The use of Thermodynamics and Phase Equilibria for Prediction of the Behavior of High Temperature Corrosion of Alloy 617 in Impure Helium Environment

Thermodynamic consideration was performed for Alloy 617 exposed to an impure helium ($H_2$ 20pa, $H_2O$ 0.5pa, $CH_4$ 2pa and CO 5pa) at $950^{\circ}C$. Oxidation power was decreased in the order Al > Ti > Si > Cr > Mn. Decarburization and carburization reactions were available leading to carbon activity decrease and increase, respectively, depending on carbon and Cr activities. The thermodynamic prediction was compared with the experimental results obtained in similar conditions (($H_2$ 20pa, $H_2O$ 0.05pa, $CH_4$ 5pa and CO 2pa) and $950^{\circ}C$) by others for Alloy 617. The driving force for oxidation of Al, Ti and Si is very large to be oxidized at a given impure helium and the environment is actually carburizing towards the structural alloy, which is consistent with this work.