| Magnesium alloys are extensively used in automobile, aerospace, electric appliances and structural components owing to their low density, high specific strength and stiffness, excellent castability, machine ability and damping property etc. However, the relatively poor corrosion resistance of this alloy limits its extensive utilization in many fields. Among the various corrosion protection methods, chemical conversion coating is one of the most important methods. Many methods have been proposed for the chemical conversion processing of magnesium alloys. Traditionally, conversion coatings for magnesium alloys are based on hexavalent chromium (Cr6+) compounds, which are toxic and caustic. Due to the environmental impact of hexavalent chromate and the imminence of associated restrictions, there is a great need for the development of environment-friendly treatment methods.In this paper, a new innoxious and pollution-free chemical protective coating for magnesium alloys, phytic acid conversion coating, was prepared. The coating preparation process was optimized through orthogonal experiments. The morphology, phase structure, composition, functional group, weight gain and chemical state of the coating were carried out by means of scanning electronic microscopy (SEM), X-ray diffraction (XRD), auger electron spectroscopy (AES), Fourier transform infrared spectroscopy (FTIR), electronic balance and X-ray photoelectron spectroscopy (XPS), respectively. Electrochemical impedance spectroscopy (EIS) was tested and the equivalent circuit was simulated. The coating formation process was analyzed and the formation mechanism was put forward. The corrosion behavior of phytic acid conversion coating in 3.5%NaCl solution was investigated by EIS and SEM, and the coating protection mechanism was also analyzed. The chemical Ni plating was deposited on phytic acid conversion coating combined with the active Pd2+ by silane coupling agent. This process was characterized by XPS, FTIR. The morphology, structure, corrosion resistance and mechanism of the Ni plating were studied. The influence of phytic acid conversion coating as a middle layer on the properties of the epoxy varnish coating on magnesium alloys was investigated.The optimum parameters to prepare phytic acid conversion coating are determined by the orthogonal test. The results are: concentration of 5 g/l, temperature of 20℃, time of 15 min and pH of 8.The formation of phytic acid conversion coating is cooperated with the dissolution of substrate and the deposition of conversion coating. And the formation process can be divided into three steps: initial stage, metaphase stage and late stage. The phytic acid radical reacts with different number of magnesium ion or aluminum ion to form chelate compounds on the surface of magnesium alloy as a chemical conversion layer.The main failure mechanisms of the phytic acid conversion coating in 3.5%NaCl solution are dissolution and exfoliation corrosion. The failure process can be divided into three periods: Cl- absorption, conversion coating dissolution and penetration into the coating.The active Pd2+ cations are connected to the phytic acid conversion coatings on AZ91D magnesium alloy by silane coupling agent. Ni-P coatings can be plated on the AZ91D alloy coated with phytic acid conversion coatings using the catalysis of palladium atom. The nickel coating is very compact without defects such as cavities and crevices, and the coating has excellent corrosion resistance.The phytic acid conversion coating using as a middle layer between the magnesium alloy substrate and the epoxy varnish coating can obviously improve the corrosion resistance of the coating system. This can be attributed to the hydroxyl group, phosphate group, and other organic functional groups covered on the surface of the conversion coating. It greatly enhances the combination of the organic coating with the magnesium alloy substrate, and the conversion coating itself has good corrosion resistance. At the same time, the conversion coating significantly enhances the dielectric shielding property of the coating system. The above results show that phytic acid conversion coating has a unique structure. The coating itself can efficiently improve the corrosion resistance of magnesium alloy. Additionally, when acted as a middle layer, it can also improve the corrosion resistance of the nickel plating and organic coating on the surface of magnesium alloy. The study is of important theoretical guiding significance for the design of the magnesium alloy protection.
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