High-Temperature Nitridation and Degradation of Austenitic Stainless Steels and Ni-Based Alloys for Ammonia Decomposition Systems: A Critical Review and Preliminary Experimental Observations

Reliable structural materials are essential for the safety of high-temperature ammonia decomposition systems, where components may experience complex NH₃/H₂/N₂ atmospheres and associated nitridation-induced degradation. This review critically evaluates the high-temperature nitridation and associated degradation mechanisms of austenitic stainless steels and Ni-based alloys exposed to ammonia-containing environments relevant to ammonia decomposition systems. The available literature is analyzed by distinguishing direct ammonia-containing evidence from mechanistic evidence obtained under non-ammonia nitriding conditions, thereby clarifying the applicability and limitations of each evidence base. The review establishes the relationships among nitrogen supply, nitrogen ingress, nitride formation, elemental redistribution, defect evolution, and material degradation. The results demonstrate that nitridation responses are governed by the coupled effects of gas chemistry, temperature, exposure history, alloy chemistry, surface-scale stability, and defect-assisted transport rather than by alloy category alone. Austenitic stainless steels generally exhibit nitrogen enrichment, expanded-austenite evolution, and Cr-rich nitride precipitation, whereas Ni-based alloys tend to develop internal nitridation associated with strong nitride-forming elements such as Cr, Al, Ti, and Nb. These behaviors represent condition-dependent tendencies rather than strict distinctions between alloy classes. Preliminary NH₃ exposure experiments on 310S stainless steel at 500 °C and Inconel 600 at 700 °C, each conducted for 300 h at an initial NH₃ pressure of 3 MPa, are included as separate case studies to illustrate nitridation-related responses rather than to rank alloy resistance. This review highlights the need for more controlled NH₃/H₂/N₂ exposure studies and coupled microstructural–mechanical characterization to support material selection for future ammonia decomposition systems.