nitrogen diffusion barrier. In Alloy 800H, because the initially formed Cr ₂ O ₃ scale did not suppress the nitrogen diffusion, an internal nitride layer formed beneath the Cr ₂ O ₃ scale, resulting in the breakaway of the Cr ₂ O ₃ scale and severe internal oxidation and nitridation.