In-situ Construction Of Coatings On The ZK60 Mg Alloy Surface And Their Corrosion Resistance

The poor corrosion resistance of magnesium alloys obstructs their application in engineering. Therefore, improving the corrosion resistance of Mg alloys is an urgent problem to be solved thesis. In this dissertation, the ceramic coating was formed on the ZK60 Mg alloy by electrochemical-assisted deposition and plasma electrolytic oxidation（PEO） to enhance the corrosion resistance of substrate. Moreover, the corrosion mechanism of the coatings was investigated to provide the theory foundation for its further engineering applications.By using electrochemical assisted deposition technology, the effects of deposition potential, deposition time and hexadecyl trimethyl ammonium bromide（CTAB） on the thickness, hydrophobicity and corrosion resistance of the SiO₂ films were explored in the hexadecyltrimethoxysilane（HTMS）/ tetraethoxysilane（TEOS） solution system. The experimental results show that the thickness of SiO₂ film is increased by the deposition potential and deposition time, while the corrosion current density is decreased remarkably. To achieve superhydrophobic state, CTAB is used to increase the film thickness and decrease the electrodeposition time. The mechanism of HTMS-hydrolysis on the magnesium alloy surface is analyzed by molecular dynamics and quantum chemistry methods. The adsorption energy of the system is-76.03 kcal/mol. Theoretical calculation results based on density functional theory show that the highest occupied molecular orbital（HOMO） could be adsorbed through contributing electrons to empty orbital of the metal atom and forming bonding orbital. Lowest unoccupied molecular orbital（LUMO） could accept the electrons of the metal atom to form the antibonding orbital. The synergistic effect between the bonding orbital and antibonding orbital leads to the adsorption of HTMS. Molecular frontier orbital theory indicates that the adsorption of active site of HTMS is silicon hydroxyl.The interfacial adhesion between SiO₂ film and metal substrate was weak and the electrodeposition process had a dependent on the shape of magnesium alloy. To solve these problems, PEO technology was used to construct ceramic coatings on magnesium alloy surface in the electrolyte contained Na₂SiO₃-KOH-Na F to enhance the applications in material processing engineering. The concentration of each electrolyte in the system had been optimized by orthogonal experimental method. The effects of current density, duty cycle, frequency, and oxidation time on corrosion resistance of the coatings were investigated to establish the optimum process parameters. On PEO coating surface had been found a large number of tiny pores and cracks. The XRD and XPS results suggested that the major mineral phases in the PEO coating were Mg₂SiO₄, Mg O and Mg. The corrosion resistance of PEO coating was analyzed by potentiodynamic polarization. The corrosion potential is positively shifted by 0.056 V under the optimum technology parameter. Therefore, the coating exhibits a good corrosion resistance in 3.5 wt% Na Cl solution.In order to achieve the surface treatments of the whorls, a based electrolyte was further optimized by addition of glycerol. The experimental results suggest that glycerol had a great influence on the PEO process. It could deduce the volume of the oxygen and eliminate the partial discharge phenomena which caused by the inhomogeneity of the electric field distribution. SEM, EDS, XRD and potentiodynamic polarization were used to analyze the effect of glycerol on microstructure, element composition, phase composition and corrosion resistance of the coatings. The coating contained less surface defects and showed better corrosion resistant with adding 100 m L/L of glycerol. Corrosion current is only 4.30×10^-8 A/cm², which is four magnitudes lower than the substrate of the ZK60 magnesium alloy under the optimum technology parameters. Moreover, the addition of glycerol could improve surface quality of whorls and the edge of connectors. The molecular dynamics methods are used to analyze the effect of glycerol adsorption on the magnesium alloy surface to the PEO process of reaction. The interface components and property between the solution and substrate have changed by this glycerol adsorption on the surface of magnesium alloy. Furthermore, as the organic molecules could increase the surface resistance and decrease the surface of active sites during the PEO process, the discharge centers are divided into smaller active sites to avoid the fierce discharges. The extent of spark discharges is softened and the diameters of discharge pores in the oxide layer are decreased.To enhance the corrosion resistance, it is vital to improve the surface quality of the PEO coating. The coatings were modified by Fe3O4 nanoparticles and grephene oxide（GO） to decrease the surface defects in this thesis. First of all, the influence of magnetic field on the preparation of PEO/Fe3O4 was studied. Under the adhesion of dopamine, array structure with Fe3O4/HTMS was immobilized on the magnesium alloy by the magnet-induce assembly technique. The super hydrophobic coating was successfully prepared with the water contact angle of 157°. The performances of the composite coating of PEO/Fe3O4 exhibited more positive corrosion potential and lower corrosion current density. The corrosion current is only 1.44×10^-8 A/cm² and it is four magnitudes lower than the substrate of the ZK60 magnesium alloy.Graphene oxide（GO） which with lots of functional groups was prepared by modified Hummer method. GO was embedded into the porous structures of the plasma electrolytic oxidation coatings to reduce the porosity. And graphene oxide was formed on the plasma electrolytic oxidation coatings of magnesium alloys via electrostatic self-assemble method, which functioned as a physical separation with inhibiting effects between the protected metal and reactants. The corrosive reactions of compound coating of PEO/GO were not taken place by dipping in 3.5 wt% Na Cl solution for 100 h. And the resulting graphene oxide films acted as a barrier for the corrosive media. As a result, the corrosion rate is significantly decreased to 1.45′10^-8 A/cm² and is fivefold of the pristine magnesium alloy. Therefore, the corrosion resistance of magnesium alloy was improved by GO film.