Electron Microscopy Study On Surface/Interface Structure Evolution Of Stainless Steels Induced By Electrochemical Treatment

Stainless steel is employed as construction materials in many fields because of its excellent plasticity,toughness and corrosion resistance.The thin and compact passivation film covered on stainless steel surface gives it excellent corrosion resistance,however,as we all know,stainless steel do not remain "stainless " in certain corrosive media,in the acidic environment,the passivation film will dissolve,in the environment containing aggressive anions(such as Cl-)stainless steel is prone to pitting corrosion,this local corrosion form is highly concealed,destructive,undetectable corrosion perforation,and even cause catastrophic accidents.Therefore,it is of great significance to improve the corrosion resistance of stainless steel materials.By changing the microstructure,composition and distribution characteristics of alloying elements at the interface between the passivation film and the stainless steel matrix,the corrosion resistance of stainless steel can be enhanced by improving the stability of the passivation film.The introduction of N into the surface layer of stainless steel by electrochemical nitriding can significantly increase the pitting corrosion resistance of stainless steel.Passivation at a specific potential can realize the atomic-scale reconstruction of the passivation film/matrix interface,forming a low-energy stable interface composed of a large number of low-energy planes,thereby significantly improving the resistance of stainless steel to dissolving in acidic media and the pitting resistance in chloride-containing media.The fine analysis of the evolution of the surface interface structure of materials in the process of electrochemical surface treatment at the atomic scale is the basis for establishing the relationship between the surface interface microstructure and macroscopic corrosion resistance,and is of great significance for further improving the theoretical system for the stability study of passivation films.For electrochemical nitriding,the surface of stainless steel as the site where the relevant electrode reaction occurs in the nitriding process,its surface structure will inevitably change,which,to some extent,will affect the corrosion resistance of the nitriding material.Detecting the structural evolution occurring in the electrochemical nitriding is of great significance for deciphering the involved electrode reactions and thus optimizing the nitriding parameters.In this work,using atomic force microscopy as well as transmission electron microscopy,we have clarified the concomitant localized reductive dissolution of passive film,anodic dissolution of metal matrix at micro-anodic sites,as well as re-deposition of the dissolved metal cations,which roughens the surface by forming the undulations at surface with undulation amplitude in the range of a few tens of nanometers.Element mapping analysis by Super EDS technique reveals that the re-deposited product is mainly comprised of iron oxide,which indicates iron is dissolved and the resultant iron cations occurs re-deposition.The interface atomic configuration of Fe-Cr-Ni alloy can be reconstructed by anodic polarization at transpassivation potential to form a low-energy dense interface.For stainless steels with high N and Mo content,Mo,as an alloying element that promotes transpassivation,may have unpredictable effects on the transpassivation film and interface structure during the transpassivation process.N participating reactions during the transpassivation dissolution process have the effect of changing the local pH value of the solution,which may affect the formation process of the oxide film.In this paper,654SMO super austenitic stainless steel was transpassivated,an amorphous oxide film with a thickness of about 300-500 nm was formed on the surface of 654SMO stainless steel after transpassivation.The STEM imaging in the high-angle annular darkfield mode shows that the oxide film is divided into two layers from the contrast difference,and the interface between the matrix and the oxide film fluctuates obviously.The tens of nanometers inner layer of the oxide film is continuously and uniformly covered on the surface of the matrix,showing a dark contrast,where the Cr and N elements are obviously enriched.The outer layer of the oxide film shows a brighter contrast,and there are vertical channels inside.Super spectrogram scanning and line scanning analysis showed that the outer layer of the oxide film contained almost all the alloying elements of the stainless steel matrix,and no obvious element enrichment was found.The inner and outer layers are loosely combined,even voids appear at the junctions,and there is a clear enrichment of Mo elements.The results show that the presence of alloying elements N and Mo has a significant impact on the evolution of oxide film and interface structure during the transpassivation process of stainless steel.Electrochemical tests showed that the super austenitic 654SMO after passivation had a lower current density in Cl-containing solution,and the passivation treatment improved the stability of the 654SMO passivation film.