The microstructural evolution of the plasma electrolytic anodic coating on AZ31 magnesium alloy in an alkaline silicate solution system was studied. Voltage as high as 300 V was observed in the constant current density anodization and caused a characteristic coating transformation from a single thin porous layer to a multilayered microstructure during the anodization process. This transformation was greatly contributed to physical breakdown behaviors of the thin dielectric, during which, closed and open types of defects were observed. Such defect distribution and density in the barrier dielectric thin film were highly likely to affect the sparking behaviors as well as the resultant coating formed. In a 10 min run of anodization, a near-amorphous, silicate-containing, dense-to-porous overcoat was observed, under transmission electron microscope, attaching to a fluoride-rich barrier layer. In chemical analyses, fluoride contents were concentrated in the barrier layer along the magnesium substrate, while silicate contents were distributed away from the barrier layer in increasing amounts during the growth evolution. (c) 2008 The Electrochemical Society.