Effects of the Graphene Additive on the Corrosion Resistance of the Plasma Electrolytic Oxidation (PEO) Coating on the AZ91 Magnesium Alloy

Graphene was added into the electrolyte during the preparation of plasma electrolytic oxidation (PEO) coating on the AZ91 Mg alloy. The effects of graphene additive on the surface morphology, phase composition and chemical composition of the PEO coating were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Additionally, the effect of the graphene additive on the mechanical property of the PEO coating was evaluated using micro-hardness tester. Furthermore, the corrosion resistance of the coatings was tested by the potentiodynamic polarization curves and the electrochemical impedance spectroscopy (EIS). The results show that the graphene was successfully added into the PEO coating, and the incorporation of graphene reduced the number and size of micro-pores and cracks in the coating. Compared with the AZ91 Mg substrate, the micro-hardness of the PEO coating increased more than 15 times, and the corrosion current density decreased about three orders of magnitude when the amount of graphene addition in the electrolyte was 250 mg/L.