High temperature hydrogen attack (HTHA) is a phenomenon which affects low carbon ferrito-pearlitic steels constituting pressure vessels under temperature (T>250°C) and Hydrogen pressure. It is characterized by the formation of methane bubbles at grain boundaries due to the reaction between Hydrogen and Carbon atoms in solid solution and in carbides. It leads to a decarburization of the surface of the steel. As ageing under hydrogen atmosphere progresses the bubbles grow and coalesce until cracking occurs leading to mechanical properties loss. A special attention is paid to this well-known phenomenon in petroleum refining and petrochemistry. It has to be predicted in the new power to gas reactors and in biomass energy recovery processes. The aim of the PhD thesis is to bring elements of understanding on the role of features causing methane bubbles creation and to study the interaction between these bubbles and the microstructure of the steels. In this purpose, several steels grades whose intial microstructure will be deeply characterized, will be aged in different temperatures conditions and under Hydrogen pressure in chambers specially developped for this thesis. The PhD student will characterized the microstructural evolutions of the steels before and after ageing thanks to the charaterization techniques available at the nano-characterization platform (Minatec) of CEA Grenoble.