Site‐Specific Hydroxide Formation and Corrosion on Mg Nanocrystals

ABSTRACT Magnesium is one of the lightest structural metals and is highly attractive for aerospace, transportation, biomedical, and energy technologies, yet its application is limited by rapid corrosion and an incompletely understood initiation mechanism. Here, using single‐crystalline Mg nanocrystals as a defect‐free model system, we identify intrinsic corrosion behavior. Results show that the alkali‐induced Mg(OH) ₂ hydration layer initially functions as a protective barrier, but its stability is determined by interfacial coherence with the Mg substrate. Once the film grows beyond a critical thickness of about 10–20 nm, incomplete coverage at crystal corners leads to local exposure. Quasi in situ TEM and 3D reconstruction reveal that corrosion starts from these exposed corners, propagates along junctions between the thick hydroxide and Mg underneath, and generates interfacial rupture that acts as pathways for Cl ⁻ ingress. Rather than dissolving the entire hydroxide film, Cl ⁻ ions preferentially attack incoherent Mg(OH) ₂ /Mg interfaces, triggering localized matrix corrosion. These findings provide direct experimental insight into the earliest stages of Mg corrosion and identify interfacial engineering as a viable strategy to enhance corrosion resistance in Mg and potentially other lightweight metals.