Theoretical Study On H₂S Adsorption/Dissociation And Diffusion Mechanism On The Surface Of Iron And Iron-sulphide Compounds

Carbon steel undergoes severe corrosion reactions in a wet H₂S environment,producing a corrosion product of different crystal forms-polymorphous iron-sulphide compounds.Among them,the mackinawite?Fe S?is the most common corrosion product in the initial stage of corrosion,and pyrite?Fe S₂?is a stable corrosion product in the late corrosion or high temperature conditions.Different crystalline iron-sulphide compounds not only have great differences in physicochemical properties,but also have different influence mechanisms for subsequent corrosion processes and hydrogen embrittlement.At present,there are still many basic scientific problems to be solved in the theoretical study of different crystalline iron-sulphide compounds.For example:the electronic structure and thermodynamic properties of iron-sulphide compounds under different conditions are still uncertain.The adsorption/dissociation and diffusion process of H₂S on the surface of iron or different iron-sulphide compounds are not clear.The influence mechanism of defects on the adsorption/dissociation and diffusion processes still unclear.The essential cause of the difference in H infiltration processes on the surface of iron and crystalline iron-sulphide compounds has not been studied.Therefore,this thesis has carried out in-depth theoretical calculations on the above basic problems through the first-principles density functional theory?DFT?.The main research contents and calculation results are as follows.First,structural,elastic and thermodynamic properties of different crystalline iron-sulphide compounds were studied.The results show that mackinawite is metallic and that pyrite is a semiconductor with a band gap of E_g=1.02 e V.Using the stress-strain method,the elastic properties including the bulk modulus?B_V?and shear modulus?G_V?were derived from the elastic data.Density functional perturbation theory?DFPT?calculations within the quasi-harmonic approximation?QHA?were used to calculate the thermodynamic properties,and the mackinawite and pyrite are found to be dynamically stable compounds.The isothermal bulk modulus,thermal expansion coefficient,constant volume/pressure heat capacities,Gibbs free energy and entropy of the Fe-S compounds were obtained.Furthermore,the temperature of the mackinawite?pyrite phase transition was predicted at 490 K.Based on the calculation results,the experimental prediction model for the formation of different Fe-S compounds in the Fe-H₂S-H₂O system was modified.The adsorption/dissociation process of H₂S and the diffusion mechanism of H atoms were calculated by spin-polarization DFT on defect-free and vacancy-defective Fe?100?surfaces.Calculated by Climbing Image Nudged Elastic Band?CI-NEB?,the maximum dissociation energy barriers?E_a?of H₂S on the Fe?100?surface of defect-free and first-layer vacancy-defective Fe are 0.35 and 0.17 e V,respectively,indicating that the reactivity of the vacancy-defective Fe?100?surface is moderately increased.The existence of vacancy defects changes the preferential H atom diffusion entrance to the subsurface and shortens the diffusion path.It is calculated that the diffusion E_a of the H atom from the surface into the inner-layers is greater than the dissociation E_a of H₂S on the Fe?100?surface,which indicates that H diffusion process acts as a control step for H to penetrate into the Fe?100?bulk structure.The surface properties of the layered Fe S were calculated by the dispersion-corrected density functional theory?DFT-D2?,and the order of different surface stability was found to be?001?>?011?>?100?>?111?.The stable adsorption sites and adsorption energy(E_ads)of H₂S on different surfaces were calculated,and weak physical adsorption was observed on the most stable?001?surface.The complete dissociation process of H₂S on different surfaces was calculated by CI-NEB method.It was found that the complete dissociation of H₂S occurred preferentially on the surface of?011?,and almost no dissociation reaction occurred on the?001?surface.Finally,the effect of pre-adsorbed atom X?X=H,O,S?on the adsorption/dissociation and diffusion mechanism of H₂S on the?001?surface was also investigated.It was found that the pre-adsorbed atoms were favorable for the adsorption/dissociation process of H₂S,and the pre-adsorption of S atoms greatly reduced the dissociation E_a.In addition,pre-adsorption of H atoms can also significantly promote the diffusion of H atoms from the surface to the bulk structure.The surface energies of Fe S₂?100?,?110?and?111?were calculated by the GGA+U method,and the surface of Fe S₂?100?was found to be the most stable.The adsorption/dissociation process of H₂S on different Fe S₂ surfaces was calculated.The results show that H₂S is chemically adsorbed on different Fe S₂ surfaces,and E_ads increase in the order of?110?<?111?<?100?.It was also found that H₂S is prone to primary dissociation on the surface of the studied Fe S₂,and complete dissociation occurs preferentially on the?111?surface.In addition,it was also found that the pre-adsorbed atom X can not only reduce the dissociation E_a of H₂S,but also the final state after dissociation is more stable on the surface.However,the effect of pre-adsorbed atoms on different surfaces is not exactly the same.Pre-adsorbed O atoms can significantly reduce the E_ads of H₂S on the?100?surface,but the pre-adsorption of H atoms has a greater effect on the E_ads of H₂S on the?110?and?111?surfaces.Finally,the order of probability of generating H₂ on different perfect surfaces is Fe S?001?>Fe S₂?100?>Fe?100?.The essential reason for the difference in H penetration between iron and different iron-sulphide compounds is theoretically revealed by simulation calculations.