Study On Corrosion Behavior Of X70 Pipeline Steel And Coating Delamination Under Stray AC And DC With Stress

With the continuous increase of transportation pressure of oil and gas pipeline and the interference of stray current,the operating environment of buried high-strength steel pipelines become more and more complicated.Researching and exploring the problem of corrosion of high-strength steel under stress has become a new problem in the research of pipeline steel corrosion.In addition,the corrosion and delamination behavior of high-strength pipeline steel under stray alternating current/direct current(AC/DC)with stress are complicated and the mechanism its unknown.Therefore,carrying out the study of corrosion behavior of high-strength steel and coating delamination under AC and DC with stress has important engineering significance.In this paper,the effects of stray AC and DC with stress on the corrosion of coated X70 pipeline steel and the delamination were investigated using COMSOL Multiphysics simulation,electrochemical impedance spectroscopy(EIS)and three-dimensional(3D)digital microscope techniques.Using the COMSOL Multiphysics software to study the propagation law of stray current in the soil near the buried pipeline steel and the coating damaged area.The influence of stray current on the corrosion behavior of buried pipeline steel was revealed,and the protective measures to reduce the harm of buried pipeline under stray current interference were proposed.The electrochemical results showed that the open circuit potential(OCP)of the samples were negatively shifted by stray AC,and the corrosion potential shifted negatively with the increase of stray current density.The negative extent of the corrosion potential caused by stray DC was more negative than that of stray AC.Also the negative extent of the corrosion potential caused by the mixed of stray AC and DC was larger than that of AC and DC respectively.The polarization curves had only activedissolution zone and no passivation zone.Anodic polarization curves become steeper with the increase of current density.After adding stray DC interfence,the anodic polarization curves showed a significant inflection point,because X70 steel samples deposited a large number of corrosion products on the surface and thus temporarily impeded the anodic polarization process.EIS constants exhibit two time constants all the time.The effect of stress on electrochemical behavior was less than that of stray current.The results of corrosion morphology showed that with the increase of current density,the maximum pit depth on the samples become deeper.the maximum pit depth caused by the mixed of stray AC and DC was deeper than that of AC and DC respectively.Under the same conditions,the order of the promotion effect of corrosion reaction and coating delamination was DC>AC>stress.There was a critical current density that maximized the delamination area of the coating and then remained the same.Based on the above research,compared to the effects of(stray AC,stray DC,mixed ACand DC,coupling AC and DC with stress)interference on corrosion behavior of X70 pipeline steel and cathodic delamination.To explore the mechanism of corrosion of X70 steel and coating delamination under various factor.