Methyl Viologen as a Redox Mediator for Scanning Electrochemical Microscopy to Study Copper Corrosion Under Deaerated Conditions

Scanning electrochemical microscopy (SECM) is a popular tool to study corrosion with high spatial resolution. The feedback mode of SECM involves the use of an added redox mediator in solution to probe local surface kinetics. Depending on the redox mediator's oxidation state and formal potential, as well as the corroding metal's corrosion potential, the electroactive species can potentially polarize the substrate and alter the corrosion behavior at the macro and/or microscale. Therefore, the choice of redox mediator is material dependent. This study explored the use of methyl viologen (MV) as a potentially ideal redox mediator for studying the local reactivity of copper. The SECM solutions were deaerated prior to SECM approach curve measurements to avoid a convoluted response from the oxygen reduction reaction at the ultramicroelectrode. Since the formal potential of MV is lower than the corrosion potential of Cu and undergoes oxidation to regenerate at the substrate's surface, the redox mediator was successfully implemented for kinetic measurements without inducing oxidative etching, ultramicroelectrode fouling, or macroscale corrosion. This work highlights the importance of redox mediator choice for SECM corrosion studies to avoid misinterpretation of data and provides a systematic method of making such decision for accurate measurements.