Corrosion resistance of one-step superhydrophobic polypropylene coating on magnesium hydroxide-pretreated magnesium alloy AZ31

Abstract Superhydrophobic coating on magnesium (Mg) alloys usually comprises an inner coating with a hierarchical structure and an outer coating with a low surface energy. However, the instability and weak interfacial adhesion of both coatings restricts their use on a larger scale. In this paper, a Mg(OH)2 coating was formed on the AZ31 Mg alloy via hydrothermal process, and subsequently one-step fabricated a micron-sized spherical structure polypropylene (PP) film on the Mg(OH)2 coating to obtain the corrosion-resistant and superhydrophobic coating. Surface morphology and chemical composition of coatings were characterized via field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) together with X-ray photoelectron spectroscopy (XPS). The contact and sliding angles were measured to determine the coating hydrophobicity. Furthermore, the corrosion resistance was investigated through electrochemical polarization, electrochemical impedance spectroscopy, and hydrogen evolution tests. Results indicated that the Mg(OH)2/PP hybrid coating exhibited superhydrophobicity due to its micron-scale spherical structure with high contact angle (165.5 ± 3.6°), a low sliding angle (4 ± 0.6°) and low surface energy (6.7 mJ/m2). Corrosion current density decreased approximately four orders of magnitude (from 6.15× 10−5 A cm−2 to 3.12 × 10−9 A cm−2) compared with AZ31 substrate, indicating that the proposed coating possessed superior corrosion resistance. Stability tests indicated that the hybrid coating showed excellent physical and chemical stability. Moreover, the scratch tests revealed that the composite coating had better adhesion.