Study On High Resolution In-situ Electrochemical Imaging Based On Submicron Ultramicroelectrode

Corrosion is a kind of classical electrochemical behavior caused by the interaction between metal materials and environmental media.It usually involves the coupling of multiple electrochemical reactions and has highly localized characteristics.Due to its good corrosion resistance,stainless steel has been widely used in many fields,such as energy,chemical industry,medical equipment and household appliances.As one of the main forms of localized corrosion,pitting corrosion has a very strong impact on the damage of stainless steel.For a long time,traditional electrochemical detection methods and microscopic observation methods is hard to realize in-situ monitoring of pitting process.It is difficult to combine electrochemical principles with high-resolution imaging.Also,it is limitation to obtain the pitting information accordingly.Therefore,it is of great significance to in situ observe the dynamic evolution of early pitting corrosion with high precision and resolution.Scanning Electrochemical Microscopy(SECM)as one of electrochemical scanning probe techniques,has high spatial resolution and chemical/electrochemical sensitivity combined with ultramicroelectrode(UME).It has unique advantages to in-situ study the local breakdown of passive film,the mechanism of pitting initiation and growth,corrosion products detection and topographic characterization.A well behaved submicron UME with controlled geometry has been manufactured by the P-2000 laser-based micropipette puller system and BV-10 micropipette beveler system.Combined with the optical microscope,scanning electron microscopy(SEM),cyclic voltammograms and approach curve to characterize sub-micron UME.The cyclic voltammograms have the ideal sigmoidal shape and the experimental curve is consistent with the simulation curve as well,which shows that the sub-micron UME fabricated by this method has a good electrochemical performance.The evolution of localized corrosion on 316L stainless steel(SS-316L)immersed in 1.0 mM FcMeOH + 0.1 M NaCl solution was investigated based on theSECM feedback mode.A submicron Pt UME with a radius of 0.7 ?m manufactured by the P-2000 laser-based micropipette puller system and BV-10 micropipette beveler system is used as SECM tip approaching down to 1 ?m from the SS-316L substrate to study the initiation and growth of the pitting corrosion of stainless steel.According to the SECM imaging results,the passive film of stainless steel was local breakdown after 4h of immersion,and FcMeOH at this place could be reduced and regenerated,resulting in a peak current at a range of about 20 pA in the scanning area.The growth of pitting corrosion and initiation of new active spot occurs on the surface of the SS-316L with the immersion time increasing,and the peak current can reach-190 pA.The growth of active sites was inhibited due to repassivation of the surface film,which further hinders localized corrosion,resulting in the decrease of peak current.The SEM ex-situ observation of pits size indicated results indicate that the spatial resolution of dynamic active pits in SS-316L is within the submicron scale.COMSOL multiphysics software was applied to build a 3D model,and the spatial size of pitting corrosion was simulated,analyzed and quantified by combining experimental data.3D COMSOL simulation results show that the lateral and vertical resolution values are 0.5 ?m and 0.42 ?m for a single active pit,respectively.Two adjacent pits had radii of 0.36 ?m?0.45 ?m and depths of 0.87 m?0.92 ?m,with approximately 7 ?m separation between the two pits.From the fitting results,the pit radius and depth on SS-316L as determined from SECM mapping have micron scale.this is first time to quantify the pit corrosion size of stainless steel using in situ SECM mapping and COMSOL simulation.The detection of Fe2+ generated in the evolution of localized corrosion on SS-316L immersed in acid solution with 6%FeCl3 was investigated based on the the substrate generation/tip collection(SG/TC)of SECM.The submicron Pt UME with a radius of 0.45 ?m is used as SECM tip approaching down to 1 ?m from the SS-316L substrate were used as SECM tip,respectively.The current peak detected by the probe corresponds to the current signal of Fe2+ oxidized into Fe3+,and the magnitude of the peak current corresponds to the concentration of Fe2+.The accurate quantitative analysis of Fe2+ concentration is also discussed in this paper.There is a concentration limit when Fe2+ is detected,and the extremely low concentration of Fe2+ may not be detected by using SG/TC mode of SECM.And the local pH change at the metal/solution interface also affects the detection of Fe2+.