Stress Corrosion Behavior Of Two Typical Stainless Steels In Saturated H₂S+Cl^- Medium

Stainless steel is widely used in petrochemical industry because of its excellent toughness,plasticity,corrosion resistance and machining performance.Duplex stainless steel is widely used to manufacture containers and pipes for conveying corrosive media.Compared with duplex stainless steel,austenitic stainless steel is cheaper and used widely.In this paper,2205 duplex stainless steel and 316 austenitic stainless steel are selected as the research objects.The stress corrosion behavior of the two materials is analyzed and compared,and the stress corrosion cracking mechanism is explored,which provides important basis and reference for the safe use of the two typical stainless steel materials.The main research contents are as follows:（1）The stress corrosion behavior of 2205 duplex stainless steel in saturated H₂S+Cl^-mixed medium and Cl^-single medium was studied by slow strain rate tensile test combined with electron microscope.The results show that in Cl^-single medium environment,the stress corrosion sensitivity coefficient of 2205 duplex stainless steel is far less than 25%.The fracture characteristics are ductile fracture,and there is no stress corrosion occurred in this medium.Comparely speaking,in the saturated H₂S+Cl^-mixed medium environment,as the Cl^-concentration increases,the tensile strength and elongation of 2205 duplex stainless steel decrease gradually,while the stress corrosion sensitivity index and fracture corrosion products keep increasing.The stress corrosion sensitivity index is not more than 25%.There is no stress corrosion phenomenon.These results indicate that 2205 duplex stainless steel has a good corrosion resistance.（2）The stress corrosion behavior of 316 austenitic stainless steel in saturated H₂S+Cl^-medium was studied by using slow strain rate tensile test and surface analysis technique.The results show that with the increase of Cl^-concentration,the elongation of 316 austenitic stainless steel decreases,while the stress corrosion sensitivity index and fracture corrosion products increases.In the saturated H₂S+15.9%Cl^-medium,the sensitivity index Fδof 316 austenitic stainless steel is far more than 25%,which belongs the stress corrosion danger zone.The fracture position has a slime pattern,which shows obvious stress corrosion characteristics.（3）According to the experimental results,the mechanical properties of the two typical stainless steels in saturated H₂S+Cl^-corrosive medium were compared and analyzed.It is found that with the increase of Cl^-concentration,the elongation and fracture time of the two typical stainless steels gradually decreased,while the stress corrosion sensitivity index increased gradually.However,2205 duplex stainless steel has higher tensile strength,while 316 austenitic stainless steel has better ductility.And the stress corrosion sensitivity index of 316 austenitic stainless steel increases more than that of 2205 duplex stainless steel.（4）In order to compare with the experimental results,the finite element software was used to establish a numerical model for the stress corrosion of stainless steel material.The simulation of the stress corrosion fracture process of stainless steel was carried out to explore the internal mechanism of stress corrosion cracking under uniaxial tensile.It is found that when the pitting corrosion occurs on the surface of the stress corrosion sample,the stress concentration occurs in the pitting pit,which causes the spherical pitting pit transforming into ellipsoid shape.During the ellipsoidal pitting,the maximum stress appears in the vertical tensile direction,which will generate cracks in this direction,and the crack expansion will eventually lead to the specimen fracture.