Corrosion Behavior Of API X80 Pipeline Steel Weld Joint In NACE Solution

With the increasing demand of oil and gas resources,the corrosion problem of oil and gas pipelines attracts much attention.Welding is used to connect oil and gas pipelines.Weld joint,which is easy to be occurred local corrosion due to the uniform of component and stress concentration,and it’s one of the weakest links in the safety of pipeline transportation.Corrosion control of weld joint has aroused people’s concern.Weld joint of pipeline steel which consists of base metal,the heat affected zone and the weld metal which is complex galvanic couple,and the mechanism and corrosion evolution in the internal pipeline are not clarified now.At present,few studies have been done on the research of the galvanic corrosion mechanism of each sub area by separating the welded joints,and the research on the corrosion mechanism of galvanic corrosion is not thorough enough.In order to solve these difficulties,the paper research the corrosion behavior of X80 Pipeline steel welded joint components(base metal/ heat affected zone/ weld metal)in NACE solution.The effects of chloride ion,acetic acid,temperature and ethylene glycol on the corrosion behavior of the weld joint were studied by electrochemical method.The main factors and secondary factors in the influence factors were analyzed by the orthogonal design of loss experiment.The most important conclusions are as follows:In this paper,it use the classical electrochemical measurement techniques to research the influence of four factors about the electrode’s corrosion electrochemical behavior,and it studied the electrode process information of each partition in electrolyte solution.The four factors incleude temperature,acetic acid,chloride ion and ethylene glycol.In the same electrolyte solution,the corrosion tendency of the weld metal is higher than base metal and heat affected zone,and the resistance of the weld metal is obviously higher than others.Under the experimental conditions,with the increase concentration of acetic acid,increase temperature or decrease concentration of ethylene glycol,the corrosion rate of the base metal,heat affected zone and weld metal gradually accelerated.The corrosion rate of the base metal,heat affected zone increased firstly and then decreased with the increase of chloride,and the corrosion rate of the weld metal gradually increased.By analyzing the change of electrochemical impedance spectroscopy and potentiodynamic polarization curves,the mechanism of various factors on electrode reaction is clarified.In this paper,the method of orthogonal experimental design is used.Four factors and four level orthogonal corrosion weightlessness experiments were designed to study the corrosion behavior weld joint in NACE solution.The influence of each factor(chlorine ion,acetic acid,temperature and ethylene glycol)on the corrosion behavior of the weld joint and each partition was studied,and the main factors influencing the corrosion behavior of the acetic acid and temperature were obtained,and the sodium chloride and ethylene glycol were the secondary factors.According to the proportion of Fe,C and O,the main components of the corrosion products are ferrous carbonate,some of which are ferrous acetate.In this paper,the corrosion electrochemical behavior and weight loss behavior of the weld joint are analyzed,and the correlation between the weld joint of X80 steel and the corrosion behavior of each partition is studied.In the same electrolyte solution,the corrosion loss rate of the weld metal is obviously lower than base metal and heat affected zone on one weld joint,which is in agreement with the analysis of the impedance and polarization curves in the electrochemistry.The galvanic corrosion process of the welded joint is taken as a whole,and there is a significant characteristic of the platform corrosion,in which the corrosion degree of the weld metal is obviously greater than that of base metal and heat affected zone.The reason is that the galvanic corrosion process makes the weld zone be activated.It is known that the corrosion tendency of the weld metal is higher than others by the classical electrochemical test.Therefore,the corrosion behavior and corrosion law of each zone are analyzed from two angles of corrosion weightlessness and classical electrochemistry,and the conclusions obtained are basically the same.