Influences of Na2SiO3 and EDTA-ZnNa2 concentration on properties of zinc-containing coatings on WE43 magnesium alloys

There exists great interest in the development of coatings that can simultaneously improve corrosion resistance and contain essential trace elements on biomedical magnesium alloys. In an alkaline solution composed of NaF, phytic acid (H12Phy), Na2SiO3 and EDTA-ZnNa2, it was demonstrated that zinc-containing coatings with improved corrosion resistance were successfully fabricated on WE43 magnesium alloys using a one-step micro-arc oxidation (MAO) procedure. The influences of Na2SiO3 and EDTA-ZnNa2 concentrations on MAO coating properties were systematically evaluated using SEM, EDS, XRD, XPS, scratch test and potentiodynamic polarization. These results demonstrate that MAO coatings containing 1.67 wt% Zn exhibit superior corrosion resistance. Na2SiO3 is one type of passive agent of magnesium alloys, and has been shown to be favorable for the formation of coatings as it enhances corrosion resistance by improving surface uniformity, coating thickness and adhesion to the substrate. Raised EDTA-ZnNa2 in the range of 1 to 3 g/L is found to increase zinc content in MAO coatings but decrease their corrosion resistance. As an acidic substance, EDTA-ZnNa2 is a corrosive agent of magnesium alloys and has shown to be harmful for the formation of MAO coatings. Additionally, the corrosion mechanism of MAO coatings was discussed by measuring electrochemical impedance spectroscopy as a function of immersion time.