Fine insight of early germinated cavities of high temperature hydrogen attack related to low carbon steels microstructure

The High Temperature Hydrogen Attack (HTHA) phenomenon affects low-carbon ferrito-pearlitic steels submitted to hydrogen at temperatures above 200°C. Those steels are used to manufacture pressure vessels or piping, and is likely to occur in the development of promising technologies close to the carbon neutrality such as the power to gas and biomass retreatments. HTHA leads to the formation of cavities full of methane in depth of the material. As ageing in hydrogen atmosphere progresses the cavities grow and coalesce until cracking occurs. The mechanical strength of the steels is thus reduced (high risk of rupture). It is necessary to understand the origin of HTHA at the early stages, and to predict the evolution of the cavities in order to prevent accidents. High resolution X-ray tomography is the most suitable technique to characterize the small cavities (size and localization) at the early stages of the attack.