Optimization of dual electrolyte and characteristic of micro-arc oxidation coating fabricated on ZK60 Mg alloy

Micro-arc oxidation (MAO) process was carried out in a dual electrolyte system of NaAlO2 and Na3PO4 to develop compact, smooth and corrosion-resistant coatings on ZK60 Mg alloy by single factor experiments. The microstructural characteristics of coatings were investigated by X-ray diffractometry (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). Test of mass loss was conducted at a 3.5% NaCl solution to assess the resistance to corrosion. The effect of every element in the dual electrolyte system on voltage-time responses during MAO process and the coating characteristic were also analyzed and discussed systematically via single factor experiments. The results reveal that the main components of NaAlO2 and Na3PO4 as well as additives of NaOH, NaB4O7 and C6H5Na3O7 demonstrate different effects on MAO process and coating characteristics. By means of single factor experiments, an optimized dual electrolyte system was developed, containing 17.5 g/L NaAlO2, 5.0 g/L Na3PO4, 5.0 g/L NaOH, 3.0 g/L NaB4O7 and 4.2 g/L C6H5Na3O7.