Analysis of the surface film formed on Mg by exposure to water using a FIB cross-section and STEM–EDS

The composition and structure of the surface film formed on pure Mg exposed at the corrosion potential in pure water for 48h was investigated using a FIB cross-section and STEM–EDS. The surface film formed is duplex in nature, consisting of a thinner, more-porous, nano-crystalline MgO-rich inner layer, and a thicker, less-porous Mg(OH)2-rich outer platelet layer. The results are consistent with the theory that a chemical breakdown (hydration) of the bulk inner MgO layer (native oxide) is a necessary precursor step to the corrosion process that leads to the significant thickening of the partially protective outer Mg(OH)2 layer.