Effects of tannic acid on properties of anodic coatings obtained by micro arc oxidation on AZ91 magnesium alloy

Anodic coatings were obtained by micro arc oxidation (MAO) on AZ91 magnesium alloy in solutions with and without tannic acid. The influence of tannic acid on formation and property of anodic coatings was studied by scanning electron microscope, energy dispersive X-ray spectroscopy, Fourier transform infrared and potentiodynamic polarization tests. The results demonstrate that tannic acid changes the coating color. Tannic acid can improve the corrosion resistance of the anodized magnesium alloy by forming an insoluble magnesium–tannate complex, improving the pore uniformity and increasing the thickness of anodic coatings.