Optimization of AZ31B Magnesium Alloy Anodizing Process in NaOH-Na2SiO3-Na2B4O7 Environmental-Friendly Electrolyte

The optimization of NaOH-Na2SiO3-Na2B4O7 electrolyte for the plasma electrolytic oxidation of AZ31B magnesium alloy was investigated through orthogonal tests. The properties of the anodized films were evaluated by film thickness, roughness measurements, salt spray tests, scanning electron microscopy (SEM), X-ray diffraction (XRD) and potentiodynamic polarization tests, respectively. The orthogonal tests revealed that the optimal formulation of the electrolyte comprised NaOH 45 g/L, Na2SiO3 50 g/L, and Na2B4O7 90 g/L. NaOH exhibited the most significant effect on film thickness, while Na2SiO3 had the greatest effect on corrosion resistance. Moreover, the optimal electrical parameters were also obtained with the values of current density 1 A /dm2, oxidation time 15 min, pulse frequency 200 Hz and duty cycle of 10%. The surface morphology of the anodized coating formed under optimal conditions was uniform and compact. Furthermore, the phase compositions of all samples were mainly composed of MgO and Mg2SiO4. The corrosion potential, corrosion current density and polarization resistance of the prepared coating by plasma electrolytic oxidation improved remarkably compared with that of the substrate.