Corrosion of alloy 800 exposed to high temperature helium

• High temperature helium exposure leads to the formation of non-uniform surface corrosion layer on Alloy 800 • Elemental segregation occurs after less than 150 h of exposure, particularly Cr • An Mn-rich oxide layer (with Cr) begins to form after 1000 h exposure • Formation of corrosion layers follows Ellingham Diagram thermodynamic predictions Alloy 800 is currently being considered as a suitable alloy for use within the next generation of High Temperature Gas Cooled Reactors (HTGRs). These reactors will operate using Helium as the heat transfer gas at temperatures of 700 °C and higher. HTGR materials need to be able to withstand these high temperatures over service lives of multiple decades in the presence of helium and impurities present either within the gas or from ingress to the system. This work exposed Alloy 800 at 750 °C in a helium atmosphere for up to 1000 h and characterised the materials using advanced microscopy techniques. Within the bulk material, Mn and Cr segregation to the grain boundaries took place at less than 150 h of exposure due to the high temperature and similar segregation was seen towards the surface, both along grain boundaries and within grains. A non-uniform Cr- and Mn-rich oxide forms on the surface of Alloy 800 after 150 hours with the uniformity and thickness increasing with exposure duration. Al, Ti, Si and Mn are all seen to oxidise ahead of the bulk material and before Cr, with the development of an Al-rich internal oxidation zone and the formation of a non-uniform Mn-rich oxide forming at the surface of the alloy. Cr later migrates to the surface and forms part of this oxide. The thermodynamics of the oxidation reactions are in agreement with Ellingham diagram predictions with the alloying elements oxidising in order of stability.