Corrosion Testing of Materials in High Temperature Supercritical Carbon-Dioxide Environment

by Kumar Sridharan, Guoping Cao, & Mark Anderson
University of Wisconsin

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The supercritical CO2 Brayton cycle is being considered for power conversion system in Generation IV Fast Reactors as well as for other energy-related applications. The interest in supercritical CO2 Brayton cycles stems from its improved economics, system simplification, and high power conversion efficiencies. In the supercritical CO2 Brayton cycle the temperatures of specific components in the system could be in the range of 30oC to 650oC. The temperatures of supercritical CO2 in the recuperator, reactor, turbine, and generator sections can be quite high (about 400oC to 650oC). Long-term materials corrosion at these high temperatures will be an important issue. Therefore, the corrosion performance limits of materials at these temperatures as well as the mechanisms of corrosion must be clearly established and understood. Even in the lower temperature sections of the Brayton cycle (~ 150oC to 300oC), corrosion evaluations will be important in order to verify the feasibility of using low alloy steels to improve the economics of the process.

To investigate the corrosion performance of various candidate materials in supercritical CO2 environment, we have designed and built a supercritical CO2 autoclave facility that can operate at any combination of temperatures and pressures up to 650oC and 3900psi, respectively. The facility is comprised of eight components: 1) gas supply system, 2) high pressure CO2 pumping system 3) CO2 pre-heat system,  4) Inconel 625 temperature and pressure controlled autoclave, 5) CO2 pre-cooler, 6) pressure controlled flow and gas sampling system, 7) sample holder, and 8) computer control and acquisition system. Using this facility the corrosion performance of a number of high temperature alloys has been evaluated at 650oC and 3000psi for exposure durations of 3000 hours, with samples being removed every 500 hours for weight change measurements. Detailed scanning electron microscopy including energy dispersive spectroscopy was performed of alloy alloys upon culmination of the entire test. Tests were performed using ultra-high purity, research grade carbon-dioxide gas.

The alloys tested in this study included: (i) two ferritic-martensitic steels (F91 and HCM12A), (ii) an oxide dispersion strengthened (ODS) steel (PM 2000) containing 5.5% Al, (iii) four Fe-Ni based alloys (316 and 310 stainless steels, IN800H, and Al-6XN), and (iv) three Ni-based alloys (Haynes 230, IN625, and PE-16). The F91 (9% Cr) and HCM12A (12%Cr) steels exhibited significant oxidation even after 500 hours of testing and were removed from the tests because of concerns of contamination. The remaining alloys were tested for the entire duration of 3000 hours. In the category of Fe-Ni based alloys, 316 stainless steel (< 20% Cr) developed the thickest oxide layer, while the performance of the remaining alloys (> 20%Cr) was comparable. The Ni-based alloys outperformed the Fe-Ni based alloys, with the Al-containing PE-16 being the most corrosion-resistant. SEM-EDS analysis showed that in the 650oC supercritical CO2 environment, the presence of Al in the alloy has a profound effect on improving corrosion resistance and Cr levels greater than 20% are very beneficial.