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.