Corrosion behavior of selected structural materials in a simulated steam-cycle high-temperature gas-cooled reactor helium environment

Corrosion of carbon steel, 2 1/4 Cr--1 Mo, Types 422, 304, and 316 stainless steel, Alloy 800 (Grade 2), Alloy 800 H, Inconel 617, and Hastelloy X by gaseous impurities in a simulated steam-cycle high-temperature gas-cooled reactor helium environment was investigated. The corrosion tests were conducted at various temperatures for up to 5000 h. Oxidation was observed in all the alloys investigated except carbon steel as predicted by thermodynamic calculations. Other gas-metal reactions observed included carbon deposition on carbon steel and 2 1/4 Cr--1 Mo, and carburization in Type 316 stainless steel with the as-received surface condition (i.e., annealed and pickled). Thermodynamic considerations were given to rationalize the chemical reaction(s) that dominates the carbon potential in the test environment and to provide a basis for making thermodynamic predictions as to the occurrence of carbon deposition, carburization, and/or decarburization