<p>LZ91 alloy is an ultra-lightweight Mg alloy for weight-reduction applications. However, its dual-phase structure (HCP α and BCC β) complicates the corrosion behavior. This study investigated the localized corrosion behavior of a commercial LZ91 alloy in 3.5 wt% NaCl solution and its relationship to the microstructure of the surface corrosion films on the two phases. Microstructure characterizations showed that the surface corrosion films on the α and β phases both consisted of an outer Li₂CO₃/Mg(OH)₂ layer, a middle Mg(OH)₂ layer, and a thin Zn-enriched interfacial MgO layer. Nevertheless, the overall thickness of the surface corrosion film on β was thicker than that on α due to the lower Volta potential of β and the resultant microgalvanic effect. Localized corrosion initiates on the alloy after 8–12 h of immersion, accelerating the corrosion rate. Noteworthily, localized corrosion preferentially propagates in α because of the thinner and less protective surface corrosion film.</p>