Density Functional Theory Study Of Brass Intergranular Stress Corrosion Mechanism In H2_S Environment

Intergranular stress corrosion cracking（IGSCC）is a special type of corrosion damage.There are no obvious signs when IGSCC occur,which can cause catastrophic effects.Brass is famous for its excellent mechanical property and wear-resisting property.It is widely used in precision instruments,ship components and military industry.But brass has strong stress corrosion cracking susceptibility in H₂S environment,which restricts its wide application.Currently,we can only surmise the mechanism and process of corrosion by the morphology and component.It is difficult to explain the mechanism of IGSCC from the molecular and atomic level by experimental method,but the development of transition state theory and first principle tensile test make it possible for people to study IGSCC mechanism by quantum chemistry calculation.In order to obtain the mechanism of IGSCC from the molecular and atomic level,this article studied the influences of grain boundary,element segregation,corrosion particle decomposition and H⁺permeation for brass IGSCC,and tested the mechanical property of brass by first principle tensile test.In the last part of article,we built a∑5（210）/[100]grain boundary model of brass and verified the accuracy of the model by surface energy.Structural and electronic analyses showed that grain boundary had messy atomic arrangements,so there were electron concentration area and electron sparse area in brass grain boundary.The sparse of atoms and electrons is an important reason to reduce the tensile strength of brass.Then,we confirmed the segregation sites and segregation concentration of Zn on brass grain boundary.The electronic analysis of segregation grain boundary showed that the segregation of Zn on grain boundary could reduce electronic concentration,because its smaller radius and weaker interaction.However,this effect could also reduce the tensile strength of brass.In addition,we analyzed the adsorption rules of H₂S、HS^-、S^2-、H⁺on ideal brass surface,ideal grain boundary surface and segregation grain boundary surface.The results showed that H₂S tended to adsorbed on No.4 top site,HS^-tended to adsorbed on No.2 hollow site,S^2-tended to adsorbed on hollow site,H⁺tended to adsorbed on No.1 hollow site.The adsorption of S^2-could improve the tensile properties of brass.The transition state theory calculation of H₂S decomposition showed that the reaction energy barrier of H₂S decomposition on segregation grain boundary surface was smaller than ideal brass surface and ideal grain boundary surface.Besides,the study of H⁺penetration showed that H⁺could capture the bonding electrons of metal,so it caused the lower tensile strength of brass.H⁺intruded into brass matrix by the larger channel on grain boundary,and the segregation of Zn could reduce the energy barrier of H⁺penetration.At the end of the article,we confirmed the favorable and harmful factors of brass IGSCC.The harmful factors were grain boundary,element segregation and H⁺penetration.The favorable IGSCC was sulphur adsorption.In this article,we studied the IGSCC mechanism of brass deeply and systematically.Our study provides important reference significance for corrosion mechanism researchs of other alloys by first principles.