Kinetic modeling of ammonia and hydrogen dissociative co-adsorption on iron surface and its effect on hydrogen embrittlement

We study the mitigation of environmental hydrogen embrittlement of iron by ammonia impurities in the hydrogen gas using combined theoretical and experimental methods. The competitive and dissociative co-adsorption of gaseous ammonia and gaseous hydrogen on the iron surface was investigated using density functional theory. The surface adsorption and decomposition of ammonia, as well as the ammonia partial pressure, were considered as influential factors contributing to the control of atomistic hydrogen uptake into the material. To elucidate the mechanism of ammonia competing with hydrogen on the iron surface and of ammonia mitigating hydrogen embrittlement, we develop kinetic modeling that can estimate the reaction rate and the dynamic surface coverage of different adsorbed species on the iron surface. The reaction rates for hydrogen and ammonia co-adsorption and dissociation were calculated using transition state theory combined with the Langmuir adsorption model, and a fracture toughness test was conducted to validate theoretical results. The adsorption rate of ammonia on iron is significantly higher compared to hydrogen, thus, ammonia hinders the hydrogen adsorption on the iron surface. However, partial pressure dependent ammonia decomposition also provides hydrogen atoms, which induce hydrogen embrittlement. The theoretical results of this study were supported well by experimental fracture toughness test results.