Investigation of Oxidation Behavior of Alloy 617 under Air/Helium Environments at 950 degrees C

Alloy 617 is a candidate Ni-based superalloy for intermediate heat exchanger (IHX) of a high-temperature gas reactor (VHTR), because of its good creep strength and corrosion resistance at high temperature. Small amount of impurities such as H2O, H-2, CO and CH4 are introduced inevitably in helium, as a coolant during operation of a VHTR. Reactions of material and impurities are accelerated with increase of temperature to 950 degrees C of operating temperature of a VHTR, leading to material corrosion aggravation. In this circumstance, high-temperature corrosion tests were performed at 950 degrees C in air and impure helium environments, up to 250 hours in this study. Oxidation rate of 950 degrees C in an air environment was higher than that of impure helium, explained by difference in outer oxide morphology and microstructure as a function of oxygen partial pressure. An equiaxed Cr-rich surface oxide layer was formed in an air environment, and a columnar Cr-rich oxide was formed in an impure helium environment.