| Corrosion of welded joints is an inevitable issue in oil/gas industry.Welded joints（WJ）,which are composed of a base metal（BM）,heat affected zone（HAZ）and weld metal（WM）,naturally possess compositional and microstructural heterogeneities;therefore,the corrosion behavior of a welded joint appears erratic and difficult to predict.In this sense,the fundamental research in the electrochemical behavior of such joint parts is required,and will be conducive to guarantee the integrity and safety of the pipeline steel,reducing the risk of corrosion failure of pipeline steel.Weight loss method,classical electrochemical method,wire beam electrode array,micro-electrochemical method and numerical simulation method was implemented to study the localized corrosion behavior of the X80 steel welded joint in CO2-saturated solutions.The main conclusions are as follows:（1）Temperature and the concentration of acetic acid（HAc）are the main factors,and the concentration of monoethylene glycol（MEG）and sodium chloride are the secondary factor affecting the corrosion rate.The weighting factor of the influence of corrosion rate of each part on the corrosion rate of X80 steel welded joint was:BM was 0.14%,inter-ciritical HAZ（ICHAZ）was 29.65%,fine-grained HAZ（FGHAZ）was 35.64%,coarse-grained HAZ（CGHAZ）was 32.01%and WM was 2.56%.Corrosion rates region X80 steel welded joint heat effect on the corrosion rate play a major role.The corrosion rate model between the X80steel welded joint and the isolated regions in the X80 steel welded joint was:CR=0.107+0.009×CRB M（10）0.831×CRICH AZ-1.066×CRFGH AZ（10）0.943×CRCGH AZ（10）0.158×CRW M.Under the same conditions,the corrosion rate exhibited the following order:ICHAZ>FGHAZ>CGHAZ>WJ>BM>WM.The relationships between the corrosion rate of the X80 steel welded joint and the solution p H was non-linear.（2）Corrosion current density of different regions in the X80 steel welded joint increased with the increase of temperature.The anodic Tafel slope of the WM was smaller than the other regions of the X80 steel welded joint,i.e.,when the same potential difference applies to different regions in the X80 steel welded joint,the changes in the current density of the WM are more obvious than those in the other regions.MEG molecular adsorped on the X80 steel surface by physical adsorption,the corrosion rate decrease with the increase of the concentration of MEG.The mechanism of HAc that affects the cathodic reaction process was found to be the buffering effect,and HAc served as the source of H+.（3）A new index,namely,the galvanic corrosion intensity factor,was proposed and verified to succeed in characterizing the degree of galvanic corrosion.HAc significantly affect the degree of galvanic corrosion of the simulated X80 steel welded joint.The degree of galvanic corrosion of the simulated X80 steel welded joint increased with the decrease of WM and increased with the increase of the HAZ and BM.The degree of galvanic corrosion of the simulated X80 steel welded joint weakened under anodic polarization condition,i.e.,the corrosion products formed by anodic polarization can protect the matrix.The intensity of galvanic corrosion increased with the increase of the concentration of Cl-,and Cl-could decrease the protectiveness of the corrosion product.（4）Based on the finite element method,the prediction model of galvanic corrosion rate of the X80 steel welded joint was established.Temperature and the concentration of MEG influenced both the anodic reaction and cathodic reaction current density of the X80 steel welded joint.The anodic reaction current density and the cathodic reaction current density increased with the increase of temperature and with the decrease of the concentration of MEG.With the increase of the insulating interval,the anodic reaction current density of the HAZ and WM decreased,while the anodic reaction current density of the BM increased.The area ratio of the BM/HAZ/WM significantly affect the anodic reaction current density of the X80steel welded joint.Increasing the width of the cathodic metal,causing the potential of the X80steel welded joint shifted positively,the anodic galvanic current density increased,and the cathodic galvanic current density decreased.Increasing the width of the anodic metal, resulting in the potential of the X80 steel welded joint negatively shifted,the anodic galvanic current density decreased,and the cathodic galvanic current density increased.In the absence of stress,the defect affected the current density distribution of the X80 steel welded joints mainly by changing the area ratio of cathode and anode.In the presence of stress,stress concentration caused by the defects,and the degree of stress concentration increased with the increase of strain,therefore causing the galvanic current density increased significantly in the defect regions.（5）A new method for the simultaneous determination of electrochemical parameters at different scales over a simulated X80 steel welded joint surface using traditional electrochemical techniques in conjunction with the WBE and SVET methods was developed.The polarization resistance of the BM and HAZ decreased with time under isolated conditions,while the polarization resistance of the WM slightly increased with the increase of immersion time.After coupled,the polarization resistance of the BM and HAZ were greater than that at the same time under the isolated conditions,while the polarization resistance of the WM was smaller than that at the same time under the isolated conditions.The WM acted as the dominant anode of the simulated X80 steel welded joint during immersion in CO2-saturated solution,whereas the HAZ and BM acted as cathodes.The maximum anodic current was observed in the WM region,and the maximum cathodic current was observed in the HAZ region.The corrosion process of the WM microelectrode in the WM/HAZ interface enhanced with the increase of immersion time.Local anodic region mainly distributed in the WM area,while the local cathodic region mainly distributed in the HAZ and BM area.With the increase of coupled time,the current density of the WM decreased first and then gradually increased,and both the anodic and cathodic ionic current density of the CGHAZ increased.The area of local anode region expanded with time,while the amplitude of anodic ionic current density was smaller than the amplitude of the cathodic ionic current density. |
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