Review of the gas/metal interactions in HTR helium up to 950 deg C

For the material investigations in the framework of development of the German nuclear process heat reactor corrosion studies in a reference helium atmosphere with impurities of H2, CH4, H2O, CO, N2 have been performed. Above 900 deg C the formation of oxide scales is mainly controlled by the H2O/H2- ratio and H2O can also cause decarburization by reacting with carbon in the alloys. These tendencies are opposed by carburization reactions caused by the splitting of methane and at lower temperatures by CO at the metal surface. The formation of stable protective oxides depends on a balance between oxidation and carburization reactions which are a function of both thermodynamic and kinetic factors. The available evidence indicates that achievable gas compositions exist which lead to minimal corrosion effects in candidate constructional alloys. (author)