Study On Interference And Corrosion Law Of Buried Pipeline By HVDC Grounding Electrode

With the increasing of energy demand in China,HVDC transmission lines and buried pipelines have been built on a large scale.Due to the similarity of their routing,local pipelines inevitably enter the HVDC interference zone.Thousands of amperes of current flow into the earth,penetrating the coating,burning down the cathodic protection equipment,accelerating steel corrosion and even injuring the operators.Therefore,it is of great significance to carry out the research of the grounding electrode’s interference on the buried pipeline and the corrosion law to ensure the safe operation of pipelines.Firstly,this paper constructed the HVDC interference model based on horizontal and vertical layered soil structure.The effects of pipeline parameters,electrode parameters and soil parameters on pipe-to-soil potential and leaking current density were studied.Then this paper proposed minmal safety distance calculation formula and comprehensive protection measures to effectively mitigate HVDC interference and the leakage current density at holiday was obtained.Finally,the corrosion behavior of the X70 steel under DC interference and after DC interference were investigated by means of electrochemical tests.The work carried out in this paper are as follows:This paper carried out a numerical simulation of HVDC interference and protection.It was indicated that the smaller the coating breakdown factor,the more concentrated the current,and the higher the local corrosion risk.The leaking current density（5.11 mA/cm²）on 1 cm²holiday is several thousands times higher than the outflow current density of the uniform coating damage model.And it was found that the most positive/negative pipe-to-soil potential did not appear on the holiday but appeared on the coated pipe near the holiday;Protective measures study proposed an effective measure of segmented isolation+cathodic protection+zinc mitigation.The corrosion behavior of X70 steel during DC interference was studied.It was demonstrated that at the range of 5-50 mA/cm²,the impedance spectrum is double capacitive arc.As the DC current density increases,the diffusion layer（between the rust layer and the solution）electric field intensity increases and the mass concentration difference is larger,resulting in rust layer/solution diffusion layer resistance decreases,the capacitance increases,mass transfer accelerates,and the corrosion rate increases.When the interference current density is greater than 50 mA/cm²,corrosion rate tends to be stable.This is because at a current density greater than 50 mA/cm²,the acceleration of the diffusion of oxygen by the electric field and the inhibition of oxygen diffusion by the corrosive product film are balanced,and the oxygen diffusion rate do not change.The corrosion behavior of X70 steel after DC interference was studied.With the increase of the DC interference time or DC current density,the EIS of X70 steel after interference shows 2 time constants,and the corrosion product layer resistance R_f and the charge transfer resistance R_ct increases continuously.