Molecular dynamics simulation of ammonia-induced nitriding of stainless steel at different temperatures

Ammonia is an attractive carbon-free fuel for decarbonizing combustion-based power and industrial heat. However, it potentially introduces nitriding and hydriding on hot metal walls through catalytic ammonia decomposition. The atomistic pathways of ammonia decomposition and nitrogen/hydrogen uptake in stainless steel as well as their impact on the material strength remain insufficiently understood. In this work, we develop an optimized ReaxFF reactive force field for the H-N-Fe-Cr-Ni system and use it to perform ReaxFF molecular dynamics simulations of 304 stainless steel (Fe : Ni : Cr = 7 : 1 : 2) exposed to pure ammonia at 500-700 K. We quantify the temperature-dependent uptake and penetration of nitrogen and hydrogen on an elemental basis, and evaluate post-exposure strength using atomistic tensile simulation performed at a reference temperature of 300 K to decouple chemical/structural effects from thermal softening. Our results show that elevated temperature enhances nitrogen penetration while suppressing overall hydrogen penetration. Among the conditions investigated, exposure at 600 K yields the highest yield stress. Mechanistically, nitrogen originates from ammonia dissociation, accumulates near the interface, and penetrates deeper as temperature increases. Hydrogen exhibits two distinct pathways: a dominant coupled pathway with nitrogen chemistry, in which most H forms surface N-H species and remains near the exterior before desorbing as H₂; and a minor independent pathway enabling deep H permeation, indicating that H behavior during ammonia-induced nitriding is sensitive to alloy composition. These findings provide an atomistic basis for assessing ammonia-metal wall interactions and their mechanical consequences in ammonia-fired combustors.