The Effects of Extended Exposure to Supercritical CO₂ on Select High-Performance Alloys

Supercritical carbon dioxide (sCO₂) Brayton power cycles are being evaluated as alternative to conventional Rankine steam cycle by the power generation industry. sCO₂ cycles have the capability to be coupled with a variety of heat sources (fossil fuels, solar, nuclear), while providing the potential for increased efficiency, compactness, and lower operating costs. However, the aggressive operating conditions remain a challenge in selecting the appropriate materials of construction. This study investigates the behavior of high-performance alloys, Inconel 740H, Inconel 625, Haynes HR-120, and stainless steel 316L, in long-term exposure to sCO₂. The materials were exposed to conditions approximating those at the turbine inlet of an indirectly fired sCO₂ cycle (700 °C and 200 bar) for just over 4800 h. Within the time window examined, the mass gain behavior showed that both Inconel superalloys exhibited an initial parabolic relationship up to 2000 h, while a slight deviation to a more linear relationship was observed for the remainder of the exposure. Between 1500 and 2000 h of exposure, Haynes HR-120 experienced breakaway oxidation behavior, while 316L stainless steel experienced significant spallation. The results of SEM, EDX, and XRD scans showed that Inconel 740H formed the most structurally stable oxide layer composed of a uniform Cr₂O₃ outer layer, with underlying discontinuous Al₂O₃ oxides. Although Inconel 625 exhibited the lowest mass gain, the presence of heavy elements within its oxide layer is expected to impact the stability of the oxide layer in the long term, whereas the continuous and structurally stable scale formed on 740H indicated superior long-term oxide integrity relative to the other alloys.