Abstract: Micro-arc oxidation (MAO) coatings were prepared in the silicate-containing electrolyte on pure magnesium and AZ91D magnesium alloy substrates, and different characterization methods were used to compare the corrosion resistance of the coatings on the two substrates. The results showed that the AZ91D-MAO coating has better formability and more Mg₂SiO₄ and MgF₂ phases, resulting in better spotting and electrochemical corrosion resistance than that of Mg-MAO coating. The Mg-MAO coating exhibits overall corrosion in the whole deterioration process, but it basically loses the protection to the matrix in the late corrosion stage, for the AZ91D-MAO coating, localised corrosion occurs at the later stage of immersion, but the coating still can protect the substrate effectively. Due to the different electrode potentials of different elements after alloying, the selective arcing occurs on the α and β phase regions of the substrate surface. This alternating discharge phenomenon is conducive to the improvement of microstructure parameters such as the compactness, composition and defects of the coating, which results in a better corrosion resistance of the AZ91D-MAO coating.

Key words: substrate materials, micro-arc oxidation, corrosion resistance, compactness