Research On SRB Corrosion Mechanism Of Cu-Bearing L245 Pipeline Steel And Welded Joint

With the wide application of hydraulic pressure technology in shale oil&gas fields and the increase exploitation of offshore oil and gas,the corrosion problems of oil&gas pipeline system caused by microorganisms such as SRB was becoming more and more serious,which brought great interference and economic loss to normal production of oil&gas industry.Corrosion scholars and experts have carried out a lot of research on SRB and other microbial corrosion problems.However,the mechanism and influencing factors of microbiologically influenced corrosion to pipeline under various conditions were still not very clear,the SRB corrosion problems haven’t been solved fundamentally,so the SRB corrosion theory and inhibition mechanism of pipeline need to be further explored.In this research,the SRB corrosion behaviors and mechanism of pipeline steel including L245,Cu-bearing L245 steel with welded joint were firstly studied in certain kinds of different environment containing SRB by static immersion test and simulant condition method.Micro detection techniques such as electrochemical test,laser scanning confocal microscope,focused ion beam-scanning electron microscope（FIB-SEM）,micro-electrochemical scanning technology were used to analyze the biofilm structure.The influence of adding Cu element on SRB corrosion behavior were investigated and the corrosion resistance property of Cu-bearing pipeline steel with welded joint were also evaluated.Finally,the influence of environmental factors on SRB corrosion behavior was clarified.The specific research contents were as follows:（1）The characteristic difference and micro-mechanism of SRB corrosion on L245pipeline steel with welded joint were studied.The existence of biofilm accelerated the pitting corrosion in stage of 168h immersion.At the initial stage of incubation（0～24h）,the growth rate of microbial film on welded metal was higher than that on base metal.The pitting corrosion rate reached the maximum in the fast growth period（24h～72h）,and decreased in the decay period（96h～168h）.The welded metal has higher sensitivity of pitting corrosion induced by SRB,especially near the fusion line area.The micro-area electrochemical scanning results showed that the pitting degree of weld zone was higher than that of base metal due to the difference of electrochemical surface properties between welded metal and base metal.（2）The SRB corrosion characteristics and mechanism on L245 under different cathodic polarization potentials were studied.The corrosion product films formed on the surface of L245 electrode plate showed different electrochemical characteristics with different cathodic potentials.With weak cathode polarization potentials(OCP～-0.875_SCE),the development of biofilm was promoted,and sessile SRB cells embedded in the biofilm showed an increasing trend with the potentials changing.The SRB metabolism activity was strengthened,which resulted in the continuous intensification of pitting corrosion and the remission of uniform corrosion.With strong cathode polarization potential(-1.05V_SCE),SRB life activity was weakened,the development of biofilm was inhibited,the quantity of sessile SRB cells in biofilm was reduced,and the pitting degree was alleviated accordingly.The cathodic polarization potential was closely related to SRB metabolic activity and pitting degree,and SRB directly gaining electrons from the metal surface led to the results of.the enhancement of SRB metabolic activity and pitting under the biofilm.（3）The influence of different concentration of Cu element alloying in L245 for its MIC resistance property was studied.With the increase of Cu content from 0wt.%to0.6wt.%,EPS clusters on surface gradually became sparser,the corrosion product film became thinner,and changed from double layers structure to single layer structure,also the anionic selectivity of the corrosion product film decreased.Accordingly,the quantity of sessile SRB in biofilm decreased,and the pitting depth,density and corrosion rate also decreased.It concluded that Cu played a significant role in inhibiting the MIC induced by SRB.This was mainly due to the release of Cu⁺,Cu²⁺from the surface of Cu-bearing L245 steel and generated Cu₂O and Cu₂S,which could continuously accumulate in the biofilm and form Cu-rich layer inhibiting the life activities of sessile SRB cells.（4）The corrosion characteristics and mechanism of Cu-bearing L245 steel and welded joint were studied in SRB culture medium and other simulated working conditions of water injection wells.The results showed that in SRB culture medium,the SRB corrosion resistance property of base metal was obviously better than that of welded metal due to the higher Cu concentration,and welded metal was sensitive to the pitting corrosion induced by SRB.In H₂S/CO₂working conditions without SRB,uniform corrosion occurred only,and the uniform corrosion rate increased from 60℃to95℃and then decreased from 95℃to 110℃.In the H₂S/CO₂ working conditions with SRB,pitting phenomenon was obvious in addition to uniform corrosion,especially on the weld zone.The presence of trace H₂S could promote the metabolism and growth of SRB,thus the pitting corrosion rate were accelerated.In H₂S/CO₂working condition where Cl^-and SRB co-exist,the pitting was further aggravated,and the presence of Cl^-could aggravate the risk of pitting.（5）The influence of environmental factors on SRB corrosion behavior of Cu-bearing steel was studied.The presence of SRB was the crucial and direct factor causing pitting corrosion.Environmental factors such as temperature,dissolved oxygen,salinity,p H value not only directly affected the corrosion phenomenon by themselves,but also indirectly promoted the pitting corrosion degree by affecting SRB cell life activity.According to the range numerical analysis,the sensitivity order of the factors affecting pitting were as follows:SRB quantity>p H>temperature>dissolved oxygen>Cl^-concentration.The corrosion rate prediction models under different influence factors were established accordingly.