On the chemomechanics of bubble growth in hydrogen attack of plain carbon steels

High temperature hydrogen attack (HTHA) is a form of degradation of carbon steels exposed to high temperature and high-pressure hydrogen whereby internal hydrogen reacting with carbides forms methane gas bubbles with an associated loss in strength and toughness due to decarburization. Grain boundary gas bubbles can grow and coalesce leading to microcrack formation and frequently to premature fracture. Current models mainly rely on the Grabke and Martin transient methane generation kinetics which is based on carburization/decarburization experiments on iron surface at temperatures between 600 °C and 800 °C, though those temperatures are much higher than those at which HTHA is observed in industrial processes. This work presents a coupled chemical kinetics and micromechanics model that addresses methane and hydrogen gas formation along with simultaneous decarburization and bubble growth over a wide temperature range. The energetics of the chemical reactions taking place at the ferrite-matrix/bubble interface are established through DFT calculations. Model calculations unveil the relationship between the rates of hydrogen migration to the bubble interface, carbon and hydrogen atom reactions for methane formation, and attendant volumetric bubble growth. The model predicts equilibrium methane bubble pressures that agree with those predicted by existing models at high temperatures. Significantly, the model predicts equilibrium methane pressures that are remarkably lower than the extrapolated predictions of the existing models at lower temperatures, e.g., 250 °C. In summary, the model establishes a methodology to understand and quantify methane pressure development and decarburization across the length and time scales that are relevant to hydrogen attack.