Oxidation of Welded Materials in High Temperature Supercritical Carbon Dioxide

Abstract Martensitic stainless steel (UNS S41000), austenitic stainless steel (UNS S31000), and nickel-based alloys (UNS N06625) specimens were exposed at 450°C and 7.6 MPa in pure supercritical CO2 (sCO2) for a total of seven months. The exposure was performed in order to assess the effect of various parameters on the oxidation of materials that may be used in oxy-combustion gas turbine systems using sCO2. Materials and environment parameters such as composition, pressure and temperature have been covered in the literature. However, engineering design will result in atypical conditions, usually localized, that are well known to affect environmental performance of materials (such as crevices, welds, stresses, or galvanic coupling). As a result of those atypical conditions, other types of failure that have not been studied in those conditions may occur, such as stress corrosion cracking, or crevice corrosion. Some of those failures resulting from engineering design are being presented in this paper. All martensitic stainless steel specimens (plain, welded, or coupled) had a matt black surface finish after the two months exposure. The austenitic stainless steel and the nickel alloy were both discolored after the exposure. Mass gain density inspection of the specimen was performed before and after exposure. The highest mass gain density was found for the martensitic stainless steel (0.5 mg/cm2), while it was close to the minimum measurable for the austenitic stainless steel and nickel alloy. The mass gain rate density decreased significantly after the first 2 months exposure from 0.35 to less than 0.1 μg/(hr·cm2) from the fourth months onward. The welded specimens of martensitic and austenitic stainless steels showed mass gain densities up to 50% higher than for the non-welded specimens. The mass gain densities of the coupled materials (galvanic coupling or similar crevice coupling) were not different from that of the single specimens but significant corrosion bonding was observed in all couples.