AbstractHydrogen attack (HA) is a degradation process occurring in steel exposed to hot, high-pressure hydrogen. It is caused by the nucleation, growth, and ultimate link-up into fissures of small methane bubbles–primarily on grain boundaries. The location and growth kinetics of the submicrometre bubbles during the incubation period before fissure formation has been studied only in the past few years. Growth is limited primarily by grain-boundary diffusion, though creep of the ligaments between bubbles can be limiting under certain conditions. The processes limiting bubble nucleation are unclear though there seems to be a critical stress (methane pressure plus applied stress) above which the bubble density increases greatly. Alloying with Cr enhances the resistance to HA, primarily by reducing the bubble density rather than by reducing the bubble growth rate. The process of bubble growth and link-up is completely analogous to that by which grain-boundary voids link-up to limit the creep ductility of alloys. Simultaneous HA and creep leads to a marked acceleration of failure, though it is unclear whether this is caused primarily by the enhancement of nucleation or by growth.MST/112