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 corroskin 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 (C) 2014 Elsevier B.V. All rights reserved.