Enhanced long-term corrosion resistance of Mg alloys by superhydrophobic and self-healing composite coating

Although Magnesium (Mg) alloys have many distinct advantages, they are vulnerable to the corrosion even in the neutral environment. Effective and feasible anti-corrosion strategies are desirable for the wide application of Mg alloys. A novel anti-corrosion composite coating, MAO (micro-arc oxidation)/[OTP][NTf2] (octadecyl triphenyl phosphonium bis(trifluoromethylsulfonyl)amide)/PSE (superhydrophobic silica/epoxy resin), is fabricated for Mg alloys by combination of MAO, ionic liquid, and superhydrophobic coating. The employed [OTP][NTf2] is incorporated into the pores of MAO coating rather than doped in the PSE coating. The successful preparation of composite coating is characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The anti-corrosion feature is measured by potentiodynamic polarization curves (PDP) and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution. The composite coatings maintain excellent corrosive resistance after 80 days immersion in 3.5 wt% NaCl solution. It is attributed to the synergistic effect of the superhydrophobic feature, the corrosion inhibition of [OTP][NTf2], and the physical barrier of the MAO coating. Even if the coating is scratched, it still exhibits the excellent anti-corrosion performance after immersion in 3.5 wt% NaCl solution for 25 days due to the repair ability of [OTP][NTf2]. The incorporation of [OTP][NTf2] not only offers the self-healing feature but also seals the pores on the MAO surface, which is favorable to enhance the overall anti-corrosion performance of composite coating.