Duplex Surface Modification Combined With Micro-arc Oxidation And Magnetron Sputtering Nickel On Magnesium Alloy And Anti-corrosion Properties Of Duplex-modified Films

Magnesium and its alloys are regarded as structural materials with low density, high strength, nontoxic and other excellent properties, famed as "green engineering materials in twenty-first Century". However, the chemical properties of magnesium and its alloys are very active, and they are very easy to be corroded, so their applications are greatly restricted. Therefore, it is very important to improve the corrosion resistance of magnesium and its alloys. In this paper, the in situ growth composite coating on AZ31 magnesium alloy were fabricated by using the technology of micro arc oxidation and magnetron sputtering, which possesses fair resistance to corrosion. And the influence of various process conditions on the corrosion resistance of the coating was discussed. In situ preparation of composite coating on magnesium alloy by micro arc oxidation and magnetron sputtering has not been reported. This provides a new path for improving the corrosion resistance of magnesium alloys.Research shows that the corrosion resistance of micro arc oxidation coating achieves optimal condition when the composite electrolyte :15 g/L sodium silicate, 5 g/L sodium phosphate, 2 g/L potassium hydroxide, 4 g/L sodium fluoride, current density 6 A/dm2, frequency 1000 Hz, 10% duty cycle, the reaction time 300 s. The thickness of optimal micro arc oxidation film is 23 μm, composed of MgO and MgSiO3 mainly, cross section morphology shows coating is mainly divided into the grounds of the transition layer,dense layer and the loose layer composition; Adhesion test showed a good combination of film and the substrate. Electrochemical impedance spectroscopy(EIS) results showed that the optimal film in the NaCl 3.5wt.% solution has strong corrosion resistance in the early and middle immersion. The pitting corrosion occurred gradually in the late stage of immersion, and the pitting corrosion of the film gradually extended throughout the whole section, and finally the corrosion layer was completely corroded with the increasing of immersion time.On the basis of this, the composite coatings were prepared by magnetron sputtering. It was found that the corrosion resistance of the coatings is optimal when the sputtering power 75 W, the sputtering power 0.8 Pa, and the sputtering time 75 min. The study showed that the sputtering nickel plating was amorphous nickel film and accompanied with a little nickel oxide formation. Whether the surface of the nickel layer is smooth or not has a great relationship with the micro arc oxidation layer. Binding force test shows that the whole film and substrate binding performance is good. The corrosion voltage of the film is greatly improved after the sputtering of the nickel coating, which makes up the deficiency of the micro arc oxidation.