Evaluation of corrosion behavior of steels for direct supercritical CO₂ power cycle applications

In order to investigate corrosion issues of heat exchanger materials at intermediate temperatures in direct supercritical carbon dioxide power cycles, a series of autoclave exposure tests and electrochemical experiments have been conducted. Potential heat exchanger materials are austenitic stainless steels and ferritic-martensitic steels. The autoclave tests were performed in fluid mixtures containing CO₂, O₂, and H₂O at 8 MPa and at two temperatures, 50°C and 245°C, to approximate heat exchanger operating conditions. Mass change measurements and characterization of the corroded surfaces using X-ray diffraction and scanning electron microscopy were performed to understand the corrosion behavior of steels. Additionally, electrochemical tests were performed in CO₂-saturated H₂O at ambient pressure and temperatures up to 50°C to obtain kinetic data in terms of the corrosion rates of the alloys. The results demonstrate that significant corrosion occurs in a pressure / temperature region where H₂O saturated with CO₂ condenses on the coupons. Larger mass change was observed when oxygen was present in the supercritical CO₂ environment. The ferritic-martensitic steel was found to be susceptible to corrosion at low temperatures in direct supercritical CO₂ power cycle environments.