Research On Corrosion Mechanism Of L360 Steel In H₂S-CO₂-Cl^- System

With the development of China’s petroleum industry,the use of pipelines for long-distance transportation of oil and natural gas has become the main means of transporting oil and natural gas resources.L360 steel is commonly used in oil and natural gas gathering and transportation pipelines due to its excellent mechanical properties and economic benefits.However,the synergistic effect between H₂S and CO₂ gases present in the produced liquid of oil and gas fields and Cl^-present in the transportation medium can lead to pipeline corrosion,posing a serious threat to safe transportation and environmental protection.Therefore,studying the corrosion mechanism of L360 steel in the H₂S-CO₂-Cl^-system can provide a basis for protective measures for oil and gas pipelines.At present,there have been many macroscopic studies on the corrosion behavior of H₂S-CO₂-Cl^-,but with the development of technology,the focus of research has shifted from macroscopic to microscopic.The use of microscopic research methods can help further reveal the corrosion mechanism fundamentally.Quantum mechanics,as an effective method,can understand corrosion dynamics at both molecular and atomic scales.If quantum mechanical simulation can be used to study the corrosion mechanism of the H₂S-CO₂-Cl^-system from a microscopic perspective,it will help deepen people’s understanding of the corrosion system and provide reference for the development of corrosion protection technology in oil and gas fields.This article starts from both numerical simulation and experimental research,focusing on exploring the corrosion situation of L360 pipeline steel in H₂S-CO₂-Cl^-environment at the micro level when corrosion occurs,and preliminarily verifies the numerical simulation results through experimental research.The main research content is as follows:（1）The adsorption model of H₂S,CO₂ and Cl^-on L360 steel surface was established based on molecular thermodynamics with density functional theory and first principles as calculation principles.Using adsorption energy as an indicator of model stability,the most stable models for different corrosive molecules and ions were selected.On the basis of selecting the most stable model,the effects of H₂S,CO₂ and Cl^-adsorption on the surface of L360 steel on corrosion were investigated using characterization methods such as adsorption energy,work function,dipole moment,and density of states.Research has found that there is a certain degree of competition and synergy between CO₂ and H₂S,but overall,their coexistence will accelerate the process of corrosion.Moreover,Cl-will cause the chemical bond between CO₂,H₂S and metal to become stronger,increasing the corrosion tendency,but will not change the reaction mechanism and corrosion products.（2）A water solution model of the H₂S-CO₂-Cl^-system was established based on molecular dynamics and gas solubility.The corrosion model was selected when the corrosion reaction reached dynamic equilibrium,using the fluctuation amplitude of temperature and energy as evaluation indicators.Taking the corrosion model under dynamic equilibrium as the research object,this study explores the corrosion mechanism in aqueous solution environment and the effects of temperature and Cl^-concentration changes on the reaction system.The results show that the increase of temperature and Cl^-concentration will destroy the chemical bond on the metal surface,and more electrons will transfer from one side of the matrix to the corrosive medium in the solution.At the same time,it was found that the corrosion products in the aqueous solution environment are mainly carbonates,sulfides,and chlorides.（3）The corrosion experiment of L360 steel was conducted using a high-temperature and high-pressure reactor.By calculating the corrosion weight loss of L360 steel sheets at different temperatures and Cl^-concentrations,and observing the corrosion morphology using SEM,it was found that the corrosion rate was positively correlated with temperature and Cl^-concentration.Based on the principle of corrosion electrochemistry,electrochemical experiments were conducted at different temperatures and Cl^-concentrations.The polarization curves,impedance spectra,and fitting circuit results showed that an increase in temperature and Cl^-concentration not only led to an increase in corrosion current density,but also weakened the stability and corrosion resistance of the passivation film.