Investigation Into Effects Of Magnetic Field On Pitting Corrosion Of 316L SS With Digital Holography

For its excellent mechanical properties and corrosion resistance,316L stainless steel（SS）is widely used in food,chemical,watch-making,aerospace,nuclear power industries.It is resistant to corrosion in most environments due to a layer of dense oxide film on its surface.Unfortunately,this layer is easily corroded by aggressive ions such as Cl^-,so it is important and necessary to study the local corrosion of 316L SS in different environments.With the widespread use of electrical facilities,magnetic fields can exist everywhere in industrial environments.To ensure the safety of countless engineering projects,it is urgent to unveil the possible effect of the magnetic field on metal corrosion in different systems and under different conditions.The digital holography has been successfully applied in metal corrosion research due to its fast imaging,ready recording and convenient/versatile reproduction.This paper aims to combine the technology with electrochemical methods to study the pitting process of316L SS under the influence of the magnetic field,and discuss the action mechanism of the field with the scanning electron microscope（SEM）and other surface analysis techniques.The aspects investigated are as follows:1.Pitting dynamic process of 316L SS in Cl^-and CO₃^2-solution under the magnetic fieldThe pitting dynamic process of 316L SS in Cl^-and CO₃^2-solution with or without the magnetic field was observed in real time and in situ by combining the digital holography with electrochemical methods.It was found that in the absence of the magnetic field,in Cl^-and CO₃^2-solutions,the carbonate film deposited on the surface of the electrode,especially around the pitting corrosion,resulting in the formation of locally blocked cells,which enhanced the autocatalytic acidification process in the pitting corrosion.The magnetic field enhanced the mass transfer process and prevented the corrosion products from forming on the pitting surface.The pitting corrosion was shallow,larger in radius and greater in number than that without the magnetic field.2.Pitting dynamic process of 316L SS in Br^-and CO₃^2-solution under the magnetic fieldThe pitting dynamic process of 316L SS in Br^-and CO₃^2-solution with or without the magnetic field was observed in situ by the digital holography combined with the electrochemical measurement.Due to the competitive adsorption between Br^-and CO₃^2-,the amount of pitting in the sodium bromide sodium carbonate solution was significantly smaller than that in the neutral sodium bromide and the alkaline sodium bromide solution with the same p H value.The mass transfer process was enhanced by the Lorentz force after the magnetic field was applied,and the pitting developed laterally and became shallow.3.Dynamic process of pitting corrosion of 316L SS in Cl^-and NO₃^-solution under the magnetic fieldThe digital holography and the electrochemical techniques were combined to observe in situ the pitting dynamic processes at the electrode|electrolyte interface during the anodic dissolution of 316L SS.Effects of the magnetic field on the pitting dynamic processes of 316L SS in Cl^-and NO₃^-solution was studied.The results showed that the pitting corrosion was inhibited by adding NO₃^-into Cl^-containing solution due to the competitive adsorption of Cl^-and NO₃^-.After the magnetic field applied,the competitive adsorption of NO₃^-to Cl^-is reduced by the Lorentz force,and the protective effect of NO₃^-on the passivated film was weakened and the pitting corrosion was enhanced.