Effect of hydrophobicity on the corrosion resistance of microarc oxidation/self-assembly/nickel composite coatings on magnesium alloys

Abstract

A microarc oxidation (MAO) film was prepared on the surface of magnesium alloy by using microarc oxidation technology. Three kinds of self-assembled films with different hydrophobicities were prepared on the surface of the MAO film by self-assembly (SAM) technology using different self-assembled molecules. The static contact angle tests determined that the contact angles of the self-assembled films were 33.71°, 93.99°, and 140.46°. After that, three layer composite coatings were successfully prepared by electroless nickel (EN) technology. SEM, EDS, XRD and XPS were used to characterize the surface morphology and composition of the samples. The Mott Schottky (M-S) curves characterize the semiconductor properties of the samples. The electrochemical test evaluates the corrosion resistance of the samples. The results show that the characteristic peaks of Ni were detected on the surfaces of the three three-layer composite coatings, and the electroless nickel plating layer was successfully prepared on the surface of MAO/SAM layers with different hydrophobicity. The better the hydrophilicity of the self-assembled layer is, the fewer morphological defects of the composite coatings there are, and the thicker the electroless nickel layer is. The composite coatings prepared by the most hydrophobic self-assembled layer only exhibited N-type semiconductor characteristics, while the other two exhibited PN dual-semiconductor characteristics. The MAO/SAM/EN three-layer composite coatings provide better protection for the Mg alloy substrate, but as the hydrophobicity of the self-assembled layer increases, the corrosion resistance of the composite coating decreases. The deposition reaction mechanism proves that the hydrophilic film is more conducive to electroless nickel plating. Molecular dynamics simulations show that the Ebinding of the three molecules gradually decreases. Both theories agree with the experimental results.