Influence of phytic acid concentration on coating properties obtained by MAO treatment on magnesium alloys

Anodic coatings were prepared by microarc oxidation (MAO) on AZ91HP in a base solution of 10 g/L NaOH with and without the addition of 0-12 g/L phytic acid (C₆H₁₈O₂₄P₆). The influences of C₆H₁₈O₂₄P₆ and its concentration on the conductivity and breakdown voltage were studied. The morphologies and compositions of anodic coatings were determined by environmental scanning electron microscope (ESEM) and energy dispersive X-ray spectroscopy (EDX). Potentiodynamic polarization test was performed in 3.5 wt.% NaCl solution to evaluate the corrosion resistance of anodic coatings. The results showed that with the increase of C₆H₁₈O₂₄P₆ concentration, the solution conductivity decreased while the values of breakdown voltage increased. EDX analysis showed that the coatings formed in solutions with C₆H₁₈O₂₄P₆ addition contained Mg, Al, O, C, P and a trance of Na. The addition of C₆H₁₈O₂₄P₆ into the base solution was helpful in coating formation and the coatings formed in the solution containing 8 g/L C₆H₁₈O₂₄P₆ exhibited the best pore uniformity and corrosion resistance.