Microstructural investigation of Alloy 617 corroded in high-temperature helium environment

The high-temperature corrosion behavior of Alloy 617, which is a candidate material for the intermediate heat exchanger (IHX) in a very high-temperature gas-cooled reactor (VHTR), was investigated. The corrosion tests were carried out at 850–950 °C in impure helium environments. All specimens showed a parabolic oxidation behavior at up to 250 h, and the activation energy for the rate constants of the mass change was similar to the activation energy for Cr/Cr2O3 oxidation in air. SEM micrographs revealed the evolution of the outer oxide layer, internal oxide, and depleted zone of the grain boundary carbides. Corrosion test in a pure helium environment showed that the minor impurity gases in helium environment affected the oxidation kinetics of Alloy 617. It is suggested that the control of minor impurities in a VHTR helium environment is necessary to apply Alloy 617 to an IHX material in a VHTR.