Microarc oxidation coating fabricated on AZ91D Mg alloy in an optimized dual electrolyte
(1. School of Materials Science and Engineering, Jiangsu University of Science and Technology,
Zhenjiang 212003, China;
2. Provincial Key Lab of Advanced Welding Technology, Jiangsu Universit)
Zhenjiang 212003, China;
2. Provincial Key Lab of Advanced Welding Technology, Jiangsu Universit)
Abstract: Microarc oxidation (MAO) process was conducted on AZ91D magnesium alloy in an electrolyte composed of Na2SiO3, NaAlO2, Na2B4O7, NaOH, C3H8O3 and C6H5Na3O7 by AC pulse electrical source. The surface and cross-sectional morphologies, film thickness, chemical composition and structure of the coatings were characterized by scanning electron microscopy(SEM), layer thickness metry, energy disperse spectroscopy(EDS) and X-ray diffraction(XRD). The corrosion resistances of the coatings in a 3.5% NaCl neutral solution were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test. The results showed that an optimized electrolyte with a composition of 15 g/L Na2SiO3, 9 g/L NaAlO2, 2 g/L Na2B4O7, 3 g/L NaOH, 5 mL/L C3H8O3 and 7 g/LC6H5Na3O7 was developed by means of orthogonal experiment. The coating obtained in the optimized electrolyte had a dense structure and revealed a lower current density, decreased by two orders of magnitude as compared with the magnesium substrate. Meanwhile, the corrosive potentials of the coated samples increased nearly by 73 mV. EIS result showed that the corrosion resistance of the coating was mainly determined by the inner dense layer. The coating primarily contained elements Mg, Al, O and Si and XRD analyses indicated that the coating was mainly composed of MgO, Mg2SiO4 and MgAl2O4.
Key words: magnesium alloy; microarc oxidation; dual electrolyte; corrosion resistance