Numerical Simulation Of Galvanic Corrosion Process For Welded Joint Of Submarine Pipeline

Oil and gas pipeline is an important part of China's petroleum and petrochemical industry.With the increasing demand for resources such as oil and natural gas in China,the laying capacity of oil and gas pipelines has increased correspondingly.However,offshore oil and gas pipelines are prone to failure due to their complex service conditions.The water in the pipeline and its dissolved CO₂,organic acids and other impurities provide a necessary environment for the occurrence of internal corrosion.In the action of medium,the welding joint is vulnerable to the failure of galvanic corrosion because of the inhomogeneity of the components and tissue of various regions,which makes it a weak link in the pipeline engineering.Because of the complex structure of the welded joint,the research on its corrosion behavior is limited to the study of the galvanic corrosion behavior through the electrochemical or weightlessness experiments,and the understanding of the dynamic development process of each region in the process of corrosion is still very short.In this paper,with the help of the second current distribution module and deformation geometry module in COMSOL Multiphysics and the measured polarization curve as the boundary condition,the galvanic corrosion process of the welded joint of the submarine pipeline in the NACE solution saturated with CO₂ was numerically simulated.The steady-state electric field distribution and the transient changes of the corrosion deformation of each component region were obtained.The influence of medium factor and material geometric factor was mainly investigated.The galvanic corrosion behavior of welded joint and its dynamic development process were discussed.The author calculates the galvanic corrosion behavior under the coupling condition of different regions.The results show that the simulation results of the complete welded joint including the weld metal,the heat affected zone and the base material are more consistent with the results of the weightless experiment.The galvanic corrosion of the welded joint is obtained by the parameterized scanning technique built in the software.It is found that the corrosion rate will decrease with the increase of the width of the weld zone and the decrease of the width of the base material area.The author used the parameterized scanning technique built in the software to get the galvanic corrosion of the welded joint when the width of the weld zone and the width of the base metal changed.The result shows that the corrosion rate will decrease with the increase of the width of the weld zone,and the decrease of the width of the base metal.The effects of temperature,HAc and MEG concentration on galvanic corrosion behavior of welded joints were further investigated.It was found that with the increase of temperature and HAc concentration and the decrease of MEG concentration,the corrosion rate of the weld zone is accelerated.The author also used the electrolysis analysis module to calculate the corrosion rate of the metal in the anode region of the welded joint,the change of the metal corrosion rate and the concentration of each substance in the solution along the direction of the metal surface,as well as the influence of the pH value and temperature on the CO₂ corrosion.The results show that the corrosion rate can be increased by the increase of temperature and the decrease of pH value.The concentration of various substances in the solution varies along the direction far away from the metal surface:Fe²⁺and HCO₃^-showed a decreasing trend while CO₂ and OH^-gradually increased.The presence of Fe²⁺and anions may lead to the formation of new phases.Numerical simulation and simulation are a powerful tool for accurate prediction of galvanic corrosion behavior and dynamic development of welded joints.However,it is difficult to accurately obtain the simulation results of galvanic corrosion behavior of welded joints under the condition that the coupling of the parent material and the weld zone or the coupling between the parent material and the heat affected zone is difficult to obtain.Only when the material is coupled with the weld zone and the heat affected zone,can the prediction results be closer to the actual corrosion behavior.