Influence of Electrolyte Concentration on the Scanning Electrochemical Microscopy Feedback Behavior of Copper

Scanning electrochemical microscopy (SECM) is a powerful technique for analyzing the local electrochemical reactivity of surfaces. The effect of electrolyte concentration is often overlooked but can be a critical parameter to consider when exploring active substrates due to its influence on the corrosion reactions taking place. In this study, hexaammineruthenium(III) chloride was employed as a redox mediator (RM) to investigate the feedback (FB) response over a corroding Cu substrate under various concentrations of sodium chloride electrolyte. At high chloride concentrations (e.g., 600 mM), the FB response rapidly transitioned towards negative behavior due to the fast depletion of RM, which accelerated the substrate’s corrosion rate. The kinetic rate constant obtained using a finite element model for the RM’s reaction at the Cu surface over time revealed a gradual decrease in surface electroactivity due to the RM reaction with the metal. By diluting the chloride concentration (e.g., <10 mM), the open-circuit potential of Cu shifted to a more positive value than the RM’s redox potential, minimizing the oxidizing behavior of the RM towards Cu. As a result, reducing the electrolyte concentration prolonged positive FB behavior for a longer duration, enhancing the reliability of SECM imaging for corrosion studies.