Corrosion Issues of High Temperature Reactor Structural Metallic Materials

Cooling helium of high temperature reactors (HTRs) is expected to contain a low level of impurities: oxidizing gases and carbon-bearing species. Reference structural materials for pipes and heat exchangers are chromia former nickel base alloys, typically alloys 617 and 230. And as is generally the case in any high temperature process, their long term corrosion resistance relies on the growth of a surface chromium oxide that can act as a barrier against corrosive species. This implies that the HTR environment must allow for oxidation of these alloys to occur, while it remains not too oxidizing against in-core graphite. First, studies on the surface reactivity under various impure helium containing low partial pressures of $H2$⁠, $H2O$⁠, CO, and $CH4$ show that alloys 617 and 230 oxidize in many atmosphere at intermediate temperatures (up to $890–970°C$⁠, depending on the exact gas composition). However when heated above a critical temperature, the surface oxide becomes unstable. It was demonstrated that at the scale/alloy interface, the surface oxide interacts with the carbon from the material. These investigations have established an environmental area that promotes oxidation. When exposed in oxidizing HTR helium, alloys 617 and 230 actually develop a sustainable surface scale over thousands of hours. On the other hand, if the scale is destabilized by reaction with the carbon, the oxide is not protective anymore, and the alloy surface interacts with gaseous impurities. In the case of $CH4$-containg atmospheres, this causes rapid carburization in the form of precipitation of coarse carbides on the surface and in the bulk. Carburization was shown to induce an extensive embrittlement of the alloys. In $CH4$-free helium mixtures, alloys decarburize with a global loss of carbon and dissolution of the pre-existing carbides. As carbides take part in the alloy strengthening at high temperature, it is expected that decarburization impacts the creep properties. Carburization and decarburization degrade rapidly the alloy properties, and thus result in an unacceptably high risk on the material integrity at high temperature. Therefore, the purification system shall control the gas composition in order to make this unique helium atmosphere compatible with the in-core graphite, as well as with structural materials. This paper reviews the data on the corrosion behavior of structural materials in HTRs and draws some conclusions on the appropriate helium chemistry regarding the material compatibility at high temperature.