High temperature corrosion of two superalloys in the impure helium environment

The helium coolant of a very high temperature gas-cooled reactor contains traces of impurities, which have an adverse effect on the structural metallic materials at elevated temperatures. The chromium-containing superalloys Inconel 617 and Incoloy 800 H have been the representative materials with outstanding performance, and, in this study, their corrosion behaviors were investigated under the impure helium, which was the typical HTGR operating environment. A continuous chromia-based surface scale can be developed in the impure helium, but after being above a critical temperature this oxide layer may be destroyed. This destructive phenomenon may be general for the high-temperature alloys such as Inconel 617 and Haynes 230, however, Incoloy 800 H can spontaneously resist this damage by the silica layer. This study was focused on the critical temperature calculations and measurements for Alloy 617 and 800 H, and the different corrosion behavior has been discussed.