Research On Forming Mechanism And Properties Of Anodizing Film Strengthened By Al₂O₃ Nano-particles On Magnesium Alloy

In order to further improve the corrosion resistance and wear resistance of the anodizing film of magnesium alloy, a low-voltage, low-temperature, environmentally friendly composite anodizing technology was proposed using alkaline silicate with NaOH as a base electrolyte system and with the addition of amino acids, Al2O3 nanoparticles and Al2O3 sol as additives. Different types of composite oxide films have been produced on AZ31 magnesium alloy.The thickness, structure, surface morphology, phase composition and corrosion properties were analyzed by eddy thickness meter, IR, SEM, XRD and potentiodynamic polarizatioin tests in 3.5% NaCl solution before and after the addition of organic additives. The results show that amino acids have remarkable effect on arc restraining, which resulted in an increase in breakdown potential and an increase in the thickness of the oxide film, and finally the improvement in the surface finish. However, amino acid-based organic additives do not alter the chemical composition and phase structure of the oxide film. Ethylenediamine tetraacetic acid(EDTA) and L-ornithine acetate(L-Orn) have much better arc restraining ability. A compact oxide films with fine holes uniformly distributed, which has good corrosion resistance, can be deposited on the magnesium alloy after adding EDTA and L-Orn separately.The results of First-principles calculations show that the strongest adsorption happened when both the amino and carboxyl groups of L- ornithine acetate standing on the Mg(0001) and formed a coordination bond. Amino acid-based organic additive can change the behavior of the interface magnesium alloy/electrolyte and influence the process of anodizing mainly through inhibition effect, arc suppression effect and surfactants. A small pore diameter, uniform and compact oxide film was obtained and thereby improving the corrosion resistance of the magnesium alloy.Composite anodic coating with superfine Al2O3 particles was prepared on AZ31 magnesium alloy by anodizing in an environmentally friendly alkaline solution with addition of Al2O3 nano-particles in the electrolyte. The resultes show that the composite anodic coating has a typical anodizing porous characteristic. The addition of nanoparticles to anodizing solution doesn’t affect significantly the process of anodizing. The composite coating is composed of a porous outer and a dense inner layer. The phase composition of composite coating is periclase MgO, forsterite Mg2SiO4 and α-Al2O3. Al2O3 nanoparticles are mainly in three forms in the composite oxide film: a) some of the Al2O3 particles are adsorbed on the oxide film surface; b) some Al2O3 particles occupy the pores; c) some of the particles are trapped in coatings through discharged channel. Compared with an ordinary anodizing coating without Al2O3, the composite anodic oxide coating with Al2O3 nano-particles exhibits excellent corrosion resistance and wear resistance.Combined sol chemical with electrochemical technology, another new composite anodic films were prepared on AZ31 magnesium alloy in NaOH-Na2SiO4 solution with varied Al2O3 sol addition. The role of sol during the anodic oxidation was analysed according to the results of the conductivity of the solution, the anodizing break voltage, the thickness, surface morphologies, phase composition, corrosion and wear resistance of the anodic films. Results show that the solution’s conductivity would become small. The anodizing break voltage increases with Al2O3 sol addition and thereby increasing the thickness of anodizing films. While adding sol into the solution has little effect on the phase composition of anodic oxidation film. The corrosion and wear resistance were enhanced after the addition of Al2O3 sol. The anodizing film has best corrosion and wears resistance when adding 10% vol Al2O3 sol in solution.Sealing processes of magnesium anodic oxide films were preliminary probed. The two new environmental friendly sealing processes, sol sealing three times and earth sealing, were optimizationed. After sealing, the corrosion resistance of the oxide film has been greatly improved. There is a strong development prospect of the two environmental friendly sealing processes.