Deciphering the Interplay between Local and Global Dynamics of Anodic Metal Oxidation

The stark difference between global and local metal oxidation dynamics underscores the need for methodologies capable of performing precise sub-μm-scale and wide-field measurements. In this study, we present reflective microscopy as a tool developed to address this challenge, illustrated by the example of chronoamperometric Fe oxidation in a NaCl solution. Analysis at a local scale of 10 s of μm has revealed three distinct periods of Fe oxidation: the initial covering of the metal interface with a surface film, followed by the electrochemical conversion of the formed surface film, and finally, the in-depth oxidation of Fe. In addition, thermodynamic calculations and the quantitative analysis of changes in optical signal (light intensity), correlated with variations in refractive indexes, suggest the initial formation of maghemite, followed by its subsequent conversion to magnetite. The reactivity maps for all three periods are heterogeneous, which can be attributed to the preferential oxidation of certain crystallographic grains. Notably, at the global scale of 100 s of μm, reactivity initiates at the electrode border and progresses toward its center, demonstrating a unique pattern that is independent of the local metal structure. This finding underscores the significance of simultaneously employing sub-μm-precise, quantitative, and wide-field measurements for a comprehensive description of metal oxidation processes.