Thermal-Stress Coupling Corrosion Behavior and Life Prediction of 304 Stainless Steel in Cl~- Aqueous Medium

Author:He Shan Lin

Supervisor:jiang da ming

Database:Doctor

Degree Year:2018

Download:24

Pages:143

Size:12329K

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304 stainless steel has excellent corrosion resistance and be widely used in many fields as the industrial production,people’s life and materials manufacture.The materials are often subjected to the comprehensive action of complex conditions such as high temperatur,corrosive ions and so on in the working environment.The protective effect of 304 stainless steel on the material matrix is derived from the dense passivation film on the surface.Under the complex working conditions,the surface film will be damaged first due to the action of many factors,and then the stress corrosion cracking of the material will lead to the safety accidents.This study deeply investigated the corrosion mechanism of material and predicted the residual life through the evolution of the protect performance of passivation film to 304 stainless steel in a variety of factors,effect of deformation martensite on the passivation properties of materials and study of stress corrosion cracking sensitivity of 304 stainless steel which contains deformation martensite.The study shows that the content of the compound will change with different soaking conditions.With the increase of soaking time,the proportion of each oxide tends to different stable value with different soaking conditions.The passivation film on the grain boundary surface will be damaged by the dissolution of the compound.When the surface morphology is damaged,the content of Cr2O3 phase does not accurately reflect the protective performance of the passivation film.The corrosion ditch formed by the dissolution of the passivation film on the grain boundary surface is the main reason for the decrease of the film protection performance.The microstructure of SUS304 stainless steel after solid solution treatment contains almost pure γ phase.The microstructure of the samples after cold-rolled deformation contains theα’martensite phase.It is found that the content of the compounds in the passivation film and the protective properties of the film first decrease and then increase with the increase of the content of the deformation martensite.When the content of α’martensite is 6%,the surface film has the lowest content of chemical compounds and the worst protective properties.The chemical content and the protective properties of the surface film decrease with the increase of the α’martensite when the content of martensite is lower than 6%,and increase with the increase of the α’martensite when the content of martensite is higher than 6%.The compound composition and semiconductor properties of surface passivation film on 304 stainless steel with martensite content of 0.6%,6%,12% and 20% are comparative study by using XPS and mott-schottky methods.The results show that the passivation film has the least amount of compounds,the highest density of donor and acceptor and the worst corrosion resistance of materials when content of α’ martensite is 6%.The density of donor and acceptor of the passivation film increase with the increase of the α’martensite,and the corrosion resistance of the material become poor when the content of martensite is lower than 6%.When the content of martensite is lower than 6%,the density of donor and acceptor of the passivation film decrease with the increase of the α’martensite,and the corrosion resistance of the material is improved.The elongation and corrosion resistance of samples after cold rolling decrease significantly during the slow strain rate test.The stress corrosion cracking sensitivity of samples with the content of α’martensite of 6% is highest.The sensitivity of stress corrosion cracking depends on the occurrence time of crack during slow strain rate test.A total of 6 groups of immersion environments are set up in the study.The damage of the original passivation film on the material surface has great Effect on the material protection performance.The damage of the surface film results in the decrease of the film resistance of samples with different martensite content,and the reduction of the rupture potential is about 0.1-0.2 mV.Through the mechanical tensile tests of the 6 groups of samples with different soaking time,the variation of yield strength as the soaking time is investigated.According to the decay law of yield strength,the use life of materials is predicted by using the grey theory GM(1.1)prediction model.According to the predicted results,the corrosion life of samples with different martensite content has obvious difference.The corrosion life of samples is significantly higher than those of other samples when the content of α’ martensite is 0%.The Effect of chloride ion concentration on corrosion life is higher than that of temperature.Under the same soaking condition,the strength attenuation rate of samples with the content of 6% martensite is highest and the life is shortest.In the environment of 3.5% NaCl solution with temperature of 90 ℃,the life of the material is only 9 months.