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The microstructure-dependent corrosion resistance of dual structured fine-grained Mg-7.5Li-3Al-1Zn has been investigated. The alloys were extruded using extrusion with a forward-backward rotating die (KoBo, a newly developed SPD method) at two different extrusion ratios. The fine-grained microstructures formed in the alloys were characterized, and the influence of grain refinement on corrosion resistance was analyzed. For fine-grained (α + β) Mg-Li alloys, a higher extrusion ratio led to more intensive grain refinement; however, this relationship did not improve their corrosion resistance in a chloride-containing solution. The corrosion resistance of the alloys was mainly controlled by the refinement of α(Mg) and β(Li), along with the distribution of second phases. The presence of MgLi2Al at grain boundaries facilitated their dissolution.