| In order to meet the mining needs of high-sulfur oil and gas wells in high temperature and high pressure environments,Nickle-based alloys are widely used in the mining of acidic high-sulfur oil and gas fields because of their excellent acid and stress corrosion resistance.The precipitation of sulfur,in the development of acid oil and gas wells,cause severe localized corrosion,and may even cause stress corrosion cracking which results in pipeline failure and catastrophic incidents.Therefore,it is necessary to research the cause of the sulfur deposition corrosion and the localized corrosion mechanism for the exploitation of high-sulfur oil and gas fields.Currently,there are many experimental researches on studying the corrosion of nickel-based alloys caused by sulfur.Electrochemical methods were used to measure the polarization curves,electrochemical impedance spectroscopy(EIS)and M-S curves.The electrochemical corrosion behavior of nickel-based alloys was studied under the environment of simulated high temperature and high pressure with sulfur and carbon dioxide.In addition,the morphology and composition of corrosion products,on the surface of nickel-based alloys after Sulfur pitting,were obtained by SEM,EDS and other technologies.However,the current theoretical study on the microscopic electronic structure and bonding characteristics of the surfaces of the Sulfur-adsorbed nickel-based alloy is still unclear.Therefore,in this thesis,we use first-principles calculation method to study the surface configuration and microscopic electronic structure of Sulfur-adsorbed nickel-based alloy 825 matrix and its passivation film under different coverages,in order to guide the Sulfur corrosion protection in practice and the development of new materials resistant to Sulfur corrosion.The main contents and conclusions are as follows:(1)Under different S coverage,through studying the S adsorption on different adsorption sites of nickel-based alloy 825(1 0 0),it can be found:at coverage of 0.25 ML and 0.5 ML,the stable adsorption site of S on the nickel-based alloy 825(1 0 0)surface is the hollow on the Ni/Fe-terminated surface,and the corresponding adsorption energies are-5.13 eV and 6.51 eV,respectively.Due to the coupling hybridization of S-S bond under 1ML coverage,the adsorption energy of S on the nickel-based alloy 825(1 0 0)surface increases to-4.68 eV,and the corresponding stable adsorption site is the bridge position on the Ni/Fe terminated surface.(2)It can be seen from the electronic structure that the S-adsorption promotes the inner layer electron activity on the surface of the nickel-based alloy 825,and the sulfur atom and the surface atoms of nickel-based alloy 825 are electronically coupled to form a bond.Under the coverage of 0.5ML,S has the strongest effect on the surface of the nickel-based alloy 825,and corrosion products FexSy are easily generated.This interaction is mainly caused by the hybridization of S-3p with Cr-3d,and the one of S-3p with Ni-3d and Fe-3d.(3)The adsorption studying of S on the passivation film of the nickel-based alloy 825 surface(NiO/Cr2O3)shows that the adsorption process is mainly endothermic,and it needs to obtain heat from the surroundings,which is one of the reasons that the NiO and Cr2O3 have protective effect on the surface of S-etched nickel-based alloy 825.(4)By analyzing the relaxation of S on the NiO(100)surface and the Cr2O3(100)surface and the two-dimensional differential charge density map,it can be seen that the S mainly interacts with the O atom on the surfaces of NiO and Cr2O3 after S adsorption.And this interaction makes the O atom own the tendency to depart from the surface of passivation film(NiO/Cr2O3).(5)By analyzing the DOS of S on the NiO(100)surface and the Cr2O3(100)surface,we can find that the interaction between the S and NiO(100)surface is mainly caused by the hybridization of S-3p,O-2p and partial Ni-3d.The interaction between the S and Cr2O3(100)surface is mainly caused by the coupling of S-3p,O-2p and partial Cr-3d. |
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