Research On Corrosion Behavior And Mechanism Of X100 Pipeline Steel In Presence Of AC Stray Current

Oil and gas pipelines are the lifeline of energy transmission in China,and have the characteristics of long distance,large throughput and high pressure.In order to achieve structural stability,safety and economy of oil and gas pipelines,the wide application of X100 and other high-strength pipeline steel has become the development trend of the pipeline industry.With the rapid growth of the length of high-voltage transmission lines and electrified railways,the occurrence of multiple pipeline AC stray current corrosion accidents in recent years shows that AC stray current corrosion has become a serious hidden danger to oil and gas pipelines.However,the current research on the AC stray current corrosion of X100 pipeline steel has not been carried out.In this paper,the related research on AC corrosion of X100 pipeline steel is carried out by means of electrochemical testing,corrosion immersion experiment,high-frequency potential measurement,surface analysis technology,thermal simulation technology and numerical calculation.The specific content and conclusions are as follows:(1)In the soil simulating solution of Korla and Yingtan in China,the AC stray current interference causes the corrosion potential of X100 pipeline steel to shift negatively.The AC corrosion rate increases with the increase of the AC current density,but the increase in corrosion rate gradually decreases,and the corrosion rate in Yingtan soil simulating solution is greater than that in Korla soil simulation.The X100 pipeline steel AC corrosion products in Korla and Yingtan soil simulating solution have poor protection to the substrate.X100 pipeline steel has local corrosion when it is affected by AC currents greater than 100 and 200 A/m~2 in Korla and Yingtan soil simulation fluids,respectively.When the current density is less than this,uniform corrosion occurs.The local corrosion tendency in Korla soil simulation fluid is greater than that in Yingtan soil simulating solution.(2)In order to study the differences in AC stray current corrosion behavior of different microstructures at welded joints,the thermal coarse grain heat affected zone(CGHAZ)and the intercritically reheated coarse-grained heat-affected zone(ICCGHAZ)of X100 pipeline steel were obtained through thermal simulation technology.In the Korla soil simulating solution,with or without AC interference,corrosion rates in descending order are ICCGHAZ,CGHAZ,and base metal.In Yingtan soil simulating solution,the natural corrosion rate is sorted from large to small as:Materials,ICCGHAZ and CGHAZ,but the corrosion rate under 200 A/m~2 AC current density interference is almost the same.The corrosion rate in acidic Yingtan soil simulating solution is greater.In the presence or absence of AC stray current,the corrosion product structure and morphology of X100 pipeline steel base metal,CGHAZ and ICCGHAZ in the same environment are the same.Under natural corrosion conditions,the base material of X100 pipeline steel is uniformly corroded,grain boundary corrosion occurs in CGHAZ and ICCGHAZ,and grain boundary corrosion is more serious in Yingtan soil simulating solution.Under the interference of 200 A/m~2AC current density,X100 pipeline steel base metal,CGHAZ and ICCGHAZ show obvious local corrosion pits in Korla soil simulating solution,and CGHAZ and ICCGHAZ show grain boundary corrosion.In Yingtan soil simulating solution,the morphology of the corrosion is pit-like,but it tends to be uniformly corroded as a whole.(3)The AC corrosion mechanism of X100 pipeline steel in Korla soil simulating solution was studied through the combination of experiments and electrochemical theory.It was found that most of the AC stray current was consumed by charging and discharging the corrosion interface capacitance.Only a small amount of the AC current component passing through the Faraday resistor causes metal corrosion.The charging and discharging of the interface capacitance by the AC current periodically oscillates the electric potential of the electric double layer,the increase of the cyclic corrosion current in the positive half cycle is greater than the decrease in the negative half cycle,and the total corrosion rate increases.The driving force of the AC current corrosion comes from the periodic polarization of the electric double layer potential.Even if no AC current passes through the corrosion interface,AC accelerated corrosion will occur when it is in an AC electric field.The electrode interface capacitance value has a great influence on the corrosion rate of AC current.With the same AC current,the larger the capacitance value,the smaller the corrosion rate.(4)The effects of AC stray current on the cathodic protection(CP)of X100pipeline steel in Korla soil simulation fluid were studied through experimental tests,theoretical analysis and numerical calculations.It was found that AC current interference caused the PC electrode potential to periodically oscillate and the average value is shifted.When the CP potential is more negative,it is shifted actively.When the CP potential approaches the natural potential,the direction of the potential shift depends on the kinetic parameters.Factors affecting CP potential shift include different Tafel slopes of electrode reactions in different potential ranges,non-linear relationship between the polarization potential and polarization current of the cathode reaction,and Tafel slope change caused by AC current interference and other dynamic parameters value.When affected by 500 A/m~2 AC stray current,the CP potential of-1.2V cannot provide effective protection.Under the CP potential of about-0.7V,the protection cannot be provided to corrosion with or without AC.The greater the density,the greater the corrosion rate.When the AC stray current is not greater than 100 A/m~2,a CP potential of-1.0V can provide effective protection.The AC corrosion mechanism under CP protection is that the AC stray current causes the electric double layer potential(the electrode potential without IR drop)to shift in a positive direction,and the active corrosion occurs when the effective protection potential of the CP cannot be reached.