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Corrosion originates from atomistic reactions occurring at dynamic solid-liquid interfaces; however, direct experimental observation of these reactions has remained elusive due to the inability to preserve transient interfacial states during characterization. To refine corrosion models, advanced techniques capable of analyzing corrosion interfaces at the atomic scale are essential. Recent advancements in cryogenic-atom probe tomography (cryo-APT) enabled 3D nanoscale analysis of frozen liquid-metal interfaces. However, challenges remain in sample preparation for cryo-APT on metals undergoing corrosion. This study introduces a microcorrosion cell fabricated using localized electrodeposition in liquid (LEL), enabling atomic-scale capture of liquid-metal reactions by integrating picoliter-scale electrolytes encapsulated within sealed metallic microvessels, subsequently analyzed using cryo-APT. This approach enables 3D, nanoscale mapping of corrosion reactions with simultaneous spatial, chemical, and temporal resolution. As a model system, copper exposed to aerated dilute sulphuric acid reveals temperature- and time-dependent interfacial evolution, including nanoscale clustering of copper-sulphate species, enhanced ion pairing at elevated temperature, and the emergence of transient carbon-based interfacial complexes inaccessible to conventional characterization methods.