<p>With the increasing demand for hydrogen energy, ammonia is gaining attention as a promising hydrogen carrier by virtue of its advantages in ease of transportation and storage, simple liquefaction, and high volumetric energy density. However, degradation of structural materials has received less attention while many high-temperature ammonia utilization systems are under development. When ammonia reacts with metals, it forms metal nitride which can degrade the material properties. This paper reviews the degradation modes and mechanisms of ferritic steels and austenitic stainless steels induced by high temperature ammonia exposure. The materials are well-known in context of ammonia catalysts and nitriding heat treatments. Thermochemical factors, including temperature, pressure, atmospheric composition, and alloy type, significantly influence the nitriding phenomena. The degradation mechanism, influenced by these factors, would be discussed. A detailed understanding of the nitriding phenomena will provide insight into material selection and consideration in emerging ammonia energy industries such as ammonia cracking and ammonia combustion systems.</p>