Visualizing the Hidden Atomic Pathways of Iron Oxidation.

Iron oxidation is a fundamental chemical transformation underpinning planetary evolution and technologies ranging from steel metallurgy and corrosion to catalysis and magnetic storage. Despite decades of surface-science research elucidating oxidation kinetics and surface dynamics, the atomic-scale transformations occurring beneath the surface during the incipient stages of oxidation have remained largely inaccessible. Here we directly visualize the atomic genesis of iron oxidation using in situ environmental scanning/transmission electron microscopy (ETEM/ESTEM). We reveal that oxidation begins with the nucleation of an epitaxial FeO layer on metallic iron. Once the FeO film reaches a critical thickness, a lattice-template-driven phase transformation is triggered at the buried FeO/Fe interface, rather than at the gas-exposed surface, leading to the formation of the higher-valence oxide phase Fe3O4. This interfacial transformation drives a concerted outward flux of iron cations, continuously regenerating a ∼two-monolayer FeO skin atop a thickening Fe3O4 underlayer. This process results in a persistent, self-regulating FeO/Fe3O4/Fe trilayer architecture that directly contradicts the prevailing assumption that oxidation states monotonically increase toward the gas-solid interface. By identifying this counterintuitive, interface-driven mechanism, our results provide the missing atomic-scale link for understanding oxide stability and phase evolution in reactive environments.