Corrosion Behavior of S30408 Pipe and Welds in Supercritical-carbon Dioxide Environment

Supercritical carbon dioxide (S-CO₂) Brayton cycle system has advantages of high thermal-electric conversion efficiency, simple structure, compact equipment, and the system has been widely studied in the field of small modular reactor. However, the degradation of candidate materials due to corrosion in the service environment hinders the widespread application of this technology. The corrosion behavior and mechanism of materials play an important role in the engineering application of S-CO₂ Brayton cycle. In order to study the corrosion problem of S30408 pipe in the S-CO₂ environment, the corrosion behavior of the steel pipe was studied. Corrosion tests at 320 ℃ and 500 ℃ were carried out on S30408 pipe and its welds in S-CO₂ environment. The corrosion specimens were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS). The effect of temperature on the corrosion products and corrosion mechanism of S30408 pipe and its welds in S-CO₂ environment was analyzed. The experimental results show that after 320 h of corrosion, obvious corrosion, corrosion crack propagation, and corrosion material shedding occur in S30408 pipe and its welds. The corrosion of pipe and welds is caused by oxidation and carburization, which can cause alloy element consumption, microstructural change, rapid failure, and its corrosion products mainly comprise of iron and chromium oxides and carbides, as well as the oxide of Si. Corrosion on the inside of the pipe is more severe than on the welds, and the surface of the pipe is cracked. Corrosion products are shed from the inner wall of the pipe, and the shedding of the corrosion products is due to the cracking of the inner wall surface of the pipe and the sawtooth shape of the cross section. The mismatch between the thermal expansion coefficient of the corrosion products and the matrix, and the fact that the experiment is in flowing S-CO₂ are the main reasons for the shedding of the corrosion products. Only large block scab-like corrosion products are produced in the welds at high temperature. At 500 ℃, obvious intergranular corrosion cracking occur in pipe and welds. The corrosion mechanism indicates that serious intergranular cracking will occur when the pipe exhibits slight surface corrosion at the temperature of 500 ℃. The strong permeability and dissolution capacity of S-CO₂ make the corrosion cracks easier to expand physically. This kind of crack is easy to expand to the inside of the pipe, causing material failure, which poses a great threat to the safety of the pipe and the entire equipment.