Effects of additives, voltage and their interactions on PEO coatings formed on magnesium alloys

Plasma electrolytic oxidation (PEO) coatings were prepared on AZ91D magnesium alloys in an alkaline phosphate-based electrolyte. The effects of K2ZrF6, EDTA-Na, voltage and their interactions on the microstructure and corrosion resistance of these coatings were investigated based on orthogonal experimental design. The thicknesses, phase compositions and morphologies of the coatings were analyzed by using TT260 eddy-current coating thickness measurement gauge, XRD and SEM, respectively. The amount of micro-pores and the porosity on the coating surface were counted quantitatively by Image J software. The electrochemical test was used to evaluate the corrosion resistances of the coatings. The results show that bringing interactions among each factor into the experimental design is the key to this study. And the coating can be remarkably affected not only by the separate functiont of K2ZrF6, EDTA-Na and voltage, but also their interaction. The interaction among K2ZrF6, EDTA-Na and voltage increases the content of the generated substances and inhibits the ejection of these substances out of the discharge channel, which improves the compactness and the thickness of the coatings as well as the content of the corrosion-resistant phase. Therefore, when high concentration of K2ZrF6 and EDTA-Na combines with high voltage, the obtained coating exhibits the optimum comprehensive properties.