Display omitted •A schematic four-layered structural model of the natural oxide film has been constructed.•A novel concept for the Pilling–Bedworth ratios of chemical compounds is proposed.•Grain refinement in the microstructure leads to a shift from pitting corrosion to overall corrosion.•A method to characterise the corrosion rate of dual phase Mg–Li–Ca alloys is proposed. The influence of the microstructure and the oxide film of the Mg–9.29Li–0.88Ca alloy on its corrosion behaviour was investigated using SEM, EPMA, XPS and corrosion measurements. The results demonstrated that the fine-grained microstructure improved the mechanical and corrosion resistance of the alloy and shifted pitting corrosion to overall corrosion. The oxide film contained a multi-layered structure, with the outer layer being enriched in lithium-bearing compounds; the interior layer predominantly consisting of oxides, hydroxides and carbonates of lithium and magnesium; and the bottom layer containing oxides. The Pilling–Bedworth ratio for chemical compounds was proposed, and the corrosion rates were characterised.