Corrosion And Protection Of Underground Pipeline By Stray Current Caused Of Rail Transit

The traction voltage of the subway is higher than 750V DC,and the reflux system mostly adopts the mode of rail reflux.Inevitably,some traction current escapes from the track and leaks into the ground and the surrounding environment.The stray current flowing into the soil will cause impressed current depolarization corrosion of the underground pipeline.In view of this reality,this paper uses MATLAB simulation calculation to obtain the current distribution of the stray current discharged by the subway rail on the buried pipeline,and measured the stray current potential of the simulated pipeline buried in the underground natural gas pipeline along the subway line,under the viaduct and 25 m away from the subway track and 33 m underground.The mechanism and influencing factors of soil corrosion under DC interference were studied by weight loss method and electrochemical method,and the feasibility and effect of protection against stray current such as sacrificial anode protection and conductive soil filling were explored.The following conclusions are obtained through the research:（1）The stray current voltage is related to the length of the pipeline,track resistance,pipeline resistance,soil resistivity,distance between the rail and the pipeline,and the spatial position of the pipeline and the rail.When the pipeline is parallel to the track,the pipeline potential varies exponentially with the pipeline length,and the stray current voltage is basically proportional to the safety distance.When the pipeline crosses at different planes with the track,the pipeline potential varies with the length of the pipeline as a parabola distribution,the safety distance increases with the discharge voltage,the increase rate decreases gradually.（2）When the electric locomotive passes on the viaduct,it will also discharge stray current to the ground along the pier.When the locomotive stops at the platform,it will also discharge certain stray current.The stray current is the largest when the electric locomotive passes through,and gradually decreases when approaching or going away.The stray current is triangular waveform.The potential of stray current discharged by locomotive under viaduct is about-0.04 V～-0.01 V.The potential of stray current in the simulated pipeline buried 33 m underground at 25 m away from the subway line is about-0.315 V～-0.28 V.（3）Soil water content,p H value,salt content,SO₄^2-concentration and Cl^-concentration have a synergistic effect on stray current corrosion.When the water content is lower than 15%,the water content pair is the main factor for corrosion,and the corrosion rate reaches the maximum when the water content is 25%～30%.If the water content continues to increase,stray current has a great influence on corrosion.The increase of SO₄^2-concentration and Cl^-concentration and the decrease of p H value will accelerate the stray current corrosion.When the ground potential caused by stray current is 0.3 V and the soil moisture is 20 wt%,the corrosion rate of steel is 8%higher than that without stray current.（4）In the range of stray current caused by rail transit,the potential reversal does not occur under the protection of Mg-Y sacrificial anode,and the protection potential is as low as-0.7 V to-0.9 V,the sacrificial anode to protection area ratio is 1:200.When the DC interference potential is 10 V,the protective radius of 1.37×10^-3m²Mg-Y sacrific anode against stray current corrosion is about 2 m.Coat the pipe with conductive soil which is composed of 75%gypsum powder,20%bentonite and sodium sulfate.When the coating thickness is 0.2 m,the pipeline potential of parallel track can be reduced by 0.1 V～0.12 V.For T-shaped or intersecting pipelines,the pipeline potential can only be reduced a little,indicating that the protection effect is poor.