X80管线钢焊接接头的模拟重构及电偶腐蚀行为表征

Reconstruction and Characterization of Galvanic Corrosion Behavior of X80 Pipeline Steel Welded Joints

利用离散、热模拟处理和模块化重构的微电极阵列对X80管线钢焊接接头进行模拟重构,并联用经典电化学测试技术和微电极阵列测试技术研究了X80钢模拟焊接接头及孤立分区在CO2饱和的NACE溶液中的电偶腐蚀行为。结果表明,在NACE溶液中,孤立的细晶区开路电位最正,腐蚀倾向小,而焊缝的电位最负,腐蚀倾向大;各部分的腐蚀电流密度大小关系为:部分相变区>细晶区>粗晶区>母材>焊缝;模拟X80钢焊接接头组织中,粒状贝氏体与铁素体的混合组织的开路电位最正,极化电阻最小。当各组成部分耦合后,模拟焊接接头中的焊缝和粗晶区作为主要阳极,腐蚀加速;细晶区和部分相变区作为主要阴极,腐蚀减缓;邻近热影响区的母材微电极以阳极性电流为主,远离热影响区的母材微电极以阴极性电流为主。提出电偶腐蚀强度因子用于表征电偶腐蚀强度。孤立的焊缝区的自腐蚀电流密度虽最小,但在电偶腐蚀的加速作用下,焊缝与粗晶区作为模拟焊接接头的薄弱环节,首先因腐蚀而失效。

Welding is widely used for pipeline connection. Composition, microstructures and properties of the welded joints are highly heterogeneous and the resultant corrosion such as galvanic corrosion between different parts is widely present and influence the long-time service and safety. In this sense, the fundamental research in the electrochemical behavior of such joint parts is required. Electrochemical corrosion behavior of simulated X80 steel welded joint, accurately modeled by wire beam electrode(WBE)technique, was investigated by classical electrochemical techniques and microelectrode array(MEA)technique. A new index, namely the galvanic corrosion intensity factor, was proposed and verified to succeed in characterizing the degree of galvanic corrosion. Results showed that microstructure of granular bainite mixed with ferrite showed the highest positive open circuit potential and lowest polarization resistance. Furthermore, the corrosion tendency of the isolated electrodes that constituted the X80 steel welded joint was found to increase in the following order: fine grain heat affected zone(FGHAZ) < intercritical heat affected zone(ICHAZ) < base metal(BM) < coarse grain heat affected zone(CGHAZ) < weld metal(WM). Due to the difference in potential and the polarization characteristics, the WM displayed the highest polarization resistance but the most positive current density. The CGHAZ possessed a lower polarization resistance and a higher positive current density. In comparison, the FGHAZ and ICHAZ performed a lower polarization resistance but higher negative current densities. The WM and CGHAZ acted as the main anode, while the FGHAZ and ICHAZ acted as the main cathode and the galvanic current polarity of some BM electrodes changed with time during the immersion test. The intensity of galvanic corrosion of simulated X80 steel welded joint plateaued with immersion time. The results revealed that WM and CGHAZ were the weak links in the simulated X80 pipeline steel welded joints during its long-term service.