Fabrication And Craft Of Micro-arc Oxidation Colouring Coating In Phosphate On AZ91D Magnesium Alloy

Fabrication and craft of micro-arc oxidation colouring coating on AZ91D magnesium alloys are studied in this article to improve the corrosion resistance and decorative purposes of magnesium alloy. Effect of department of phosphate and hexametaphosphate electrolyte composition on AZ91D magnesium alloy oxidation coloring are investigated by single factor experiments. Electrolyte components of micro-arc oxidation colouring and a better match are eventually determined combining with micro-arc oxidation coating thickness, surface morphology, and elemental composition of corrosion resistance test results.The results show that electrolyte concentration has a greater influence on coating thickness, corrosion resistance and color of the coating. Increasing of the electrolyte concentration can improve the coating thickness, also it is conducive to the deepening corlour of the coating. The concentration is increased to a certain degree, while the coating thickness will reach a limit. Corrosion resistance and coating surface quality will be reduced when continue to increase concentration, and the coating thickness also decreases to some extent. Coloring coating can be obtained by adding salts, and the showing color of the coating is colors of the composition of metal oxide or compound in the coating, or color appears when the interaction. Different color of the coating can be obtained by adding different concentrations of sodium metavanadate into two electrolyte systems. The spark state and coating thickness are changed by adding sodium citrate auxiliary additives in the phosphate electrolyte, and coating performance improves. Coating structure is effectively adjusted by adding potassium fluoride in Phosphate electrolyte, and corrosion resistance of the coating is further increased. Uniform color of the coating is obtained of thickness 41μm and corrosion resistence time for the 73 s in the best of phosphate formulations. And uniform color of the coating is also obtained of thickness 51μm and corrosion resistence time for the 211s in the best of hexametaphosphate formulations. And the coating obtained in two formulations has excellent decorative properties.EPMA results reveale that coatings contained the elements such as Mg, Al, V, O and P, and distribution of various elements in the whole coating. XRD shows that the color coating mainly MgO, Mg2Al04, a-Al2O3 and Mg2V2O7 other phase, in which hair color substances containing V, phase, but also changes the color of the main components. P phase has not been detected in the XRD.