Preparation and characterization of PLA coating and PLA/MAO composite coatings on AZ31 magnesium alloy for improvement of corrosion resistance

Abstract

To improve the corrosion resistance of AZ31 magnesium alloy in SBF (simulated body fluid), a porous MgO layer was prepared on the surface of AZ31 alloy by micro-arc oxidation (MAO) performing at a constant current density in an electrolyte containing NaOH and KF. A poly-lactic acid (PLA) layer was prepared on the surface of AZ31 alloy and on the MAO layer by dip coating in the PLA–chloroform solution. The MAO coating was composed of MgO, as confirmed by X-ray diffraction (XRD) analysis. The adhesive strength at the interfaces of the PLA–MgO–AZ31 is 45MPa as measured by the pull-off method, which is larger than that at the interface of the PLA–AZ31. A model of mechanical interlocking and MgoO chemical bond to illustrate the bonding mechanisms at the interfaces of the PLA–AZ31 and the PLA–MgO is proposed in the present work. The scanning electron microscope (SEM) reveals that the PLA coating is smooth, uniform and nonporous. Potentiodynamic polarization, electrochemical impedance spectroscopy and immersion tests show that the PLA coating significantly improves the corrosion resistance of the bare AZ31 alloy and the AZ31 micro-arc oxidized alloy in SBF. The PLA/MAO composite coatings could be a good choice for surface modifications of magnesium alloy to decrease its degradation rate in body fluids.