Effect Of Cold-working Induced Martensite Transformation On Corrosion Behavior Of 304 Stainless Steel

304 stainless steel is a widely used austenitic stainless steel.Due to the dense passive film on the surface,304 stainless steel shows excellent corrosion resistance,so that it plays an important role in the sea,oil and other harsh service environment.However,due to the low strength and hardness of 304 stainless steel,in the processing and manufacturing process of equipment,coldworking methods such as cold rolling are often used to improve the strength of the material,which will inevitably cause deformation-induced martensite transformation.After the phase transformation,there are a large number of defects such as dislocations and vacancies in the lath martensite.These defects will serve as a source of pitting corrosion for stainless steel,making it more susceptible to pitting and stress corrosion cracking in the marine environment containing chloride ions.The series of reactions led to the premature failure of equipment and pipelines,even catastrophic accidents.In this paper,the structure,phase composition,mechanical properties and corrosion behavior of 304 stainless steel before and after coldworking in 3.5% Na Cl solution were studied.The effect of chloride ion concentration on the corrosion behavior of 304 stainless steel before and after coldworking was also studied.Optical microscopy and transmission electron microscopy were used to characterize the structure of the material;X-ray diffraction was used to calibrate the material phase composition;microhardness was used to measure the hardness value of the material;dynamic potential polarization curves and electrochemical impedance were used to evaluate the pitting performance of materials;micro-area electrochemical impedance spectroscopy,M-S curve and capacitance potential curve were used to evaluate the electrochemical performance of passive film;XPS method was used to analyze the composition of passive film;SEM and LSCM were used to characterize the morphology of the material.The results show that before coldworking,the structure of 304 stainless steel was a single austenite phase.After coldworking,the 304 stainless steel undergoed a martensitic transformation,and the structure was composed of the original austenite phase and the deformation-induced martensite phase;after the martensitic transformation,the hardness of 304 stainless steel increased significantly,and the hardness value was more than twice that before untreated;the pitting resistance of 304 stainless steel decreased,the diameter and depth of pits increased during the steady-state pitting stage and the shape of pits was still deep hole-shaped;simultaneously the electrochemical performance of passive film was reduced,and the semiconductor performance of passive film was the same as that before the treatment,which was n-type;after the martensitic transformation,the composition of passive film of 304 stainless steel was composed of iron-chromium compounds or simple substances,but the composition was slightly different from that before untreated;with the increase of chloride ion concentration,the pitting resistance of 304 stainless steel and the electrochemical performance of passive film were reduced.And when the chloride ion concentration was the same,the pitting corrosion resistance of 304 stainless steel and the electrochemical performance of passive film were poorer than before.