Stress Corrosion Cracking Behavior And Mechanism Of X80 Pipeline Steel Under Alternating Current Application In Near-neutral PH Solution

X80 steel is widely used in west-east gas process.Due to the limit of the geographical location and space,pipeline steel with high voltage power lines and electrified railway is in the public corridor in many places.Alternating current(AC)corrosion is more and more obvious,so the research is in this background.The effect of alternating current on stress corrosion cracking behavior and mechanism of X80 pipeline steel in near-neutral solution was studied.The effect of full wave AC,constant potential,half wave AC,the coupling effect of AC and Cl-,Bacillus cereus on stress corrosion cracking behavior and mechanism of X80 pipeline steel were studied through data acquisition,electrochemical,immersed test and slow strain rate tensile(SSRT)test in combination with the method of surface analysis.Finally,the electrochemical cyclic voltammograms combined with finite element Comsol Multiphysics simulation were used to analyze AC interference on stress corrosion electrochemical mechanism.The results showed that AC could accelerate X80 steel corrosion.In near neutral pH solution,with the increasing of AC density,the corrosion morphology of X80 steel changed from general corrosion to localized corrosion.The AC improved stress corrosion susceptibility,the stress corrosion mechanism of X80 steel was mixed by anodic dissolution and hydrogen embrittlement mechanism in near neutral pH solution.When the AC density Was less than 10A/m2,the AC vibrate effect improved SCC susceptibility.When the AC density was more than 30A/m2,AC can promote hydrogen precipitation and improved SCC susceptibility.Negative half wave AC promoted the cathodic and anodic reaction of X80 steel.The positive half wave AC promoted the anodic reaction,therefore,the positive and negative half wave AC both promoted X80 steel corrosion.When the negative half wave AC density was 30A/m2,it promoted hydrogen precipitation and lead to pitting corrosion because of hydrogen induced anodic dissolution.The shape of pitting showed acuity.The SCC susceptibility significantly enhanced and hydrogen embrittlement act a major role in the SCC mechanism.When the positive half wave AC density was not less than 30A/m2,it promoted the anodic dissolution of X80 steel.The pit show wide and shallow concave shape.X80 steel show some SCC susceptibility in those positive half-wave AC density.The synergy AC and Cl-promote X80 steel corrosion and show the obvious SCC susceptibility.The waveform between the working electrode and reference electrode had obvious changes and shifted negative after being applied AC.AC participated in corrosion process.AC promoted Cl-migration and led to the Cl-concentration at crack tip.AC promote ion migration and then promote corrosion.In the system of AC and Bacillus cereus,AC promoted the migration of organic matter,which result in the surface of X80 steel absorbed much organic matter.At the bottom of the organic matter and corrosion product film,Bacillus cereus can transfor NO3-into NH4+ through denitrification in the anaerobic environment.Bacillus cereus prompted local corrosion.The synergy AC and bacillus cereus accelerated the X80 steel corrosion and improved the stress corrosion susceptibility.The reason for AC improving corrosion and stress corrosion was studied by combination of cyclic voltammograms and finite element simulation.AC will change the original polarization potential,do cyclical vibrate in the metal surface and destroy the metal surface state.Besides,AC caused anodic oxidation curve and cathodic reduction curve was asymmetry.The cyclic voltammograms overlay results showed anodic dissolution rate was greater than the cathodic reduction rate,and then generated localized corrosion.Cathodic current increase mainly in the stage of cathodic polarization.Combined with the finite element simulation,it can be found that the increase of cathodic polarization current promoted hydrogen local precipitation,and cause hydrogen embrittlement of X80 pipeline steel.Under some conditions,X80 steel will produce cracks and show high stress corrosion susceptibility.