Self-sealing micro-arc oxidation coating on AZ91D Mg alloy and its formation mechanism

Micro-arc oxidation (MAO) coatings were obtained on AZ91D Mg alloy in an alkaline Na2SiO3-NaF base solution with and without K2ZrF6. Then, the surface and cross-sectional morphologies, elemental compositions and phase structure of the resulting coatings were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The corrosion behaviour of the coatings was evaluated by potential dynamic polarization (PDP) curves and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results show that the self-sealing MAO coating which consists of MgO, MgF2, t-ZrO2, MgSiO3 and amorphous phosphate related to Mg can be fabricated in a K2ZrF6-containing electrolyte solution. With increasing K2ZrF6 content, the coating exhibits a more dense cross-sectional morphology and less coating defects, along with better anticorrosion properties than the traditional MAO coating. The ZrF62−  plays an important role in producing the self-sealing MAO coating, which results in a three-layer structure: MgO-rich inner layer, MgF2-rich sealed pore layer and porous outer layer. The hydrolysed ZrO2 can promote the formation of the seal pores during MAO process due to its high boiling point.