Electrochemical Corrosion Behavior Of API X80 Steels In NACE Solution

Welded joints,which is easy to be occurred local corrosion due to the uniform of component and stress concentration,is one of the weakest links in the safety of pipeline transportation.Corrosion control of welded joint has always aroused people's concern.Welding joint of pipeline steel which consists of base metal,the heat-affected zone(HAZ)and the weld metal is a short circuit of battery,and the mechanism and corrosion evolution in the internal pipeline are not clarified now.The traditional method of electrochemical test can only obtain the statistical average electrochemical information respectively of base metal,HAZ and weld metal,and has certain limits in representing the process of local corrosion of welded joint which is highly localized.In order to solve these difficulties,traditional electrochemical test was used in this paper to research the effects of the immersion time and the thickness of liquid layer on the electrochemical corrosion behavior of X80 Pipeline steel welded joint components(base metal/HAZ/weld metal)in NACE solution.Welded joint simulated by arrayed electrode and multichannel galvanic corrosion test system developed by our laboratory was used to research multi-body galvanic corrosion behavior of welded joints;the effects of area ratio of welded joints components and the concentration of acetic acid on electrochemical corrosion process of simulate welded joints.The most important conclusions are as follows:1?Corrosion behaviors of single material electrode make up of base metal/ HAZ/ weld metal are similar in NACE solution—active dissolution,but the kinetic data have some difference.With the extension of immersion time,there is corrosion product film generated on the electrode surface.The film restrains the electrode reaction,but it's loose and the effect of protecting is limited.By the loose film and chloridion in solution impact,the electrode surface happens severe local corrosion in the end.2.Corrosion behaviors of single material electrode make up of base metal/ HAZ/ weld metal are similar in different thickness of NACE solution layer.The oil film on the surface of waterline blocks diffusion and dissolution of CO2 from gas to liquid,and then refrain the cathode reaction;be influenced of corrosion product film and chloridion in solution,electrodes in waterline develop severe local corrosion,then lead to self-corrosion potential more negative than electrodes in other position of different thickness of liquid layer.The other electrodes only show that anode was activated and dissolved in experiment time,and there is no corrosion product film generated.3.When welded joints simulated by arrayed electrode immerse in NACE solution,weld metal is always advocate main anode in shorted cell,the HAZ is always advocate main cathode,the polarities of base metal are alternant with the distance extended between base metal and HAZ,and the current are different.Taking accelerated corrosion into consideration,some active points of corrosion serious maybe appear in the area of base metal which nearby HAZ,and threaten the safety of welded joints.4.Keep the area ratio of base metal and HAZ is 10:2 constant in simulate welded joints,when decrease the area of weld metal gradually,current intensity and potential difference between main anode and main cathode will increase obviously;the base metal which far away from HAZ showed state of self-corrosion basically;galvanic corrosion is highly localized and intensity increased,but the range of impact narrow.5.When the contents of acetic acid in NACE solution increase from 0 to 1.0mass% gradually,polarization degree of main anode and main cathode will be strengthen;absolute of current will increase obviously;galvanic corrosion strength will also increase;the range of cathode protection will narrow.It's indicated that acetic acid can intensify the galvanic corrosion of welded joints.The research of this paper contributes to deepening understanding of localized corrosion on pipeline welded joints,and providing scientific basis and reference for welding technology and corrosion control of pipeline steel.