Challenges in Quantitative Scanning Electrochemical Microscopy for Corrosion Studies of Ferrous Materials: The Role of Redox Mediator–Substrate Interactions

A key challenge in using scanning electrochemical microscopy (SECM) in feedback mode for corrosion studies is decoupling the redox mediator's (RM) influence from the intrinsic reactivity of the substrate. In this work, macro‐ and microelectrochemical experiments are combined with finite element modeling to investigate how two widely used RMs, ferrocenemethanol (FcMeOH) and hexaammineruthenium (III) chloride ( ), affect the corrosion behavior and SECM response of iron and stainless steel (SS‐316L). The apparent rate constants extracted from SECM measurements highlight a clear dependence on substrate passivation. SECM measurements over iron revealed that the applied potential required to induce FcMeOH oxidation causes ultramicroelectrode fouling via iron oxide deposition, thereby compromising measurement reliability. In contrast, undergoes reduction at the active Fe surface, leading to local RM depletion and a feedback response characterized by a steeper current decay than typically observed over passive surfaces. On SS‐316L, negative feedback was observed for both mediators, reflecting the presence of a stable passive film. This study identifies key pitfalls in SECM corrosion analysis and demonstrates how RM–substrate interactions can affect interpretation. These findings offer practical guidance for improving the quantitative reliability of SECM in probing localized corrosion processes of ferrous alloys.