Study On CO₂/Cl^- Coadsorption Characteristics Of γ-Fe（111） Surface Based On First Principles

CO₂ corrosion has been a hot topic in the exploitation of oil and gas field,and the effect of Cl^-on steel corrosion has always been a problem that can not be ignored in the process of CO₂ corrosion.However,most studies only discussed with experiments to investigate the corrosion of steel under the coexistence of CO₂ and Cl^-.This paper combained theoretical method with experimental method.First,the first principle calculation method based on density functional theory was used to analyze the adsorption characteristics and corrosion mechanism of CO₂/Cl^-coadsorbed on γ-Fe（111）surface under the coverage of 1/6 ML,1/4 ML and 1/2 ML.And the corrosion behavior of 316 L stainless steel under the coexistence of CO₂/Cl^-was studied by high temperature and high pressure test and electrochemical test.The results of the first principle calculation showed that when the CO₂ was adsorbed on the γ-Fe（111）surface under the coverage of 1/6 ML,1/4 ML and 1/2 ML,the most stable adsorption position of was the bridge（B）;when CO₂/Cl^-was coadsorbed,the stable adsorption sites of CO₂ and Cl^-were all in the hole position（H1）under the coverage of 1/6 ML and 1/4 ML,but Cl^-was not adsorbed under the coverage of 1/2 ML.According to the calculation of the electronic properties of the adsorption system,such as the surface work function,the overlapping population number,the density of states and the charge density difference,the results showed that: CO₂ and Cl^-mainly interacted with the first layer of atoms on the surface of Fe（111）,there were covalent bonds between surface Fe and Cl^-ion,C and O atoms,in addition,the covalent interaction between Cl^-ion and Fe atoms was greater than the covalent interaction between CO₂ and Fe atoms,indicating that CO₂ and Cl^-had competitive adsorption on Fe（111）surface;the interaction between the 3d orbit of Fe atom and the 2p orbital electron of C,O1,O2 and Cl^-atoms resulted in the adsorption of CO₂ and Cl^-on the surface of Fe（111）.The adsorption of CO₂ molecules reduced the electron stability of the surface of Fe（111）,while the addition of Cl^-ions promoted the activity of electron of the Fe（111）surface,and made the surface of Fe（111）more unstable,which was conducive to corrosion electrochemical reaction with the corrosive medium.The results of high temperature and high pressure test showed that the influence of Cl^-concentration,temperature and CO₂ partial pressure on corrosion rate of 316 L stainless steel decreased in turn.The results of electrochemical experiments showed that under the coexistence environment of CO₂ and Cl^-,and the concentration of Clions was within the range of 0 mol/L¹.5 mol/L,with the increase of Clconcentration,the passivation range of the 316 L stainless steel decreased,the density of the blunt current increased,the breakdown potential decreased,and its corrosion resistance decreased.