Novel dual Pt-Pt/IrOx ultramicroelectrode for pH imaging using SECM in both potentiometric and amperometric modes

Potentiometric Scanning Electrochemical Microscopy (SECM) allows the pH distribution at electrode/electrolyte interfaces to be mapped at the micrometer and submicrometer scale using ion-selective ultramicroelectrodes (UMEs) as scanning probes. However, this technique lacks precise control of the tip-to-substrate distance. Herein we propose a novel dual Pt-Pt/IrOx UME for local in situ pH visualization of 316L stainless steel (316L-SS) corrosion processes with precise tip-substrate distance control using both amperometric and potentiometric SECM modes. Simulations based on COMSOL and experimental approach curves obtained with this dual UME configuration are used to precisely establish the tip-substrate distance. Higher pH values were observed for smaller tip-substrate separations when this probe configuration is used with bulk electrolyte of the same pH. Furthermore, during the corrosion process local anodic and cathodic zones were observed to form, disappear and regenerate at a vertical tip-substrate distance of 12 mu m as a result of the localized corrosion and repassivation of 316L-SS.