Corrosion resistance of self-cleaning silane/polypropylene composite coatings on magnesium alloy AZ31

Abstract Magnesium (Mg) and its alloys have been widely used in a variety of industrial fields, however, the high corrosion rate and surface contamination restrict their applications. In this study, a corrosion-resistant polymer coating with self-cleaning properties on Mg alloy AZ31 was successfully fabricated via a pretreatment of amino-silane (poly(3-aminopropyl)trimethoxysilane, PAPTMS) and subsequently covered with a polypropylene (PP) film. Surface morphology and chemical compositions were examined using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and Fourier transform infrared spectrophotometry (FT-IR) and X-ray photoelectron spectroscopy (XPS) as well. Contact angle was measured to determine hydrophobicity of the composite coatings. Water permeability of the coatings was evaluated through electrochemical impedance spectroscopy (EIS). Corrosion resistance of the coating was investigated using electrochemical and hydrogen evolution tests. Results indicated that PAPTMS/PP coatings possessed a micrometer-scaled porous spherical microstructure, and super-hydrophobicity with high water contact angle (162 ± 3.4°) and low sliding angle (5 ± 0.6°) due to the low surface energy (10.38 mJ/m2). Moreover, the coating exhibited a smaller water diffusion coefficient (8.12 × 10−10  cm2/s) and water uptake volume fraction (24.5 %), demonstrating low water permeability and good physical barrier performance. As a result, the corrosion current density of PAPTMS/PP coating exhibited approximately three orders of magnitude lower than that of the AZ31 substrate, suggesting excellent corrosion resistance. Finally, corrosion-resistant mechanism of the hybrid coating was proposed.