Research On CO₂ Corrosion Characteristics Of Gathering Pipeline Steel In The Medium Containing Crude Oil

At present,laboratory study of CO₂ corrosion problems of oil and gas pipe usually consider only pure-water system to simulate the corrosive environment,seldom regard the effect of crude oil on the corrosion process.But the actual production and transportation of fluid is often multiphase mixed media such as oil,gas and water,crude oil is one of the important factors that affect corrosion behavior,related scholars study of impact of crude oil on CO₂ corrosion is still relatively small,further studies of the relationship between oily corrosive environment of gathering pipeline and corrosion behavior are needed.In the present dissertation,the CO₂ corrosion behavior and mechanism of X65 steel in the CO₂/oil/water environment of gathering pipeline were studied using high temperature and high pressure reaction kettle and electrochemical testing device.The macro analysis methods?SEM,XRD,EDS?and electrochemical test?EIS?were used to research the influence factors,corrosion mechanism and product film characteristics,the effects of electrode reaction process on the corrosion mechanism of X65 steel were discussed.The main research work and the results are as follows:1.Corrosion factors of CO₂/oil/water environment were studied by weight loss test and macro analysis.The results show that water cut is the main controlling factor and it influences corrosion behaviour significantly.Crude oil in the corrosive medium can weaken the dissolution of corrosion scales and affect the grain size,piling pattern,thickness and structure of corrosion scales on the X65 steel surface.Consequently,crude oil has an obvious effect of corrosion inhibition and it can reduce the corrosion rate greatly.2.The influence laws of different factors on X65 steel product film were studied,the relationship between corrosion behavior and product film characteristics in the presence of oil was discussed,corrosion mechanism model of CO₂/oil/water system was established.The results show that corrosion rate of X65 steel increases with the increase of water cut.When the water cut is low,the corrosion product is relatively dense,which has certain protective effect on the steel substrate.With the increase of water cut,the barrier effect of crude oil on the metal surface is weakened,product film becomes thickened and loose,and binding force with the substrate is poor,the corrosion rate increased significantly.The corrosion rates of X65 steel all show the trend of increase at first and then decrease with the temperature,flow rate and CO₂ partial pressure rise,and the maximum values occur at the critical values?critical temperature,critical velocity,critical CO₂ pressure?.Below the critical value,the corrosion products formed on the surface of the substrate is loose and unstable,and the adhesion strength with the substrate surface is weak,higher than the critical value,the product film is dense,strong adhesive,and has a certain protective effect.3.The change of water cut leads to the change of corrosion rate and morphology of X65steel,essentially the corrosion mechanism has changed.The graph of the X65 steel corrosion rate changes with the water cut is divided into four different intervals,respectively represent four different corrosion mechanisms,the corrosion mechanism model of water cut influence on the product film structure and corrosion morphology of X65 steel is established accordingly.4.The properties of X65 steel corrosion product film under different factors were studied,the effect mechanisms of product film in the presence of oil on the CO₂ corrosion behavior of X65 steel were explained.Under the condition of different water cut,the corrosion process of X65 steel is mainly controlled by the diffusion control and activation control,the corrosion rate increases with the increase of water cut.The AC impedance spectroscopy of the corrosion product film has two time constants,which is consisted of high frequency capacitive arc,low frequency capacitance and the Warburg impedance.With the increasing of water cut,corrosion product film becomes loose and porous,corrosion active points increase,resulting in the low frequency capacitive arc shrinks and the corrosion degree increases.Under the conditions of different temperature,the corrosion process of X65 steel is mainly affected by the activation control,the AC impedance spectroscopy has two time constants,which is consisted of high frequency capacitive arc and low frequency capacitive arc.Temperature can accelerate the reaction rate,but also accelerate the deposition of corrosion product film,with the increase of temperature,the maximum corrosion rate appears.When the specimen surface is covered with tiny grains and relative density product film,the corrosion rate starts to decrease.Oily corrosion system will enhance the reaction activation energy,inhibit the corrosion of X65 steel.The increase of temperature and the corrosion inhibition of crude oil can strengthen the protection of the corrosion product film,which makes the peak temperature shift to the lower temperature segment.Under the conditions of different velocity of flow,the corrosion process of X65 steel is mainly controlled by the activation control and diffusion control,the AC impedance spectroscopy is consisted of high frequency capacitive arc,low frequency capacitance and the Warburg impedance.When the flow rate is low,the oil-water mixture is not uniform,and the crude oil can not be absorbed and enriched evenly on the surface of the specimen,which may cause local corrosion.The product film is loose and easy to fall off,the flow rate increases will accelerate the diffusion rate of corrosive ions and the dissolution of the steel substrate,the corrosion rate increases.When the velocity exceeds the critical value,relative density product film with small grains comes into being,and flow rate increases will make the oil film uniformly adsorb on the surface of the specimen,reduce the corrosion active points and protect the metal from corrosion,the corrosion rate begins to decline.Under different CO₂ partial pressure conditions,the X65 steel corrosion process is mainly controlled by the cathodic control,the corrosion rate shows a trend of first increases and then decreases with the increase of CO₂ partial pressure.The AC impedance spectroscopy is consisted of high frequency capacitive arc and low frequency capacitive arc.When the CO₂partial pressure increases,the dissolution of H₂CO₃ concentration increases,the depolarization reaction is faster,and the corrosion speed is faster.The increasing of CO₂partial pressure also accelerates the deposition rate of FeCO₃.When the CO₂ partial pressure exceeds the critical value,product film formed on the metal surface becomes compact and has strong adhesion,which plays a protective effect on the metal substrate.With the increasing of CO₂ partial pressure,the solubility of CO₂ in the crude oil increases,which makes the crude oil swell.The viscosity of crude oil decreases and the flowability becomes better,the probability of adsorption and contact with the substrate increases,the corrosion rate goes down.5.The influences mechanism of electrode reaction process in crude oil emulsion on the corrosion behavior of X65 steel were studied,the cathode and anode impedance model of CO₂ corrosion process was established,and the connection between substrate surface state and impedance spectrum characteristics was discussed.It can be seen from the polarization curves that the effect of oil content on the cathodic process is greater than the anodic process,indicating that CO₂ corrosion in the emulsion is a typical cathode control reaction.The forming conditions of corrosion product film on the sample surface is related to the electrode reaction process,the changes in low frequency impedance arc and capacitive arc can be used to conclude the electrode reaction processes and the product film formation conditions.When the emulsion oil content is 0%,2.2%,the anode polarization impedance spectrum of X65 steel has three time constants,which is consisted of high frequency capacitive arc,low frequency inductance arc and capacitance arc.When the oil content is 3.1%,the anodic polarization impedance spectrum consists of high frequency capacitive reactance arc and low frequency capacitive reactance arc,there are only two time constants,the inductance arc disappears.With the increase of oil content,crude oil adsorption opportunities increase,under the synergy of oil film adsorption and product film deposition,the covering area of product film layer is more and more big,the anode activation area is more and more small,the low frequency capacitive reactance arc gradually expands,but the inductive arc gradually shrinks and finally disappears.Under the conditions of different emulsion oil content,the cathode polarization impedance spectrum of X65 steel is always made up of high frequency capacitive reactance arc and low frequency capacitive reactance arc,there are two time constants.Oil content will not affect the cathode reaction type,just influence the cathode reaction speed.With the increase of oil content,high and low frequency capacitive arc expand.The cathodic reduction reaction is controlled by the activation control of H₂CO₃ andHCO₃^-.With the increase of emulsion oil content,on the one hand,the solution resistance increases,and the liqu idity becomes poor because of the emulsion viscosity increases,which will slow the mass transfer rate of H2CO3andHCO₃^-,and inhibit the cathodic reaction.On the other hand,the adsorption opportunities of crude oil increase,so that the corrosion rate decreases.