Corrosion Process and Mechanism of Micro-arc Oxidation / self-assembly / nickel Composite Coating on Magnesium Alloy

Magnesium alloys, which are the lightest metal construction materials used in industry, play a vital role in future development. Magnesium alloys exhibit outstanding qualities such as low density, efficient electromagnetic shielding, and dimensional stability, making them highly valuable across a wide range of applications in automotive, medical, and electronic communication sectors, among others. However, Mg alloys are highly active and readily corrode in aqueous solutions or humid atmospheres. These alloys have limited applications because of their poor corrosion resistance. Composite coatings can improve the defects of a single coating to achieve better substrate protection. To improve the corrosion resistance of magnesium alloys, a micro -arc oxidation (MAO) / self -assembly (SAM) / nickel composite coating was fabricated on the surface of a magnesium alloy (AZ91D), via MAO, self -assembly by ethyl acetate (C4H8O2), and chemical plating with nickel. SEM and EDS were used to characterize the surface morphology and corrosion product content of the corrosion -processed samples. XRD and XPS tests were employed to analyze the changes in the surface material of the sample during corrosion. AFM was used to characterize the surface roughness of the sample during corrosion. Polarization curve and electrochemical impedance spectroscopy was used to assess the corrosion resistance of samples at various corrosion durations. The corrosion behavior of the composite coating in 3.5 wt.% NaCl environment was studied by morphological structure analysis, electrochemical tests, and corrosion product analysis, and the corrosion process model of the composite coating was established. The results show that the presence of Cl- accelerates the onset of corrosion. Polarization curves and impedance tests showed that the corrosion resistance of the MAO / SAM / Ni composite coating was significantly improved compared with that of the magnesium alloy matrix. The corrosion current density of the composite coating decreased by three orders of magnitude compared with that of the magnesium alloy. After 120 h of corrosion, the corrosion current density of the composite coating was still one order of magnitude lower than that of the magnesium alloy substrate, and the electrochemical impedance reached 1.16x10(4) Omega center dot cm(2). The results indicate that the composite coating significantly improved the corrosion resistance of the Mg alloy. The Mg alloy matrix corrodes within 24 h and generates corrosion products, including MgO and MgCl2, in an environment of 3.5 wt.% NaCl. The corrosion of the MAO / SAM / Ni composite coatings can be divided into three stages, namely early, middle, and late stages. The surface structure of the Ni layer remained dense when the composite coating was exposed to a salt -spray environment for 0-96 h. In the early stages of corrosion, the corrosion resistance of the coating improved, mainly owing to the formation of the corrosion product NiO, on the surface of the coating. As the corrosion time increased, trivalent NiOOH formed on the surface of the coating, and the coating gradually deteriorated. When the composite coating was exposed to a corrosive environment for 120 h, the Ni layer started deteriorating, and the corrosive ions penetrated and formed channels. Subsequently, the protection capabilities of the SAM and MAO layers diminished. After 144 h, the corrosive ions directly penetrated the composite coating, rendering the substrate coating ineffective. Once the outer layer of the electroless nickel plating was compromised, corrosion ions easily penetrated the composite coating, forming MgCl2 corrosion products. The results provide an experimental basis and theoretical foundation for the development, preparation, and application of such coatings.