Influence Of Stress On The Electrochemical Behavior Of Pipeline Steel Welded Joints

At present, there are a lot of research about the pipeline stress corrosion cracking at domestic and abroad. Research of various methods for pipeline strengthening is also quite deep. The whole performance of X80 pipeline steel can be improved through perfecting the surface condition, which has great significance for long-term safty operation of pipeline.In this experiment, the surface machining of X80 pipeline steel and its welding joints are realized by the technology of ultrasonic surface rolling processing(USRP). Microstructure was observed by optical microscope after USRP, surface residual stress of X80 steel welded joints was tested with X-ray diffraction before and after USRP.A stress corrosion Device was designed to test the electrochemical behavior of welded joints and the base materialng before and after USRP. A comparative study is made between the electrochemical behavior of X80 steel welded joints in nearly neutral solution and high p H solution under different compressive stress before and after ultrasonic surface rolling processing by potentiodynamic polarization curves and electrochemical impedance methods.(1) The result of the metallographic photographs shows that the surface layer of X80 pipeline steel and its welding joints grain was finered after USRP, refinement of a certain degree of plastic flow in the surface layer.(2) By use of non-destructive measuring method-X ray diffraction method,we test the residual stress of the steel plate before and after ultrasonic impact treatment. The result shows that, the experimental result indicates: ultrasonic processing. can largely reduce the residual stress of welded joints. After ultrasonic impact treatment, the residual tensile stress near the weld all converted to beneficial residual compressive stress, the compressive stress away from the weld area remains compressive stress,but the value has increased significantly.(3)When it comes to the electrochemical tests, the process of cuting the sample will result in the release of surface residual stress.To solve this problem, we designed a device that can appliy compressive stress on the specimen,so that the electrochemical testing can be done on the surface of specimen under the effect of stress.These are to sduty the influence of stress on the electrochemical behavior of pipeline steel welded joints.(4) The electrochemical tests show that, in nearly neutral solution and high pH solution, as the applied stress increases, the polarization resistance of base metal and weld after USRP treatment is improved and the corrosion current density is reduced, resulting in better corrosion resistance.