Corrosion Mechanism,prediction And Inhibition Control Of N80 Carbon Steel In The Supercritical CO₂ Aqueous Environment

Recently,global warming caused by the massive emission of greenhouse gas CO₂ has become increasingly prominent.Carbon Capture,Utilization and Storage（CCUS）technology is an important measure to solve this issue.The captured CO₂ from CCUS can be injected into oil reservoirs for enhancing oil recovery（EOR）.However,the injected CO₂usually would cause the CO₂ corrosion problem of carbon steel pipelines,especially the more serious corrosion under supercritical CO₂（SC-CO₂）conditions,which poses a great threat to the security of oil exploitation.Therefore,it is of great theoretical significance and practical value to carry out the research on the mechanism,prediction and inhibition control of the corrosion of carbon steel in the SC-CO₂ environment for the safe service of carbon steel pipelines during the oil exploitation.This thesis focuses on the corrosion problem of carbon steel in the SC-CO₂environment.The corrosion mechanism of carbon steel in the SC-CO₂ aqueous environment was in-depth revealed by in situ electrochemical test,weight loss test and surface analysis.Meanwhile,a water chemistry model of the SC-CO₂ aqueous environment was developed.Based on this water chemistry model,a corrosion prediction model of carbon steel in the SC-CO₂ aqueous environment was further constructed.Finally,a high-efficient corrosion inhibitor was designed for the harsh SC-CO₂ corrosion environment.Combined with theoretical calculations,the corrosion inhibition mechanism of the developed inhibitor in the SC-CO₂ aqueous environment was essentially revealed.The main contents of this thesis are as follows:1.The corrosion behavior of N80 carbon steel in the O₂-containing SC-CO₂ aqueous environment（O₂ content less than 12.5%）was investigated by in situ electrochemical tests,mass loss measurements and surface characterization.Meanwhile,based on fugacity-activity method,a water chemistry model of SC-CO₂-O₂-mixed salts-H₂O system was developed,and thus the influential mechanism of O₂ on the corrosion of N80 carbon steel in the dynamic SC-CO₂ aqueous environment was comprehensively revealed.It is demonstrated that the corrosion of N80 carbon steel is inhibited in the presence of O₂.According to the developed water chemistry model,the decrease in the corrosive species concentration caused by the introduction of O₂ is not responsible for the inhibition to the corrosion of N80 carbon steel.In the presence of O₂,a ferric hydroxide/oxide film was formed on steel surface in the initial period.Its diffusion barrier effect promotes the formation of protective Fe CO₃ film underneath the ferric hydroxide/oxide film,and thus inhibits the SC-CO₂ corrosion of N80 carbon steel.2.Based on Pitzer theory for describing and calculating the interaction parameters between species,a high-accuracy water chemistry model with a simple form was developed by combining the solubility model（activity coefficient-fugacity coefficient type）and species equilibrium model to determine the mutual solubilities of CO₂ and H₂O,and the corresponding species concentrations in the CO₂-mixed salts-H₂O system(mainly contains Na⁺,Ca²⁺,Mg²⁺,K⁺,Cl^-,and SO₄^2-)with the temperature range of 15～250 ^oC,the pressure range of 1～600 bar and the salt molality range of 0～6 mol/kg.The computed solubilities of CO₂ and H₂O,and p H values are in good agreement with the experimental data reported in the literature.Based on this accurate water chemistry model,a model for predicting the corrosion of carbon steel in the SC-CO₂ aqueous environment（without considering the formation of corrosion products film）was established by using finite element method.The modeling corrosion rate agrees well with the measured corrosion rate.3.A prediction model on the corrosion evolution of carbon steel in the SC-CO₂ aqueous environment,which involves mass transfer process,electrochemical corrosion reaction process at the steel/solution interface,homogeneous/heterogeneous chemical reactions and the evolution of Fe CO₃ film,was developed based on finite element method.By introducing a porous domain to describe Fe CO₃ film,the evolution of Fe CO₃ film structure（thickness and porosity）and its influence on the corrosion process of steel were incorporated into the model.Different from the existed CO₂ corrosion prediction models,this model could not only predict the time-dependent corrosion rate,but also track the movement of corroding surface and depositing interface in real time via the arbitrary Lagrangian-Eulerian technology.The predicted evolution of corrosion rate and Fe CO₃ film thickness shows a good agreement with the experimental data.4.For the harsh SC-CO₂ aqueous environment,a novel thiadiazole derivative,1-phenyl-3-（5-thioxo-4,5-dihydro-1,3,4-thiadiazol-2-yl）thiourea（PTT）was synthesized with5-amino-1,3,4-thiadiazole-2（3H）-thione（ATT）through a simple one-step method.The inhibition performance and mechanism of ATT and PTT for the corrosion of N80 carbon steel in the dynamic SC-CO₂ aqueous environment were investigated by mass loss and electrochemical measurements,surface characterization,and theoretical calculations.It is found that ATT and PTT present the outstanding inhibitive performance,especially for PTT with an inhibition efficiency up to 99.58%,which is significantly higher than those reported corrosion inhibitors in the dynamic SC-CO₂ aqueous environment.The theoretical calculations based on density functional theory（DFT）reveal that the adsorption of PTT on steel surface is implemented through the bonding of the three S atoms of the thione-thiadiazole and thiourea fragments,as well as the benzene ring.Compared to ATT,the significant improvement in the inhibition performance of PTT is attributed to the stronger adsorption of PTT by forming more S-Fe bonds and the extra C-Fe bonds.