High-temperature corrosion of steels by nitridation in ammonia: Degradation mechanisms and comparison between steel grades

• Three steel grades were exposed to high-temperature gaseous NH 3 to study nitridation corrosion. • Nitridation was observed in all three steel grades. • Low chromium alloying led to more severe nitridation at 500 °C than at 400 °C. • Stainless steel underwent much more significant nitridation at 400 °C than at 500 °C. • Nitridation mechanism is proposed for the studied steel grades. Ammonia is an integral part of hydrogen economy and a viable carbon-free fuel for marine combustion engines. The combination of ammonia, high temperatures and long exposure times can cause nitridation corrosion in steels relevant for combustion engines, but limited research has been conducted on the topic. In this work, three steels, 34CrNiMo6 (low-alloy steel EN1.6582), X40CrSiMo10-2 (alloy steel EN1.4731), and 316plus (stainless steel EN1.4420), were exposed to gaseous ammonia atmospheres at 400 °C and 500 °C for up to 1000 h. The specimen surfaces were characterised by a variety of techniques, e.g., electron backscatter diffraction, glow-discharge optical emission spectroscopy, and micro-indentation, while the system thermodynamics was modelled with Thermo-Calc Software making use of compositional depth profile data. All materials underwent nitridation under the test conditions, and the formed nitride surface film was in most cases brittle, porous, and cracked, and typically tens of micrometres thick. In all investigated alloys, the structure, phase and elemental composition of the surface films were function of the alloying elements. For the studied stainless steel grade, the surface film compositions were dependent also on temperature, with a protective chromium nitride film being formed at 500 °C compared to an iron nitride film at 400 °C, in agreement with thermodynamics of nitride formation. The obtained results can be used to tailor the film composition in the desired direction. The study highlighted the importance of careful material selection for the conditions in ammonia combustion engines.