Comparison of coating properties obtained by MAO on magnesium alloys in silicate and phytic acid electrolytes

Anodic coatings were prepared by using microarc oxidation (MAO) on AZ91HP in silicate containing solution (Si-solution) and phytic acid containing solution (P-solution), respectively. The influence of the electrolytes on coating structure, morphology and composition was studied by using X-ray diffraction (XRD), environmental scanning electron microscope (ESEM) and energy dispersive X-ray spectroscopy (EDX). Potentiodynamic polarization test and immersion test were employed to evaluate the corrosion resistance of anodic coatings. Different electrolytes caused the differences in the MAO process and coating properties. The breakdown voltage and the final voltage in P-solution were higher than those in Si-solution. The pore uniformity of anodic coatings obtained in Si-solution (Si-film) was worse than that in P-solution (P-film). XRD analyses indicated that Si-film was amorphous, while P-film consisted of MgO. The corrosion resistance of the sample coated with P-film was better than that with Si-film.