Nitridation Resistance of Alloys in Pressurized Ammonia Operation: Effects of Temperature and H2O

Ammonia is emerging as a promising carbon-free fuel that can be converted to energy using combustion engines and fuel cells or converted to hydrogen in a cracker. The structural alloys in these applications are exposed to harsh conditions, notably high temperature and presence of NH 3 and H 2 O, under which their behavior remains insufficiently understood. In this work, the nitridation behavior of seven alloys (625, 600, 800HT, AISI 316L, AISI 310S, AISI 444 and EF101) was systematically studied as a function of temperature (495–670 °C), NH 3 concentration (6–85%) and H 2 O content (0–2%). Mass gain measurements combined with microstructural characterization were used to quantify nitridation and identify degradation mechanisms. Nitridation resistance increased with decreasing Fe-content and increasing Ni-content of the alloy, with alloy 625 showing the highest resistance across the investigated conditions. Increasing temperatures and the presence of 2% H 2 O accelerated nitridation in alloys 600, 800HT and 316L, but suppressed it in alloy 444. The improved nitridation resistance of alloy 444 is attributed to the formation of a protective oxide scale, highlighting the critical interplay between oxidation and nitridation in environments containing both NH 3 and H 2 O.