In-situ Spatially Electrochemical And Chemical Imaging Over The Corroding Metal Surface

The metal corrosion process is a typical interface electrochemical reaction.With the development of the corrosion,a series of anodic and cathodic active sites/regions formed on the metal surface,resulting in the change of micro-chemical environment(such as pH,Cl-and metal ions concentration)at the metal/solution interface.These micro-chemical environment changes would affect the progress of corrosion process.Since the size of these anodic and cathodic active regions is usually in the scale of micrometer,or even in nanometer,it is difficult to measure the local interfacial chemical environment changes with conventional electrochemical methods.Scanning electrochemical microscopy(SECM)is a kind of in-situ spatially resolved electrochemical technique,which enables in-situ measurement of the surface topography,location and size of surface "active spots" and species concentration distribution of metal/solution interface.SECM has been widely used in the study of metal corrosion processes.Amperometric mode is the most widely used mode in SECM,which enables electrochemical or topographic imaging of the metal/solution interface,but does not respond to the species of interest that are not electroactive.Compared with amperometric mode,potentiometric mode of SECM allows quantitative measurement of the local concentration of the specific species(including H+,OH",alkaline earth metal ions,such as Mg2+,Zn2+).However,the main disadvantage of this mode is the inability to control the gap of tip-substrate precisely.Due to the limitations of the conventional probe,we cannot obtain the in-situ spatial electrochemical/chemical information of the metal/solution interface at the same time.In this thesis,the microelectrochemical pH sensor associated with the potentiometric mode of SECM was firstly adopted to monitor the local pH distribution at the metal surface over time.A solid-state combination Pt/IrOx-pH micro-sensor electrode was prepared by anodic electrodeposited iridium oxide film(EIROFs)onto a 10 μm platinum ultramicroelectrode(UME).The morphology of the film was observed by SEM,and the valence state of the membrane were analyzed by XPS.The prepared Pt/IrOx-pH micro-sensor electrode has a very good linear response(R2=0.999)in the range of pH=1.00-13.00,good stability,long lifetime(it still showed good linear response even after 110 days of aging).The response time to pH is fast and the potential changes immediately at the moment of pH change.The CV curve and XPS results show that the ratio of Ir4+/Ir3+ in the IrOx film increases with the positive shift of the cyclic voltammetric scanning potential range,which leads to an increase of the slope of the Pt/IrOx-pH micro-sensor electrode response(super-Nernstian response).Then the sensor was employed to explore the localized pH distribution during the corrosion of 316L-SS in NaCl solution at open circuit potential.SECM mapping results show the anodic and cathodic active regions are detected separately.The dissolution of iron at anodic active region results in the generation of ferrous ions.The ferrous ions undergo hydrolysis reactions,resulting in the local acidification of the electrolyte solution.While proton reduction occurs in the cathodic active region,resulting in the increase of local pH.With the prolonging of immersion time,the pH difference between the anodic and cathodic zones increases from 0.22 to 1.27 in the range of 500 μm× 500μm over the 316L-SS.At the same time,in this paper a sub-micron Pt/IrOx-pH micro-sensor electrode was also fabricated to enable the probe to move to a height of 1 μm over the 316L-SS substrate surface.Associating with the potentiometric mode of SECM,the pH measured by the probe at different distances over the 316L-SS,could be converted to proton flux and finally we achieve to monitor the in-situ corrosion current density.The results show that the corrosion current densities of the 316L-SS are jt=4h=7.90×10-3 A·cm-2,jt=6h=2.67×10-3 A·cm-2,jr=8h=1.54×10-3 and jt=10h=2.45×0-14 A·cm-2 respectively,after immersion in the acidic solution containing 6%FeCl3 for 4,6,8 and 10 h.Therefore,in-situ localized corrosion current density of the metal can be monitored in nondestructive detection using this method.A novel solid-state dual Pt-Pt/IrOx-pH micro-sensor electrode,combined with SECM in amperometr:ic/potentiometric composite mode,was proposed to study the localized corrosion behavior of 316L-SS in three NaCl solutions with different pH(pH=2.00,6.00 and 11.00 respectively),with precise control of tip-substrate distance.A typical normalized theoretical approach curve in 1 mM FcMeOH+0.1 M KNO3 solution was simulated with COMSOL Multiphysics software based on the true shape and size of the dual Pt-Pt/IrOx-pH micro-sensor electrode.This normalized theoretical approach curve was used to precisely establish the tip-substrate distance in SECM experiment.Results show that,higher pH values are observed with smaller tip-to-substrate distance in the three NaCl solutions with different pH,proving the importance of the precise control of the tip-to-substrate distance.The pH value of the electrolyte adjacent to the substrate surface,in the acid solution,increases with the prolonging of immersion time.The anode and cathode active sites can be detected after 5 and 20 h immersion at a vertical tip-substrate distance of 12 μm,indicating the formation of pit during the immersion.The pH distribution becomes almost even after immersion for 8,32 and 48 h,because the formed pit during the immersion is a kind of metastable pitting,and the growth of the pit will be terminated as it was covered with a passivation film.However,no obvious "active sites" were detected in the neutral and alkaline solution,indicating that the passivation film over the stainless steel surface is stable in neutral or alkaline solution,which can protect the substrate from pit corrosion.This novel solid-state dual Pt-Pt/IrOx-pH micro-sensor electrode was also employed for quassi-simultaneous in-situ spatial electrochemical and chemical imaging over the metal corrosion surface,using SECM in both potentiometric and amperometric modes.We performed quasi-simultaneous imaging of the localized Fe2+ concentration and pH over the 316L-SS,as well as the quasi-simultaneous imaging of surface morphology and pH distribution over the AZ91 magnesium alloy,in the same solution without changing the probe.Obvious anodic and cathodic active site are observed over the 316L-SS surface during the immersion process.The local anodic site are observed to be close to the local cathodic site,and the apparent distance between the local anode peak and cathode peak after 1 hour immersion is approximate 84 μm at the height of 15μm.With the prolonging of the immersion time,the anodic and cathodic active region can be detected at the same position,which is consistent with the SEM results due to the formation of stable pit over the 316 L-SS in 6%FeCl3 solution.This method is also used for studying the localized corrosion behavior of AZ91 magnesium alloy.The current mapping results show that the corrosion products are accumulated over the substrate surface.The decrease and increase of localized pH,corresponding to the corrosion active anode and cathode region respectively,are observed adjacent to the corrosion product according to the pH mapping results.