Research On Corrosion Behavior Effect Of Buried Pipeline Steel Under Metro Stray Current

The petroleum and natural gas pipelines are the major arteries of national energy,and whether their safe operation is a major issue related to the national economy and people’s livelihood.In recent years,with the rapid development of the economy,there are more and more high-voltage transmission lines,high-speed railways,and urban subway lines,which facilitate people’s travel and also bring stray currents to buried pipelines,especially DC stray currents,which cause particularly serious corrosion to buried steel pipelines.In order to clarify the corrosion mechanism and corrosion control technology of subway stray current,this article combines actual working conditions and simulates interference signals to study the corrosion behavior of buried pipeline steel.Through numerical simulation and field experiments,the interference law of subway stray current and corrosion control technology in the field are analyzed,Innovative research results have been achieved in terms of corrosion behavior pipeline steel,corrosion prediction models,and the determination of anode zone under stray current interference.In the simulated soil solution,the corrosion mechanism of L290 pipeline steel was studied by using triangular wave alternating polarization with cycles of 60s,120s,180s,300s,600s,and 900s.Research has shown that the corrosion rate and anodic current efficiency of L290 pipeline steel increase with the increase of interference period.The surface product composition of the corroded coupon includes Fe₃O₄,Fe₂O₃,Fe OOH,and Fe（OH）₃.As the interference period increases,the corrosion degree of the coupon gradually deepens and the corrosion becomes more severe.During forward half cycle polarization,the coupon first generates Fe₃O₄.As forward polarization progresses,Fe₃O₄oxidizes to Fe₂O₃,with intermediate products including Fe OOH and the hydrated product Fe（OH）₃of Fe OOH.In the experiment of triangular wave superimposed cathodic polarization to-0.85V（vs CSE）and-1.10V（vs CSE）levels,at low interference cycles,the main corrosion products were Fe₂O₃,Fe（OH）₃and Fe OOH;At high interference cycles,the corrosion products are mainly Fe₂O₃,Fe（OH）₃,and Fe OOH,Fe₃O₄and Fe（OH）₂also begin to appear.Low cycle corrosion rate is low,with few products.During the forward half cycle,the corrosion product Fe₃O₄is completely transformed into Fe₂O₃and intermediate products;At high cycles,the degree of corrosion deepens,and at negative half cycles,there may be a process of Fe³⁺reducing to Fe²⁺.The intermediate products Fe（OH）₂and Fe（OH）₃cycle,resulting in a decrease in corrosion rate.According to the relationship between the corrosion rate of the coupon and the period,the power function relationship between the corrosion rate and the interference period is obtained by data fitting.Under experimental conditions,a corrosion rate prediction model without cathodic polarization and a corrosion rate prediction model with cathodic polarization reaching levels above-0.85V（vs SCE）were obtained.In a soil environment,buried pipeline steel is corroded by soil and AC/DC stray current,and the corrosion of pipeline steel becomes more complex when cathodic polarization is superimposed.In the AC corrosion simulation experiment,the changes in electrochemical parameters of the sample are analyzed by regularly changing the applied voltage of the simulated cathodic protection external current system.The coupon’s off-potential and DC current density change asymmetrically as cathodic polarization proceeds.The AC current density of the coupon gradually decreases with time,while the diffusion resistance gradually increases with time.During the cathodic polarization reaction,the coupon interface increases diffusion resistance and reduces AC interference current.The minimum AC/DC density ratio of the coupon is 14,but still greater than 5.The risk of AC corrosion is high,and it is difficult to reduce the risk of corrosion only by improving the cathodic protection effect.Under actual operating conditions,although the test coupon continues to undergo cathodic polarization,the test coupon undergoes corrosion.The study found that the corrosion of the coupon was mainly caused by AC stray current,and the corrosion products were mainly iron oxides,mixed with soil impurities and calcium magnesium refractory salts.Ribbon sacrificial anodes are commonly used as drainage protection materials for pipelines,which are directly connected to the pipeline and are an accessory part of the pipeline.Through square wave polarization of the ribbon magnesium anode and the ribbon zinc anode,it was found that the corrosion rates of the ribbon magnesium alloy and the zinc alloy sacrificial anode increased with the increase of the interference period.The product of magnesium anode under stray current interference of different interference periods is only Mg（OH）₂.When positive anodic polarization occurs,the magnesium alloy anode potential shifts in a positive direction instantaneously,and the deviation is large,but then it quickly drops back,because the passive film formed on the surface increases the IR drop.During cathodic polarization,an alkaline environment is formed,resulting in the formation of Mg（OH）₂on the magnesium anode surface.With the continuation of anodic polarization,the surface facial mask film breaks to expose the matrix,and the IR drop disappears or decreases to the normal value,and the potential changes steadily.The main corrosion product of zinc anode is Zn O.Zn（OH）₂appeared in the 60s and 120s cycles.At low cycle interference,the positive and negative interference time is shorter,and the cathodic process during negative interference causes the corrosion products on the surface of the coupon to remain in the form of hydrated products,leading to the appearance of Zn（OH）₂.As the interference period increases,the instability of hydration products leads to anodic current during forward interference,which oxidizes Zn（OH）₂into Zn O products.Therefore,only Zn O corrosion products appear in XRD at higher periods.Based on the analysis of the weight loss of ribbon anodes,numerical fitting was used to predict the weight loss formula of anodes.Theoretical support was provided when conducting current drainage design calculations for ribbon anodes.Under actual working conditions,the buried pipeline is interfered by the dynamic stray current of the subway.Based on the assumption of homogenization,a joint model of the subway and pipeline is established,and numerical simulation is performed on the established model.Based on the basic principle of stray current interference,a method for determining the anode area of a pipeline affected by stray current interference has been developed.CS-1,CS-7,and CS-11 test post have been determined as feed locations for simulating cathodic polarization experiments.Through numerical simulation of the three locations,it has been determined that cathodic polarization at CS-11 is the most reasonable location.Through field experiments,the monitoring results show that when the current reaches 15A,the cathodic polarization is good,and the pipeline section potential meets the requirements of AS2832.1.The off-potential data of 1.5%at CS-7 test post,0.6%at CS-10 test post,and 0.1%at CS-6 test post do not meet the cathodic protection criteria,which is less than 5%of the standard AS2832.1.