Mechanism Study On Localized Corrosion Of 304 Stainless Steel Triggered By Typical Inclusions

304 stainless steel is widely used in aerospace,marine,construction and other industrial fields because of its excellent mechanical properties,corrosion resistance,and also in medical,food and other industries because it has easy to clean,beautiful and other characteristics.However,in some special environments(chlorine)in the long-term service process,the pitting-based local corrosion is still the most important reason for limiting its application.The occurrence of pitting corrosion is hidden,difficult to be detected in time and directly observed,pitting corrosion along the internal expansion of the metal matrix leading to material perforation,fracture,resulting in major sudden accidents.Therefore,clarifying the pitting mechanism of 304 stainless steel is an important theoretical guidance to predict the corrosion of stainless steel in certain environments in advance and prevent the occurrence of pitting.In this study,the typical MnS and oxy-sulfide inclusions in stainless steel are selected as the research objects,and the in-situ,quasi in-situ and ex-situ studies on the pitting corrosion of stainless steel induced by the inclusive particles are carried out by combining electron microscopy,atomic force microscopy,laser confocal Raman spectroscopy and other testing techniques,and a more comprehensive mechanism of the local corrosion of stainless steel matrix induced by the inclusions is proposed based on the experimental results.It is hoped that this work can provide effective experimental parameters for the related simulation work,with a view to establishing a complete corrosion prediction system by combining experimental work with simulation work.Specifically,this paper first explored the general process of 304 stainless steel pitting expansion including pitting nucleation,metastable pitting growth and expansion,and stable pitting growth through electrochemical studies.Pitting nucleation often originates from the MnS inclusions exposed on the stainless steel surface,forming trenches at the boundary between MnS and the matrix;metastable pitting then begins to grow and expand along the trenches,with some eventually stopping at sub-circular corrosion pits of a few microns to a dozen microns in size and about 1-5 μm in depth,and some entering the stable pitting growth stage;eventually forming pits of more than 100 μm in size and more than 50 μm in depth on the surface of the sample with a typical "lace cover" structure.The metastable pits and stable pits can be clearly distinguished from their morphologies,and a preliminary mechanism of corrosion pit evolution is established based on the pitting morphological features at different stages.Secondly,the localized corrosion behavior of stainless steel induced by MnS in 3.5 wt.%NaCl solution was investigated using in situ and quasi-in situ multianalytical methods.It was found that the dissolution of MnS started from the active dissolution site in its edge region,and gradually evolves into a larger dissolution area on the MnS surface with the increasing number of dissolution sites,leading to the total dissolution of the MnS surface layer.The dissolution of MnS directly produced elemental sulfur covering the MnS surface,followed by the further reaction of S into the solution to expose the new MnS surface,and MnS entered into a new dissolution process,correspondingly,the elemental sulfur showed a periodic rule of appearing first and the disappearing.The dissolution of MnS induced the dissolution of the surrounding matrix and eventually formed a corrosion pit with a diameter of about 20μm.The deposition of copper-containing compounds inhibited the continued dissolution of MnS and resulted in some MnS remaining in the pit.The transparent thin Cr2O3 found in the corrosion pit is ion-permeable,and some needle-like corrosion products are deposited on top of it through the film over time.Based on the consideration and analysis of the effects and roles of S,Cucontaining compounds and Cr2O3,the local corrosion mechanism of MnS dissolution from its edge active sites until pitting corrosion is induced is further refined.Subsequently,the corrosion products and their evolution rules were studied in the process of MnS-induced localized corrosion,and the characteristics of the corrosion products and their effects on localized corrosion were analyzed.It was found that a porous corrosion product film composed of a large number of needlelike substance formed on/around MnS.The corrosion product film is mainly composed of spinel structure Fe3-xCrxO4 and thick in the middle and thin on both sides,and its thickness reaches to 600 nm.From the perspective of galvanic corrosion,the corrosion product film has a significantly higher surface Volta potential than the surrounding matrix,indicating that it has a more stable nature than the surrounding passive film.There is a local occluded zone was formed below the film,where the dissolution of MnS and the matrix was still in slow progress and a "cavity layer" was formed.The existence of corrosion product film is conducive to the occluded zone below the film to maintain its aggressiveness,delaying the termination process of MnS-induced local corrosion and triggering the occurrence of secondary pitting at the MnS/substrate boundary.Finally,a comparative study of the micro-area corrosion behavior confirms that the mixed inclusions(oxy-sulfide)other than MnS formed in stainless steel also have the ability to induce pitting corrosion nucleation.Oxy-sulfides consist of oxides with polygonal structure(Al,Cr,Mn,Ti,O)and spherical/rod-shaped sulfides(Mn,S).The dissolution mechanism of the sulfide portion of the oxysulfide dose not differ from that of the individual MnS inclusion;as for the oxide portion,its surface Volta potential is close to that of the surrounding matrix and hardly dissolves,eventually falling into the corrosion pits formed by the dissolution of the sulfide and exposing the other side.Micro-area polarization test results show that the passive film on steel matrix without inclusions is intact,as reflected in the polarization curve in the passive region is smooth and flat and without current fluctuations;while the steel matrix containing a single MnS or oxy-sulfide has obvious current fluctuations in the passive region,indicating that the presence of inclusions destroys the integrity of the passive film.Accordingly,the dissolution mechanism of oxy-sulfide is discussed in depth and the susceptibility of stainless steel local areas containing MnS and oxy-sulfide to pitting corrosion is evaluated.