High-Temperature Corrosion Behavior of Martensitic Steels in Ammonia Environment

Sammanfattning

Globally, energy consumption remains dominated by hydrocarbons (coal, oil and natural gas), while the shipping industry remains a hard-to-abate sector reliant on petroleum-based fuels and a major contributor to greenhouse gas emissions. Against this backdrop, ammonia has emerged as a promising carbon-free fuel and hydrogen carrier due to its favourable chemical properties, established industrial supply chain and alignment with decarbonisation strategies. However, materials used in ammonia-fuelled engines face a particularly demanding degradation environment: elevated combustion temperatures and cyclic pressure loads combine with ammonia-induced corrosion, such that thermo-mechanical fatigue, stress-corrosion cracking and corrosion-fatigue may act concurrently and accelerate damage even at stresses well below yield strength. This thesis investigates the corrosion behavior of three martensitic steels (327F – 42CrMo4; 280T – 19MnV6; 297A – Hybrid 55) in both a dilute ammonia aqueous solution at ambient temperature and a gaseous ammonia environment at elevated temperature. Specimens were prepared as conventional coupons and Hybrid 55 additionally as stress-concentrated forms (designed U-bend and clip coupon). Corrosion resistance in 5% dilute ammonia was ranked: Hybrid 55 > 327F > 280T. In the stressed U-bend coupon of Hybrid 55, aggressive corrosion was observed in the stress-concentrated surface region where micro-pittings and accumulation of corrosion products were spotted, attributed to passive-film rupture. In ammonia-nitrided specimens, a stratified nitrided layer structure was revealed. Porous bands within the compound layer provided favourable sites for crack initiation and propagation. Though crack networks were confined within the compound layer in the conventional plate sample, several branched cracks penetrated into the substrate under tensile stress, indicating that a nitrided layer under stress is prone to SCC failure.