Research On Pitting Corrosion Behavior Of 304L Stainless Steel Under Elastic Stress

304L stainless steel is widely used in chemical,petroleum,energy and power industries because of its excellent corrosion resistance,high temperature oxidation resistance and intergranular corrosion resistance.But in salt corrosive media,especially in the common Cl-,stainless steel is prone to local corrosion.Moreover,most of the existing industrial equipments operate under the condition of pressure,which makes the corrosion behavior significantly intensified.The occurrence of local corrosion often leads to running,emitting,leaking and dropping of equipment,resulting in the reduction of equipment life and the disastrous consequences,and even unpredictability personnel casualties and national economy loss.Corrosion behavior research at home and abroad under high stress has been studied thoroughly.However,few reports have been reported about the germination and development of localized corrosion of stainless steel under elastic stress.Therefore,research of the influence of elastic stress on the pitting corrosion behavior of stainless steel is particularly significant.On this basis,304L stainless steel was selected as the research object.Electrochemical methods(electrochemical impedance spectroscopy and potentiodynamic polarization test,potentiostatic polarization test),acoustic emission and surface morphology observation technique were used to study the pitting corrosion behavior of 304L stainless steel in 3.5 wt.%NaCl solution under elastic stress.The main research contents include the influence of stress level of tensile stress and compressive stress on the pitting corrosion behavior of 304L stainless steel in 3.5 wt.%NaCl solution and theirs differences,influence of pH uner elastic stress on pitting corrosion behavior,the influence of the concentration of SO42-and CO32-on pitting corrosion behavior under elastic stress,and stable and metastable pitting corrosion behavior of 304L stainless steel at different passivation time under elastic stress.The research results showed that with the increase of stress level,pitting potential shifted negatively and the pitting sensitivity increased under tensile stress and compressive stress,which retarded the growth rate of early anode current density,and eventually led to the increase of Eb100.The influence of compressive stress on the development of pitting corrosion was generally greater than that of tensile stress.Effect of compressive stress on the increase of early anode current density of pitting corrosion was stronger than that of tensile stress in the stress range of 0-30%?0.2,.With the increase of stress level,the pits amount of sample surface under tensile stress and compressive stress increased,and the surface morphology also changed,including the changes from single pit to multiple pits and secondary pits,and the increase of pit size.The impedance value of passive film increases with the increase of pH value.The existence of elastic stress has no significant effect on the properties of passive film at the early stage.At the same pH value,the shift of pitting potential is more negative under elastic stress compared to that of no stress.When pH equals 1,the disparity is most significant.The effect of elastic stress on the pitting morphology under different pH values is different from the typical pitting morphology under no stress,from the typical regular circle shape to irregular shape,and the growth along cracks occurs at all pH values.The impedance modulus and integrity of 304L surface passivation film increased with the increase of SO42-concentration at the early immersion stage and the change was not significant in the range of 0 mol/L to 0.035 mol/L,but a significant increase was observed from the range of 0.035 mol/L to 0.60 mol/L.With the increase of SO42-concentration,the pitting potential shifted positively,and the minimal pitting sensitivity was obtained in the range of 5.0 to 0.60 mol/L.For CO32-,the impedance increased slightly at the early stage immersion from 0 mol/L to 0.0070 mol/L.with the CO32-concentration increased from 0.0070 mol/L to 0.035 mo1/L,the impedance increased significantly,and then no longer increased with the increase of CO32-concentration and tended to be stable.The pitting potential rised first and then decreased with the increase of CO32-concentration,and the concentration for minimal pitting sensitivity was about 0.070 mol/L.Elastic stress had certain influence both on stable and metastable pitting behavior of 304L stainless steel at different passivation time,the influence was more significant in the bad passive film property including thickness and density.The existence of elastic stress weakend the repassivation ability of 304L stainless steel to some extent,and at the same time,increased the sensitivity of steady pitting.The effect of elastic stress on metastable pitting was mainly reflected in the prolongation of survival time of metastable pit and the decrease of the growth rate of metastable pitting current.This result was closely related to the 'stable current plateau'near 0.1 ?A/cm2 under elastic stress.Acoustic emission monitoring reflected very well about the influence of stress levels on pitting corrosion behavior,and the effect of elastic stress on the pitting corrosion behavior under different pH,exhibiting an excellent response and verify to the electrochemical results.In electrochemical accelerated pitting test,the accumulative hits of acoustic emission increased with the increase of stress and the decrease of pH.The existence of elastic stress increased the acoustic emission events.