Mechanical and Corrosion Response of 316SS in Supercritical CO2

The supercritical carbon dioxide (s-CO2) Brayton cycle is currently being explored as a replacement for the steam Rankine cycle due to its potential for higher efficiency and lower cycle cost. 316 stainless steel is a candidate alloy for use in s-CO2 up to roughly 600 degrees C, but the mechanical effects of prolonged exposure of base and welded material in s-CO2 have not been analyzed. The potential for carburization makes this an important concern for the implementation of 316 and similar austenitic stainless steels in the s-CO2 environment. In this study, welded and base material of two types of 316-316L and 316H-were exposed in either s-CO2 or argon at 550 degrees C or 750 degrees C for 1000 h. 550 degrees C s-CO2 exposure yielded a thin (< 1 mu m) Cr oxide with occasional nodules of duplex Fe oxide and Fe-Cr spinel that were up to 5 microns thick. However, tensile results from s-CO-2 exposure matched those of 550 degrees C thermal aging in Ar, indicating that no mechanically detrimental carburization occurred in either 316 variant after 1000 h exposure. Conversely, 750 degrees C s-CO2 exposure produced roughly 10 x the oxide thickness, with a more substantial Fe oxide (3-5 mu m) on the majority of the surface and nodules of up to 40 mu m thick. In comparison to aged samples, tensile testing of 750 degrees C CO2-exposed samples revealed ductility loss attributed to carburization. Projections of 316L performance in s-CO2 indicate that mechanically detrimental carburization-equal to that shown here for 750 degrees C, 1000 h-will likely be present after 7-14 years of service at 550 degrees C.

Automated summary

The study found that exposure to s-CO2 at 550 degrees Celsius did not result in detrimental carburization for the 316 stainless steel variants, while exposure at 750 degrees Celsius led to ductility loss attributed to carburization. Projections suggest that after 7-14 years of service at 550 degrees Celsius, mechanically detrimental carburization similar to that at 750 degrees Celsius may likely occur for 316L in s-CO2.