Investigation of corrosion resistance and formation mechanism of calcium-containing coatings on AZ31B magnesium alloy

The developed micro-arc oxidation (MAO) coatings with good corrosion resistance and high calcium content are desirable for medical magnesium alloys. In this study, the influences of NaOH, sodium phytate (Na(12)Phy), Ca (H2PO4)(2)center dot H2O and treatment duration on corrosion resistance and calcium content of MAO coatings were systematically investigated using an orthogonal experiment of four factors with three levels. It is found that the corrosion resistance of MAO treated samples is synergistically determined by coating characteristics, though the developed microcrack and the coating roughness play a main role. The prolonged treatment duration or excessively high concentrations of NaOH and Na(12)Phy decrease the coating corrosion resistance due to intensive spark discharges and the subsequent development of microcracks on coating surface. The increased NaOH concentration has a negative impact on the calcium amount due to the improved solution pH value and the decreased Ca(H2PO4)(2)center dot H2O solubility in water solution. Proper Na(12)Phy is beneficial to improving the calcium amount attributed to the chelating reaction between Na(12)Phy and calcium ions. The increased Ca(H2PO4)(2)center dot H2O concentration and prolonged treatment duration improve the calcium content in MAO coatings, demonstrating that calcium ions enter into MAO coatings by both diffusion and electromigration.