Effect of Al Content on General Corrosion Behavior of Alumina-forming Austenitic Stainless Steel in High Temperature Supercritical Carbon Dioxide

Supercritical carbon dioxide is one of the most promising candidate working fluids for nuclear power plants. Structural materials using in this kind of system face severe challenges due to its high temperature and high pressure. Corrosion, especially oxidation and carburization, will occur, which leads to the failure of material. Traditional metallic materials like austenitic stainless steel, which is regarded as the candidate cladding materials, will break away because of their limited properties. To further improve the corrosion resistance of austenitic stainless steel as the candidate cladding material for supercritical carbon dioxide nuclear reactor, the general corrosion behavior of three kinds of alumina-forming austenitic stainless steel with different Al contents and their Al-free steel was investigated in supercritical carbon dioxide at 650 ℃/20 MPa. The purity of carbon dioxide was 99.99%. All samples were firstly ground, and then polished to eliminate the surface damage regions. Scanning electron microscopy equipped with backscattered electron, electron backscatter diffraction detector and energy dispersive spectroscopy was used for microanalysis. Focused ion beam system was used to prepare the cross-sectional transmission electron microscopy foils. Selected area electron diffraction was also carried out on the transmission electron microscopy foils for revealing the detailed microstructural characterization. Glow discharge optical emission spectrum was used for carburization analysis. The results show that the weight gain of materials decreases with the increase of Al content, and the weight gain of 3.5wt% Al steel is only about 0.022 mg/cm2. The weight gain against exposure time of different materials follows the parabolic growth law approximately, which indicates that the corrosion behavior is mainly controlled by diffusion. When the content of Al is less than 1.5wt%, a dual-layer Fe-rich oxide film is formed on the surface with poor protection. Some pores were observed within it, which can cause oxidation and carburization. Carburization occurs on Al-free steel and the depth of the carburization region can be up to about 12 μm. When the content of Al is higher than 2.5wt%, protective oxide films are formed on the surface. Its outer layer is mainly rich in Cr and the inner layer is rich in Al. The Al layer on 2.5wt% and 3.5wt% Al containing steel is more continuous. These oxides own better corrosion resistance than that formed on 1.5wt% steel. Although more protective oxides are formed, carburization still exists in the oxide film and matrix of this kind of materials, while the thickness of the carburization region decreases to about 6 μm. Higher Al content, above 2.5wt%, can effectively facilitate the formation of protective Al-rich oxide film, which hinders the outward diffusion of Fe and the inward diffusion of C. It further improves the oxidation and carburizing resistance of materials, which will be useful in the supercritical carbon dioxide nuclear power plant.