Study On Initiation And Propagation Behavior Of Localized Corrosion Of Austenitic Stainless Steel

Austenitic stainless steel is prone to local corrosion in harsh service environments such as seawater.Pitting corrosion is one of the important failure forms with strong destructiveness,high concealment and random distribution.It is necessary to analyze the corrosion mechanism.In this thesis,the initiation and propagation of localized corrosion of 316L austenitic stainless steel are studied from macroscopic and microscopic perspectives by combining traditional electrochemical methods and Scanning Electrochemical Microscopy（SECM）.The specific research work is as follows:Firstly,the electrochemical performance of the probe in the SECM test was studied to clarify the pretreatment measures for the probe before the experiment.At the same time,the optimal parameters in the subsequent SECM experiments were determined,including using Fc Me OH/Fc Me OH⁺as the redox mediator and the optimal spacing of 30μm in the surface scan test.Next,the macroscopic and microscopic electrochemical characteristics and evolution of316L stainless steel in 1M NaCl solution were investigated by traditional electrochemical methods and SECM,respectively.The experimental results show that 316L has strong corrosion resistance in 1M NaCl solution;when the characteristic potential polarization of 316L is carried out,with the positive shift of the polarization potential,the stainless steel exhibits the characteristics of self-passivation,and the degree of self-passivation deepens.When the polarization potential reaches the pitting potential of 316L,pitting corrosion occurs rapidly on the surface of the material.Then,the pitting corrosion behavior of 316L stainless steel in 6%FeCl₃solution was studied.The first 24 hours of immersion was the pitting induction period.After entering the pitting corrosion extension period,the corrosion rate of stainless steel gradually accelerated.The driving forces for the expansion of steady-state pitting include small anode-large cathode cells formed inside and outside the pitting pores,oxygen concentration cells,and autocatalytic acidification effects.Finally,the electrochemical activity distribution and evolution of open pits in 1M NaCl solution were observed by prefabricated micro pits.When the pit is activated,the current detected by the probe tip at the mouth of the pit is the sum of the reduction current of the cathode oxygen and the oxidation current of the redox mediator Fc Me OH.Conversely,when the pit is passivated or partially passivated,the maximum feedback current of the probe tip above the pit is about the steady-state limit diffusion current.In order to exclude the influence of air passivation,the pitting pit was reactivated by continuous immersion method,and the self-corrosion current density at the pitting pit was calculated to be 2.8116×10-6A cm² when the sample was soaked for 264h.Steady-state pitting will continue to expand to the interior of the material according to the law of"complete activation of primary pits-local passivation of primary pits and initiation of secondary pits-complete activation of secondary pits".