Study On Pitting Behavior Of 316L Austenitic Stainless Steel During Tribocorrosion

Tribocorrosion is a form of failure of metal materials caused by the combined action of electrochemical corrosion and mechanical wear.The material loss caused by the interaction of the two is far greater than their simple summation,which will lead to premature failure of components or equipment,resulting in economic losses and even accidents.Pitting is a localized corrosion phenomenon commonly found on passive metal surfaces in contact with corrosive media containing halide ions.Pitting corrosion can lead to local rapid dissolution of materials,causing serious consequences such as perforation,stress concentration,and cracking.It is one of the most typical and most harmful corrosion forms in marine corrosion.Austenitic stainless steel is widely used in marine equipment due to its excellent corrosion resistance and good mechanical properties.Some of the shafts,cables,impellers and pump valves made of austenitic stainless steel need to be exposed to chlorine-containing environments.during operation,which in turn leads to the occurrence of tribocorrosion.At this time,pitting corrosion will still occur on stainless steel,and the superposition of tribocorrosion and pitting corrosion seriously threatens the reliability and safety of stainless steel during service.At present,there are few studies on the pitting corrosion behavior of stainless steel when it is subjected to tribocorrosion,and there is no clear understanding of the key issues such as the influencing factors,evolution law and formation mechanism of pitting corrosion behavior under tribocorrosion conditions.Based on the above problems,this paper takes 316 L austenitic stainless steel as the research object,controls the corrosion state by electrochemical means,and conducts tribocorrosion experiments combined with friction and wear tester,using white light interferometer,micro Vickers hardness tester,scanning electron microscope and transmission electron The surface morphology,surface structure and chemical composition were characterized by microscope,etc.,and the pitting corrosion behavior of 316 L stainless steel in tribocorrosion was studied.The main findings are as follows:(1)The tribocorrosion behavior and pitting mechanism of 316 L stainless steel in 3.5wt.% Na Cl solution were studied.It was found that when the stainless steel is abraded,pitting corrosion occurs at conditions well below the pitting potential,and the pitting exhibits a "collapsed" appearance.Gradient nanostructures are formed on the surface of stainless steel during the tribocorrosion process,which plays a decisive role in the nucleation and growth of pitting corrosion.Pitting corrosion first nucleates and grows on the gradient nanosurface,forming a conventional shape pitting corrosion,and then anodic dissolution-type stress corrosion cracking occurs under the action of frictional shearing force in the pitting corrosion pit,and the catalytic effect of the "occluded cell" on both sides of the crack It dissolves under the surface,causing the surface to collapse,thereby forming "collapse pitting".(2)The effect of tribocorrosion on the metastable pitting behavior of 316 L stainless steel was studied.The research shows that the increase of surface roughness will not significantly reduce the pitting corrosion potential,but will lead to a decrease in the metastable pitting potential and slightly increase the frequency of metastable pitting;the gradient nanometer formed on the surface during plastic deformation.The structure can enhance the passivation effect of the surface,thereby reducing the frequency of metastable pitting corrosion,but the gradient nanostructure can act as a fast diffusion channel of Cr element,resulting in a large loss of Cr element,and finally the Cr element content on the surface of the stainless steel decreases,resulting in the surface of the material.The upper metastable pitting appears earlier,and the current peak value and survival time of the metastable pitting are increased,so that these metastable pitting have a metastable pitting stability product that exceeds the critical value and can be far lower than that of the pitting corrosion.The potential is transformed into steady-state pitting corrosion.(3)The effect of sulfate on the tribocorrosion behavior and pitting behavior of 316 L stainless steel in chlorine-containing environment was studied.The research results show that there is a competitive adsorption between sulfate ions and chloride ions on the passivation film,resulting in the binding site of chloride ions and passivation film being occupied by sulfate ions,which weakens the corrosion effect of chloride ions on stainless steel passivation film,Reduce the corrosion degradation rate of stainless steel;sulfate ions can also promote the passivation of stainless steel to produce a thicker gradient nano-layer on the surface,improve wear resistance,and provide a certain lubricating effect,reducing the wear rate of stainless steel;sulfate ions The ions reduce the pit size and bring the stress concentration area closer to the surface with higher yield strength,thereby inhibiting the SCC germination and expansion inside the pit.