Corrosion of various engineering alloys in supercritical carbon dioxide

Abstract

The corrosion resistance of ten engineering alloys were tested in a supercritical carbon dioxide (S-CO 2) environment for up to 3000 hours at 610°C and 20MPa. The purpose of this work was to evaluate each alloy as a potential candidate for use in the S-CO2 cooled next generation nuclear reactors. The alloys that performed well in these tests will undergo further testing and those that performed poorly will be disqualified from future deployment in S-CO2 applications. The ten alloys tested in this work were classified into four categories: Ferritic-martenitic steels, austenitic stainless steels, nickel alloys, and special materials. The majority of the alloys were focused on the five alloys within the austenitic stainless steel series, followed by three nickel alloys. These alloys were F91, HCM12A, 316SS, 31OSS, AL-6XN, 800H, Haynes 230, Alloy 625, PE-16, and PM2000. The experimental procedure consisted of placing multiple samples of each alloy in an autoclave and exposing them to S-CO2 for up to 3000 hours, in 500 hour increments. At every 500 hour increment each alloy was removed from the autoclave, photo documented and weighed. One sample from each 500 hour test was reserved for future analysis while the other samples were returned to the autoclave for further testing. The 3000 hour samples were sectioned, mounted in epoxy, and polished oriented normal to its oxide growth to document the thickness and structure of each oxide layer formed. Alloys F91 and HCM12A performed poorly and experienced substantial weight gain. Each of these alloys formed a duplex oxide layer with the outside layer being iron rich and chromium depleted and the inside layer being iron depleted and chromium rich. The oxide layers were porous and were susceptible to spallation. The 3000 hour weight gain for both of these alloys was approximately 5x10-3 mg/cm2, which was two orders of magnitude higher than the remaining eight alloys. Alloys PM2000, 316SS, 31OSS, AL- 6XN, 800H, Haynes 230, Alloy 625, and PE-16 were stable oxide formers with thin, dense oxide layers and were resistant to corrosion. The weight gain of these eight alloys was on the order of 4x10 5 mg/cm 2 at 3000 hours of exposure. Overall, the alloys with high chromium and nickel contents performed the best, followed by the stainless steels with intermediate chromium content.