Study On The Pitting Corrosion Behavior Induced By Oxide Inclusions In 316L Stainless Steel

Stainless steel with excellent comprehensive corrosion resistance often suffers from pitting in the chloride ion environment.Due to its concealment and sudden destructiveness,pitting has caused great safety risks for the normal use of stainless steel materials.The vertical development is rapid,which causes the material to penetrate vertically.It is the most destructive form of corrosion.There are many factors that induce pitting corrosion of stainless steel.The existence of inclusions is an important factor inducing pitting corrosion of stainless steel.Oxide inclusions,as a large number of inclusions existing in stainless steel,mostly exist in the form of composite inclusions.The study of the influence of different components on the overall resistance to chloride ion erosion of the inclusions is very important to find out the mechanism of inclusion-induced pitting corrosion.In this paper,with in-situ analysis and chemical immersion corrosion experiments were applied,combined with scanning electron microscopy and energy spectroscopy analysis,three typical oxides of 316L stainless steel,such as Al₂O₃,MgO,and?Cr,Si,Mn?-oxide,were immersed in the chloride ion environment.The changes of morphology and composition before and after corrosion,as well as the changes of the morphology of the stainless steel matrix around the inclusions,were explored to induce the pitting mechanism.Secondly,this article uses electrochemical impedance spectroscopy and polarization curves,combined with chemical immersion experiments to comprehensively analyze the law of the three elements of aluminum,silicon,and magnesium in inducing pitting corrosion.The study reached the following conclusions:?1?Alumina exists in two forms:a single alumina inclusion and an alumina/manganese sulfide composite inclusion.The results of etching show that in the chloride ion environment,alumina inclusions in 316L stainless steel hardly induce pitting corrosion.The difference in thermal expansion of alumina and stainless steel substrate causes micro-gap at the interface,but the alumina and stainless steel substrate interface is still tightly bonded.Micro-crevices did not show significant corrosion.?2?In the composite inclusion of alumina and manganese sulfide,manganese sulfide dissolves in a short time,but alumina does not dissolve.After the dissolution of manganese sulfide,pitting pits are generated,and the direct contact between the stainless steel substrate and the corrosive medium and the ion exchange in the pitting pits are hindered,which further deepens the pitting corrosion.After the dissolution of the manganese sulfide pits,the stainless steel substrate was exposed to the chloride ion corrosion medium.The inner wall of the dissolution pit was cracked under the action of chloride ions.As the etching time prolonged,the inner wall became smooth and the dissolution pit deepened.?3?Silicon oxide mainly exists as chromium-silicon-manganese composite oxide inclusions,and the dissolution of the composite oxide inclusions in chloride ions mainly depends on the content of elemental components in the chromium-silicon-manganese composite inclusions.In the chloride ion environment,the corrosion resistance of chromium-silicon-manganese inclusions in 316L stainless steel is in the following order:single chromium-depleted phase inclusion?Cr<15 wt.%?,And single chromium-rich phase inclusion?Cr>15 wt.%?,Complex phase inclusion.For a single chromium-depleted phase inclusion,too low chromium content?Cr<6 wt.%?Will cause its corrosion resistance to decrease.?4?In the chloride ion environment,the corrosion of single-phase chromium-silicon-manganese oxide inclusions in 316L stainless steel originates from the interior of the inclusions in the form of small pores.For chrome-silicon-manganese composite phase inclusions,the sulfide wrapped in the outer layer is shorter Dissolution occurs within time,causing micro-crevices at the interface between the inclusion and the substrate,and the corrosive solution further penetrates into the micro-crevices,which promotes the dissolution of the chromium-rich phase and the substrate,leading to the occurrence of pitting corrosion.?5??Mg,Al?-oxide inclusions are discretely distributed in stainless steel,and the boundary between the inclusions and the steel substrate is straight,and each element is evenly distributed.In the chloride ion environment,Mg and Al elements were not lost before and after the corrosion,the inclusions themselves did not dissolve,the metal at the boundary between the inclusions and the stainless steel substrate did not dissolve,and the presence of magnesium elements did not exist for pitting corrosion induced by magnesium aluminum oxide.Accelerating effect,?Mg,Al?-oxide inclusions hardly induce pitting.?6??Al,Si,Mg?-oxide inclusions composite inclusions usually contain silicon-manganese elements,in which the chromium-containing silicon portion will dissolve in chloride ions,and the presence of silicon element may cause grain boundary divisions inside the inclusions,resulting in?Al,Si,Mg?-oxide inclusions,whether pitting is induced depends on the chromium content and silicon in the inclusions.