Research On The CO₂ Corrosion Behavior Of Carbon Steel Welded Joint And The Development Of Corrosion Resistance Welding Materials

The CO₂ corrosion problem of oil and gas pipeline welded joint has been a long time.Although there was the related research,so far the CO₂ corrosion problem of carbon steel pipeline welded joint is still not resolved and it often causes the accident of oil and gas leakage and environment pollution.As the corrosion media of the oilfield produced fluid containing CO₂,the paper carried out the research of CO₂corrosion behavior of carbon steel welded joint and the development of corrosion resistance welding materials.Using the lost weight method,the long time corrosion experiment is conducted under 5 kinds of CO₂ partial pressure,and it is found that at least 60 days are needed to acquire the accurate CO₂ corrosion rate.Fitting the CO₂ corrosion rate data with the power function,and it is found that corrosion process has three stages:the initial stage of corrosion（within 15 days）,corrosion products growth stage（15³0 days）and dynamic equilibrium stage（after 30 days）.The weld metal with different heat input is characterized with EBSD,combining the polarization curve and impedance results,and it is found that the weld metal corrosion resistance with the{101}‖RD-TD grain orientation and grain size distribution concentrate is better.But grain boundary type of sample has a little influence on the corrosion resistance.The influence of alloy elements（Ni、Cr、Cu and Mo）adding amount to the ratio of phase,the content of alloy elements in ferrite,mechanical property,corrosion resistance,weldability and economy is comprehensively evaluated,and the composition of resistancing CO₂ corrosion of carbon steel filler metal is determined.The mechanical property of welded joint with the filler metal meets the standard of DNV-OS-F101,2000.The surface morphology of two kinds of welded joint different zone immersing different time is observed by AFM and VEECO optical profiler.Combing the test result of grain size,crystal orientation,grain boundary types and alloying element distribution of the grain and grain boundary,it is found that the uniform corrosion of grain and grain boundary happens in the weld metal of joint W1,while the grain is corroded firstly in the other zones of two kinds of welded joints（joint W:welded joint with commercial filler metal;joint W1:welded joint with new filler metal）.The corrosion rate of joint W and W1 weld metal are compared systematically by the weight loss and electrochemical method.The morphology and composition of two kinds of corrosion film is analyzed by the SEM/EDS/GIXRD/XPS,and it is found that the outer corrosion film of joint W weld metal mainly includes the FeCO₃ and CaCO₃,while the outer corrosion film of joint W1 weld metal mainly includes the FeCO₃ and Cr（OH）₃.In addition,anion can pass the film of FeCO₃ and CaCO₃ to react with the substrate,but it is blocked by the film of FeCO₃ and Cr（OH）₃.Combining the galvanic corrosion principle,the relationship between stress and corrosion electrochemical parameter（potential and current）,Abaqus and COMSOL Multiphysics^?software,numerical model of predicting welded joint（with stress and without stress）corrosion mopholgy evoluting is presented.