S30408管材及其焊缝在超临界CO_(2)环境下的腐蚀行为研究

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

在超临界CO_(2)环境下对S30408管及其焊缝进行了320℃和500℃两种温度下的腐蚀实验,利用XRD、XPS、FE-SEM和EDS对腐蚀产物进行了表征,分析了温度对S30408管及其焊缝在超临界环境下的腐蚀产物及腐蚀机理的影响。实验结果表明,经320 h腐蚀后,高温下S30408管及其焊缝发生了明显的腐蚀,同时观察到了腐蚀裂纹扩展及腐蚀物脱落的现象。管道及焊缝内壁的腐蚀是由氧化和碳化造成的,其腐蚀产物以Fe、Cr的氧化物及碳化物为主。焊缝在高温下产生大块结痂状腐蚀产物,管道及焊缝在500℃时出现了明显的沿晶腐蚀开裂。这种裂纹很容易扩展到管道内部,造成材料失效,对管道和整个设备的安全构成极大威胁。

Supercritical carbon dioxide(S-CO_(2))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-CO_(2) Brayton cycle.In order to study the corrosion problem of S30408 pipe in the S-CO_(2) 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-CO_(2) 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-CO_(2) 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-CO_(2) 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-CO_(2) 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.