Summary of Structural Alloy Compatibility in Supercritical CO2 at 450°-800°C

Supercritical CO₂ (sCO₂) is of interest as a working fluid for several concepts. One of the most intriguing is the direct-fired supercritical CO₂ (sCO₂) power cycle, which has the potential to revolutionize natural gas as a low-emission power source. Over the past 10 years, laboratory exposures at 300 bar sCO₂ have found reasonably good compatibility for Ni-based alloys including precipitation strengthened 740H and 282 in sCO₂up to 800°C. Even less expensive wrought compositions like 825 with 31wt.%Fe have shown protective scale formation with no measurable C ingress. The introduction of 1%O₂ and 0.1-0.25%H₂O impurities at 300 bar increased the reaction rates ~2X at 750°C. However, initial screening tests at 1 and 20 bar CO₂ at 900°-1200°C showed poor compatibility for most materials, especially at 20 bar.

At lower temperatures, most steels are more susceptible to C ingress and embrittlement, especially at 650°C. Creep strength enhanced ferritic steels may be limited to <550°C and conventional stainless steels to <600°C. Two strategies to increase those temperatures are higher Ni and Cr alloying additions and Al- or Cr-rich coatings. Generally, thin protective Cr-rich oxide scales appear to prevent C ingress and results for alloy 709 (20Cr-25Ni) showed no C ingress or room temperature embrittlement after 2,000 h at 650°C in sCO₂. However, with the addition of O₂ and H₂O impurities, some less-protective Fe-rich oxide formed. Pack aluminized and chromized Gr.91 (9Cr-1Mo) and 316H show some promise. For Cr-rich coatings, carbide formation in the coatings on both alloys in sCO₂ at 650°C suggests their potential is limited to lower temperatures. Aluminide coatings on G91 with ~20wt.%Al were not able to form a thin Al-rich oxide at 600° or 650°C in sCO₂.