Galvanic Corrosion Behavior Of 110SS Tubing And Downhole Tools Materials

The actual working conditions in oil and gas field was simulated,coupling of downhole tool materials 925 and 9Cr1Mo with sulfur-resistant tubing steel 110SS at four ratios of cathode and anode area of 1:1,1:5,1:10,and 1:30,respectively.Electrochemical tests and immersion corrosion tests were used,and the scanning electron microscopy(SEM),energy disperse spectroscopy(EDS)and X-ray diffraction(XRD)were supplemented to explore galvanic corrosion behavior and galvanic corrosion effect,and clarify the influence of cathode and anode area ratio on galvanic corrosion.Conclusions of study as follows:From the corrosion rate results,it is known that 9Cr1Mo was severely corroded before coupling and moderately corroded after coupling,while the 110SS was extremely severely corroded before and after coupling,there is no obvious linear law of the corrosion rate changing with the ratio of cathode to anode area.The electrochemical test results show that the driving force for galvanic corrosion is insufficient after 9Cr1Mo and 110SS are coupled.Therefore,the9Cr1Mo/110SS galvanic pair mainly undergoes general corrosion caused by CO₂ corrosion,H₂S corrosion,and pitting corrosion caused by the enrichment of Cl^-in the pitting pit under the synergistic effect of high temperature,high acidity and high Cl^-.Obvious passivation zone appears in the anode of 9Cr1Mo,and the resistance polarization of the anode controls the progress of the reaction.The polarization process of 110SS is controlled by cathodic activation polarization,and the process of combining hydrogen ions with electrons controls the progress of the reaction.After coupling,the galvanic potential and the tendency of electrochemical corrosion of the 9Cr1Mo/110SS galvanic couple will increase with the increase of the anode-cathode area ratio.At the same time,the overall corrosion resistance and corrosion rate will decrease.The corrosion rate results of the 925/110SS couple show that 925 is slightly corroded and110SS is extremely severely corroded before and after coupling.The corrosion rate of 925 is less than before coupling.The corrosion rate of 110SS after coupling with different area ratios is greater than that of the corrosion alone.The rate increases as the area ratio of cathode to anode increases.Electrochemical test results show that after 925 and 110SS are coupled,galvanic corrosion will occur with high potential 925 as the cathode and low potential 110SS as the anode,which will increase the anode corrosion and reduce the cathodic corrosion rate.The intersection of the cathodic polarization curve and the anodic polarization curve of 925 is located in the stable passivation zone,indicating that the nickel-based alloy 925 has good self-passivation ability under high temperature and high acid corrosion conditions.After coupling,the cathode polarization is greater than the anode polarization.The entire reaction is controlled by the cathode.When the polarization is stable,the cathode current is equal to the anode current.The larger the cathode-to-anode area ratio,the smaller the 110SS area,the greater the anode galvanic current density.The degree of anode corrosion increased.The coupling of 925 and 110SS should be avoided as far as possible.The downhole tool materials 9Cr1Mo and 925 conform the production requirements under four area ratios of 1:1,1:5,1:10,and 1:30.The 110SS after coupling with 9Cr1Mo shows slightly better corrosion resistance than when coupled with 925.110SS should be coupled with9Cr1Mo at 1:30 and used after protection under the high-temperature and high-acid corrosion conditions in this article.