Understanding Localized Corrosion on Metal Surfaces Using Scanning Electrochemical Cell Impedance Microscopy (SECCIM)

Understanding the electrochemical properties at a localized scale is critically important to comprehend the origin of corrosion and develop multifunctional materials with robust corrosion resistance, particularly at conjoined metal interfaces typically encountered in automobile manufacturing. Scanning electrochemical cell microscopy (SECCM) is an emerging technique which enables to study the corrosion of metal surfaces to be visualized at the microscopic level. In this work, we developed scanning electrochemical cell impedance microscopy (SECCIM) by combining SECCM with electrochemical impedance spectroscopy (EIS) and explored the unique advantages of using SECCIM to measure the corrosion kinetics on single-crystal Mg (0001) as the model surface using direct current and alternating current polarization techniques. Specifically, a theta capillary with a tip diameter of 10 μm filled with a 0.01 M NaCl electrolyte was used as a probe to perform spatially resolved potentiodynamic Tafel polarization and EIS. The combination of traditional SECCM with EIS led to the development of SECCIM and enabled us to study small interfacial events such as charge transfer, adsorption, and emergence of resistive oxide films on the surface using the distribution of relaxation time analysis. Furthermore, by comparing localized SECCIM measurements with bulk electrochemical measurements, we establish the reliability of SECCIM for the mapping of corrosion potential and associated charge-transfer resistance on the Mg (0001) surface. Our results indicate that SECCIM measurement with Tafel and EIS analysis will provide an unparalleled ability to characterize the pitting corrosion mechanism on the heterogeneous surface of mixed-metal alloys and metal joints.