Study On Corrosion Mechanisms Of X80 Pipeline Steel And Its Laser Welded Joints In CO₂ Environments

Modern oil and gas transportation pipeline engineering has been applied on a large scale,large caliber,high pressure,long distance advanced pipeline steel has become the primary choice for pipeline transportation,X80 pipeline steel because of its high strength,good mechanical properties and low laying cost is widely used in pipeline transportation engineering.Due to the influence of service conditions and environment,corrosion and hydrogen penetration are very easy to occur during pipeline operation.CO₂,as the most common gas in pipeline operation,is one of the important factors causing corrosion and hydrogen penetration.Therefore,in order to improve pipeline safety and service life,it is necessary to study the corrosion behavior and hydrogen penetration behavior of X80 pipeline steel in CO₂environment and their mechanism.In recent years,ionic liquid corrosion inhibitors have been widely used in CO₂corrosion protection of pipelines due to their advantages of high efficiency,high thermal stability,low toxicity and biodegradation.It is of great significance to develop an ionic liquid corrosion inhibitor to inhibit the corrosion and hydrogen penetration behavior in CO₂environment.Welding is the most common and effective method for connecting pipelines.Laser welding has been applied to welding pipeline steel because of its high flexibility,high energy density and high efficiency.The microstructure and properties of pipeline steel can be significantly changed and improved by adjusting the heat input of laser welding.In addition,due to differences in the microstructure of welds and heat-affected zones in welded joints,it is easy to induce galvanic phenomena in welded joints in a CO₂environment,which promotes corrosion in specific areas.Therefore,it is necessary to investigate the influence of heat input on the microstructure and properties of laser welded joints of X80 pipeline steel as well as the differences in corrosion resistance in CO₂environment.In this thesis,the effects and differences of temperature,pressure and flow rate on the corrosion behavior of X80 pipeline steel in CO₂environment were studied through orthogonal test and electrochemical testing technology.The results showed that:velocity has the greatest influence on the corrosion driving force,while temperature has the greatest influence on the corrosion rate.With the increase of temperature and velocity,the corrosion driving force and corrosion rate gradually increase.With the increase of pressure,the corrosion driving force and corrosion rate first decrease and then increase.Among them,temperature mainly affects the activation energy and generation of corrosion products to affect the corrosion rate;pressure mainly affects the solubility and p H of CO₂in water to affect the corrosion rate;flow velocity mainly affects the corrosion medium,charge transfer rate and wall shear stress to affect the corrosion rate.An ionic liquid corrosion inhibitor was synthesized by ion exchange method,and its effects on the corrosion behavior and hydrogen penetration behavior of X80 pipeline steel in CO₂environment were studied by electrochemical testing technology and electrochemical hydrogen penetration technology.The results showed that:With the increase of corrosion inhibitor concentration,the corrosion rate and hydrogen permeability of X80 pipeline steel first decrease and then increase.The minimum value is obtained when the corrosion inhibitor concentration is 500 mg·L^-1,and the highest corrosion inhibition efficiency is 97%.When there is no corrosion inhibitor,the corrosion resistance of X80 pipeline steel first decreases and then increases with the extension of soaking time,and the minimum value is obtained at 72 h.The permeability of hydrogen atom increased first and then decreased,and reached the maximum value at72 h immersion.When corrosion inhibitor is present,the corrosion resistance of X80pipeline steel gradually decreases with the extension of soaking time,and the minimum value is obtained at soaking 336 h,while the permeability of hydrogen atom is gradually enhanced and the maximum value is obtained at soaking 336 h.However,the corrosion resistance and inhibition of hydrogen atom are much higher than that without corrosion inhibitor.When the concentration of corrosion inhibitor is more than 100 mg·L^-1,The corrosion inhibition efficiency is still above 91%.It is found that the adsorption mechanism of corrosion inhibitors is mainly physical and chemical adsorption.In the absence of corrosion inhibitors,a dense Fe CO₃ product film will be produced on the surface of X80 after a long time soaking,thus slowing down corrosion and hydrogen diffusion.In the presence of corrosion inhibitors,no corrosion products will be produced,and partial desorption of corrosion inhibitor molecules will occur,resulting in enhanced corrosion rate and hydrogen diffusion.By changing the welding speed,the microstructure evolution and corrosion behavior difference of laser welded joint in X80 pipeline steel were studied.The results showed that with the increase of heat input,the proportion of ferrite in each area of welded joint increased,the sum of the proportion of martensite and bainite decreased,and the proportion of bainite in weld and coarse crystal area increased.And the hardness of weld center is the highest,and the hardness decreases with the increase of heat input.In addition,the corrosion resistance of welded joints increases with the increase of heat input,and the corrosion resistance of welded joints in different areas is different.When there is less heat input,the corrosion resistance of the heat affected zone is greater than that of the weld,and as the heat input increase,the corrosion resistance of the weld is greater than that of the heat affected zone.The corrosion law of welded joints in immersion test is consistent with that without immersion,and the corrosion product film produced by immersion can improve the corrosion resistance.After the addition of corrosion inhibitor,the corrosion resistance of each welded joint has been greatly improved,but it does not affect the difference of corrosion behavior in each region.Among them,the region of ferrite and bainite combination has better corrosion resistance,low hardness;The region of martensite and bainite combination has poor corrosion resistance and high hardness.According to the relationship between heat input and microstructure and corrosion resistance,the function models of corrosion current density and charge transfer resistance with heat input were established.