湿气环境中CO_(2)-H_(2)S在α-Fe(110)密排面上吸附与点蚀机理研究

Adsorption and Corrosion Mechanism of CO_(2)-H_(2)S onα-Fe(110)Close-packed Plane in Humid Environment

目的从微观尺度探究CO_(2)-H_(2)S(CO_(2)和H_(2)S共存)在湿气管道顶部的吸附特性,进而揭示点蚀机理。方法基于密度泛函理论的第一性原理,利用Materials Studio构建CO_(2)、H_(2)S和CO_(2)-H_(2)S在α-Fe(110)密排面的吸附模型,对CO_(2)、H_(2)S和CO_(2)-H_(2)S在α-Fe(110)面的吸附能、局域态密度、分波态密度和差分电荷密度进行仿真;利用高温高压釜模拟CO_(2)-H_(2)S-Cl^(-)腐蚀环境,分析L360钢在湿气环境中的腐蚀行为;最后,揭示含Cl^(-)湿气管道顶部CO_(2)-H_(2)S吸附机制与点蚀机理。结果CO_(2)、H_(2)S、CO_(2)-H_(2)S及CO_(2)-H_(2)S-Cl^(-)在最稳定位置时的吸附能分别为-4.065、-3.961、-8.538、-12.775e V,表明相较于CO_(2)与H_(2)S单独吸附,CO_(2)-H_(2)S在α-Fe(110)面的吸附能更负,Cl^(-)会进一步降低CO_(2)-H_(2)S的吸附能;且CO_(2)在与H_(2)S竞争环境电子中占优势;Cl^(-)会使CO_(2)-H_(2)S的局域态密度峰值降低,转移趋势为失去电子,基体和腐蚀介质的电子向着低能级跃迁释放出更多能量,进而加强了Fe与CO_(2)-H_(2)S间的化学键强度;Cl^(-)的2p轨道与Fe的3d轨道在-6.8 eV和-5.7 eV发生重叠,Cl^(-)被吸附到Fe表面并与Fe形成化学键生成氯化物,进而改变腐蚀产物膜的组分与结构,削弱产物膜的致密性和稳定性,减弱腐蚀阻抗力。在含Cl^(-)湿气的CO_(2)-H_(2)S环境中,液相中的Cl^(-)浓度升高,使L360钢的气相平均腐蚀速率逐渐增大,最高达2.935mm/a,点蚀越发严重。结论CO_(2)与H_(2)S在α-Fe(110)面吸附存在一定的协同和竞争作用,协同促进金属的腐蚀,FeCO3会优先沉积成膜,但H_(2)S会抑制FeCO3的生长,腐蚀产物以FeS为主;Cl^(-)会增强CO_(2)-H_(2)S与α-Fe(110)面间的作用力,弱化腐蚀产物膜层的保护性,进一步加速金属腐蚀、尤其是点蚀。

CO_(2)and H_(2)S gas and the droplets containing Cl^(-)often co-exist on the top of the pipelines used for transporting oil and gas,which is different from the bottom environment of the pipelines,resulting that the corrosion processes are complex and the pipelines suffer from more serious local corrosion.Up to now,there are numerous studies about the effects of CO_(2)or/and H_(2)S on pipeline corrosion,but most of them mainly focus on the corrosion protection and control,material selection and corrosion protection systems,etc.,there are relatively few reports on the generation and evolution of corrosion from the perspective of theory,especially from the electron structure at the microscopic level.While revealing the mechanism of corrosion occurrence and evolution from the microscopic level is an important basis for formulating effective anti-corrosion measures.In this paper,the adsorption and corrosion characteristics of CO_(2)-H_(2)S on theα-Fe(110)close-packed plane in the humid environment containing Cl^(-)were studied,and the adsorption and pitting mechanism was explored from the microscopic scale,in order to provide theoretical basis for the corrosion mechanism study and effective protection technology application of pipelines in oil and gas fields.Based on the first-principle method of Density Functional Theory(DFT),Materials Studio software was applied to establish an adsorption model of CO_(2),H_(2)S and Cl^(-)onα-Fe(110)surface.The adsorption energy,local density of states(LODS),partial density of states(PDOS)and differential charge density of CO_(2),H_(2)S,CO_(2)-H_(2)S and CO_(2)-H_(2)S-Cl^(-)onα-Fe(110)surface were calculated,respectively.The results showed that the adsorption energy of CO_(2),H_(2)S,CO_(2)-H_(2)S and CO_(2)-H_(2)S-Cl^(-)at the most stable position ofα-Fe(110)was−4.065 eV,−3.961 eV,−8.538 eV and−12.775 eV,respectively indicating that the adsorption energy of CO_(2)-H_(2)S on theα-Fe(110)surface was more negative than that of CO_(2)and H_(2)S alone,and Cl^(-)further reduced the adsorption energy of CO_(2)-H_(2)S.When CO_(2)or H_(2)S were adsorbed onα-Fe(110)surface,for CO_(2),the 2p orbital of C and the 2p orbital of O overlap with the 3d orbital of Fe was in the energy range of 1.6-4.7 eV and−6.4-5.2 eV,respectively,indicating that adsorption energy of CO_(2)-H_(2)S onα-Fe(110)surface was smaller than that of CO_(2)and H_(2)S alone,and Cl^(-)further reduced the adsorption energy of CO_(2)-H_(2)S;While for H_(2)S,the 2p orbital of S overlaps with the 3d orbital of Fe was in the range of−6.7-−3.1 eV;However,when CO_(2)and H_(2)S were co-absorbed on theα-Fe(110)surface,there was a certain competitive effect between CO_(2)and H_(2)S for electrons in the corrosion environment,and CO_(2)had an advantage,the adsorption energy of CO_(2)-H_(2)S onα-Fe(110)surface was smaller compared with the adsorption of CO_(2)and H_(2)S alone,Cl^(-)further decreased the adsorption energy of the system.The presence of Cl^(-)not only further complicated the corrosion environment but also generated soluble chloride.Therefore,Cl^(-)reduced the peak of LODS of CO_(2),H_(2)S,and CO_(2)-H_(2)S,and the transfer trend was to lose electrons,the electrons of the matrix and corrosive medium could release more energy,and transition to lower energy level,thus the chemical bond strength between Fe and CO_(2)or H_(2)S was strengthened.In addition,the 2p orbitals of Cl^(-)overlaps with the 3d orbitals of Fe were at−6.8 eV and−5.7 eV,Cl^(-)would be adsorbed to the surface of Fe and form chemical bonds,forming soluble chloride.Then the structure and composition of the corrosion product film were changed,and the density and stability would also be weakened,leading to more severe corrosion of the metal.In addition,the corrosion behavior of L360 steel in the humid environment containing CO_(2)-H_(2)S and different Cl^(-)concentration was studied with a high temperature and high pressure autoclave.The results showed that with the increase of Cl^(-)concentration,not only did the average corrosion rate increase gradually,even up to 2.935 mm/a,but also the pitting corrosion would become more and more serious,which was in good accordance with the simulated results mentioned above.Therefore,the co-adsorption of CO_(2)and H_(2)S onα-Fe(110)close-packed plane has a certain synergistic and competition effect,and increases metal corrosion,FeCO3 will form preferentially,while the growth of FeCO3 is effected by H_(2)S.And the presence of Cl^(-)enhances the force between CO_(2)or/and H_(2)S andα-Fe(110)surface,and weakens the protection of corrosion product film,further accelerating metal corrosion,especially pitting corrosion.