Behavior of Metallic Materials Between 550 and 870°C in High-Temperature Gas-Cooled Reactor Helium Under Pressures of 2 and 50 bar

In order to estimate the influence of the helium pressure on the corrosion of ferritic and austenitic materials, tests were carried out under 2 absolute bar in a circuit without helium recirculation and under 50 bar in the AIDA loop. In both cases the partial pressures of impurities were 1.500, 50, 450, and 50 μatm for H₂, H₂O, CO, and CH₄, respectively. The interruption of the French high-temperature gas-cooled reactor R&D program has only produced limited results: 1. At 650°C the behavior of 11% chromium ferritic steel HT 9, Types 304 and 316 austenitic steels, and Incoloy Alloy 800H is excellent; the oxidation rates are low and decrease with time. 2. At 750 and 870°C, Hastelloy-X offers better resistance to external and intergranular oxidation than alloys 800H and Inconel-617. 3. At these three temperatures, the oxidation kinetics are appreciably faster under a pressure of 50 bar than under 2 bar. 4. Whereas carbon steel is subject to decarburization at 550°C, a carburization phenomenon is observed for alloys 800H, Inconel-617, and Hastelloy-X at 750 and especially at 870°C. 5. As for the influence of the initial surface preparation, mechanically polished specimens generally present a lower oxidation rate than those polished electrochemically.