Study On The Fabrication And Properties Of Micro-arc Oxidation Colouring Coating On AZ91D Magnesium Alloy

Magnesium alloys is low density, high strength/weight ratio, large coefficient of energy attenuation, excellent electromagnetic shield and capability of emitting heat and so on. Therefore it becomes one of the most important metal in the way of its development and application. However, corrosion resistance of magnesium alloys is very poor, which mainly effects its wide application in the field of automotive and electro-mmunication. This thesis is to use MAO coloring to obtain the good corrosive resistance and colouring ceramic coating on magnesium alloys.Firstly, how the processing parameter including current density,treating time,colourant's concentration,thepropotion of potassium permanganate and complexing agent effect on magnesium alloy cecamic coating by the micro-arc oxidation colouring in sodium silicate solution which is added potassium permanganate and complexing agent has been investigated. The condition being able to make the outstanding colouring ceramic coatings by the experiment have been found out.Draw support from method such as SEM and XRD, morphology, microstructure and phase composition of colouring ceramic coatings were observed. Thickness and corrosion-resistance of colouring ceramic coatings in different experiment condition were tested and analysed. The results show that, The element discovering a coloration by TEM testing is Mn element mainly; but being able to affirm the coloration thing further by that XRD checks is Mn₂O₃; Colourant's concentration influence coating's colour most in sodium silicate solution. Both too high and too low colourant's concentration is disadvantageous for the quality of colouring ceramic coatings, so selecting and using the medium colourant's concentration solution will be suitable. Keep identical current density and colourant's concentration, the samples thickness is enhancing by prolonging treatment time; But under identical concentration of potassium permanganate and treatment time condition, the more current density of colouring ceramic coatings, the thickness increase trend is more quick.Secondly, in the electrolytic colouring process, the coloring samples with micro-arc oxidation were placed in nickel sulfate solution. With changing reaction time and colourant's concentration, the changes of phase composition, morphology and thickness for colouring ceramic coatings were observed. Because the samples were assigned as cathode, the effect of loading undershoot has been on the colouring ceramic coatings, and the ablation phenomenon of the colouring ceramic coatings can be observed; the treated-time was not too long in electrolyte, otherwise, it can result in the colouring ceramic coatings of surface exfoliated, during the electrolytic colouring process, prolonging the electrolytic process or improving nickel sulfate content, that both can improve the content of nickel in the coating. In the electrolytic colouring process, prolonging the electrolytic process or improving nickel sulfate content, that both can improve the content of nickel in the coating. The coloring samples' corrosion-resistance have been improved with electrolytic colouring.Lastly, in the electrolytic colouring process, the coloring samples with micro-arc oxidation were placed in nickel sulfate solution. With changing reaction time and colourant's concentration, the changes of phase composition, morphology and thickness for film layer were observed. The treated-time was too long in electrolyte can result in the oxide film of surface exfoliated. The coloring samples' corrosion-resistance have been improved with electrolytic colouring. On the whole, The coloring samples' corrosion-resistance is great by the micro-arc oxidation colouring first and then electrolytic colouring treated.