Study Of The Corrosion Inhibition Mechanism And The Synergistic Corrosion Inhibition Effect Of Inhibitors In CO₂/H₂S System

Corrosion is a spontaneous process,which affect the production and living of people.The corrosion of carbon steel induced by CO2/H2S is common in the oil and gas field industry leading to many incidents.The corrosion can influence the normal production,and simultaneously bring threats to people's life and property security.The incidents induced by corrosion failure can lead to economic losses and casualties ruining the stable and harmonious social environment.Various protection technologies have been used to retard the C02/H2S corrosion,among which the injection of corrosion inhibitors is one of the most simple,economical and effective methods to inhibit the corrosion of carbon steel induced by CO2/H2S.However,there are two problems during the application of corrosion inhibitors.(1)The performance of inhibitors can be influenced by the corrosive environment.For example,the performance of some inhibitors,which can effectively inhibit the CO2 corrosion,deteriorated with the introduction of H2S into the corrosive environment.(2)Single inhibitor cannot own effective inhibition performance,and the discovery of the synergism between inhibitors become necessary to develop a composite inhibitor.However,many experiments are needed to research on the synergistic corrosion inhibition effect because of the lack of theoretical direction.In this work,many experiments were performed to study the corrosion inhibition mechanism of inhibitors and the synergism between them in the CO2/H2S coexistent brine solution.The main research contents are listed as follows:1.The characters of the corrosion products were investaged by XRD,SEM and EDS in the CO2/H2S coexistent brine solution.The results show that the corrosion products formed in this corrosive environment were composed of mostly FeS and limited FeCO3.The thickness of the corrosion products layer increased with the increasing of immersion time of the samples,and many defect was observed in the corrosion products layer.2.The influences of HS-ions on the inhibition performance of 9 corrosion inhibitors were investaged by electrochemical measurements,weight loss method and SEM surface analysis.The results show that the introduction of H2S can enhance the performance of imidazoline quaternary ammonium salt(IAS),N-benzyl pyridinium chloride(BPC),dodecyl trimethyl ammonium bromide(DTAB),tetradecyl trimethyl ammonium bromide(TTAB),hexadecyl trimethyl ammonium bromide(CTAB)and octadecylamine(OCT)and deteriorate the performance of thiourea(TU),L-cysteine(CYS)and sodium dodecyl sulfonate(SDS).In acidic solution with H2S,the HS-ions can first be adsorbed on the steel surface and negatively charge the steel surface.While,the inhibitors,IAS,BPC,OCT,DTAB,TTAB and CTAB,which are effective in the C02/H2S coexistent brine solution all can be ionized to inhibitor cations.Therefore,the electrostatic interaction between inhibitor cations and negatively charged surface is conducive to the adsorption of inhibitors.According to the results of molecular dynamics simulation,a significant enrichment of the HS-ions can be observed around the inhibitor cations of IAS,TTAB and BPC resulting in the good inhibition performance of these inhibitors in the CO2/H2S coexistent brine solution.3.Among the 9 corrosion inhibitors mentioned above,IAS and OCT can effectively retard the corrosion of carbon steel both in the CO2-saturated brine solution and the CO2/H2S coexistent brine solution.In other words,the changing of the corrosive environment cannot significantly affect the performance of IAS and OCT.Therefore,these two inhibitor were selected as the main components for the development of composite inhibitors in the CO2/H2S coexistent brine solution.And the surfactants of DTAB,TTAB and CTAB were regard as auxiliary components in the composite inhibitors because they may promote the dissolution of IAS or OCT.The synergistic inhibition effects of OCT or IAS and the surfactants were investigated by electrochemical measurements,XPS analysis and molecular dynamics simulation.The results show that a good synergistic inhibition effect exist between OCT and TTAB as well as IAS and DTAB.According to the molecular dynamics simulation results,the inhibitor films own different fractional free volume(FFV)with the changing of the composition of the composite inhibitors,and the FFV values were negatively correlative to the S values which can evaluate the degree of the synergism,namely,a relative high S value can be obtained when FFV value is relative low.The relationship between S and FFV values is conducive to predict the synergism of inhibitors and achieve the optimum proportion of them.4.Mang scholars focused on the development and application of green inhibitors because people had paid more attention to the environmental protection.The inhibition performance of the orange peels extract(OPE)and the synergistic inhibition effect between OPE and IAS on Q235 steel were investigated by electrochemical measurements,FTIR,XPS and SEM in the CO2/H2S coexistent brine solution.The results show that the OPE has good inhibition performance on Q235 steel in the CO2/H2S coexistent brine solution,and the performance of OPE can be enhanced after adding IAS into the solution indicating a good synergistic corrosion inhibition effect between OPE and IAS.OPE is rich in oxygen-and nitrogen-containing groups and functions via the adsorption of these groups on the steel surface according to the XPS analysis.Simultaneously,IAS can be also detected on the steel surface by combined using with OPE indicating a co-adsorption of IAS and the inhibitors in OPE.In conclusion,the aim of this work is to select some inhibitors with good inhibition performance in the C02/H2S coexistent brine solution and study the inhibition mechanism of them,and then discover the synergistic inhibition effect between these inhibitors and further research the synergistic inhibition mechanism.