Research On Corrosion Behavior Of X65 Steel Welded Joints Under Different Environmental Pressures In Simulated Seawater

X65 pipeline steel is widely used in submarine pipeline system, and high pressure dry underwater welding is an effective method for underwater maintenance of ocean engineering. In this paper, the microstructure of X65 welded joints of API GMAW pipeline steel under four environmental pressures (0.1 MPa, 0.3 MPa, 0.5 MPa, 0.7 MPa) conditions was observed, and the corrosion performance of the welded joints and the properties of the passive film formed on the welded joint were studied by means of polarization curves, electrochemical impedance spectroscopy (EIS) and Mott-Schottky curves, the corrosion morphology and the product were analyzed by soaking weight loss, SEM and XRD, and finally the influence of environmental pressure on the corrosion performance of the welded joint was discussed. The main conclusions are as follows:(1) There is no significant difference in the microstructure of the weld zone under different environmental pressures, which indicates that the change of environmental pressure have no effect on the weld structure, but the bainite structure in the heat affected zone increases with the increase of welding pressure. The microstructure of heat affected zone is the most coarse and irregular, and the microstructure of the weld zone is smaller than the heat affected zone, which is similar to that of the base metal.(2) The corrosion rate of the welded joints under different welding pressure immersed in the same number of days is not significant. The corrosion form of the welded joint is pitting corrosion, and the corrosion product layer is observed on the surface of the sample. In the later stage of immersion, the corrosion product layer was naturally peeled off, and there was serious local corrosion. The corrosion products of X65 pipeline steel welded joints in simulated seawater are mainly composed of FeOOH, Fe2O3 and Fe3O4.(3) Relative to the base metal and weld zone, the heat affected zone has higher electrochemical activity, so the polarization resistance of the heat affected zone is the smallest,which leads to the worst corrosion resistance. There is no significant difference in the polarization resistance of the heat affected zone under different welding environment pressure.(4) Compare with the passive film formed by 0.2 V and 0.6 V film potential on the welded joint, the charge transfer resistance of the passive film formed by the 0.4 V film formation potential is the largest, which indicates that the anodic reaction of the surface of the sample is more difficult to carry out, so the best protection potential of the anode protection in the seawater is 0.4 V.(5) The semiconductor type of passive film is N type semiconductor, and the defect types are oxygen vacancies. The passive film formed at the 0.4 V film potential is the thickest and the donor density is the lowest. The diffusion coefficient of the passive film formed on the welded joint under different welding pressure was calculated,the results showed that the diffusion coefficient of 0.1 MPa and 0.5 MPa was smaller than the 0.3 MPa and 0.7 MPa. However, their values are the same order of magnitude, indicating that there is little difference in the diffusion coefficient of the passive film formed under four pressures.