Preparation And Corrosion Resistance Of Black Film Of Micro-arc Oxidation Of AZ91D Magnesium Alloy Under Zirconium Salt System

Magnesium and magnesium alloy are treated by micro-arc oxidation process to obtain a black oxide ceramic layer.Films prepared with different process parameters and different coloring salts can improve the apparent decoration and corrosion resistance of magnesium alloys,thereby improving the apparent spraying of magnesium and magnesium alloys in sports equipment and 3C electronic products,etc.In this paper,AZ91D magnesium alloy was used as experimental material,and it was colored by micro arc oxidation in zirconium salt solution with Cu salt,Fe salt and Cu Fe compound salt.By adjusting the amount of colorant,the prepared film is comprehensively evaluated in terms of blackness,roughness,hardness and thickness,and the optimal colorant and the amount of colorant are selected.Adjust the electrical parameters of micro-arc oxidation in the optimal electrolyte formulation,and conduct experiments under different termination voltages and oxidation times.The apparent color,microstructure morphology,phase composition and other aspects of the film layer are analyzed by 3nh colorimeter,scanning electron microscope,X-ray diffractometer and other test methods.The corrosion resistance of the black oxide ceramic film was tested by electrochemical corrosion experiment and neutral salt spray experiment.Through the analysis and research of the experimental data,the following conclusions are obtained that the film prepared by adding Fe salt has the best overall performance,the film has the lowest blackness,and has no pits or peeling on the surface.The microscopic morphology of the film prepared by adding different coloring salts to the electrolyte is similar,which is a typical micro-arc oxidation volcanic pore morphology,with small cracks distributed at the joints between the holes.The main constituent substances in the uncolored salt film layer are Mg O,Zr O₂ and Mg Si O₃;after adding the colored salt,an oxide formed by the reaction of the colored salt metal ions is added to the film layer,and the oxide is the coloring substance in the film layer.The colored salt oxides are partly deposited on the surface of the film and partly in the discharge channels.The termination voltage and oxidation time have great influence on the thickness,roughness,hardness and surface state of the film.As the termination voltage and oxidation time increase,the colored salt oxides are partially deposited on the surface of the film and partially deposited in the discharge channels.As a result,the colored oxide content in the film layer gradually increases,and the color of the film layer becomes darker.The electrochemical test of AZ91D magnesium alloy black film was carried out,and the polarization curve data were analyzed.It was found that the corrosion resistance of the film was improved after microarc oxidation treatment.The addition of coloring salt increased the corrosion potential of the film,reduced the corrosion current density,and further improved the corrosion resistance of the film.The corrosion potential of the film prepared by adding Fe salt was the highest,and the self corrosion current density was the highest The minimum polarization resistance and the maximum polarization resistance indicate that the corrosion resistance of the film is the best.Through the neutral salt spray experiment,it is concluded that the black film prepared by adding Fe coloring salt takes the longest time to appear five corrosion points,and its corrosion resistance is the best.