Preparation Of Color Ceramic Coating By Micro-arc Oxidation On Aluminum Alloy And Magnesium Alloy

. The technology of micro-arc oxidation (MAO) is a new surface treatment technique, which is developed from anodic oxidation. It is one of the research hotspots of aluminum and magnesium surface treatment at present. In this paper, the uniform color and compact ceramic coating was obtained through Micro-arc oxidation on LY12 aluminum alloy and AZ91D magnesium alloy.The effects of different electrolyte systems and different stain on the color of ceramic coating were investigated,and the basis composition of the micro-arc oxidation electrolyte were established. The orthogonal design method was applicated to optimize the MAO electrolytes systems, then the better technical composition and operation conditions of the color coatings were gained. The electrolytes system of the yellow micro-oxidation coating on aluminum alloy was gaid as follows: Na2WO4 2g/L, Na3PO4 6g/L, Na2B4O7 4g/L, KMnO4 0.5g/L, pH=10. The electrolytes system of the black micro-oxidation coating on aluminum alloy contains NaSiO3 10g/L, Na4P2O7 15g/L, NaOH 4g/L, NH4VO3 8g/L, EDTA2Na 5g/L.The electrolytes system of the green micro-oxidation coating on magnesium alloy is consist of NaAlO2 10g/L, KMnO4 0.3g/L.The electrolytes system of the yellow micro-oxidation coating on magnesium alloy includes NaAlO2 10g/L, NH4VO3 2g/L. And the oxide film by the color was uniform and dense. It's discovered that the ceramic coatings color were deepening with the time prolonging and the voltage ariseing. When the micro-arc oxidation time is 50 minute at the voltage 400-420V, the deepest color of the green micro-oxidation coating was obtained. The oxide films'surface and section morphology were tested by SEM. The results showed that micro-oxidation coating was rough and porous. It was composed of the close and the loose layer. There were comparatively dense micro-pores with the diameter of micron size in it. XRD (X-ray Diffraction) analyses revealed that the compounds of the color coating. The mini-crystal Mn5O8,Mn3O4,MnO2,VO,V3O5 was detected. They were countribute to the ceramic coatings'color. Corrosion experiment and electrochemical treatment showed that the corrosion resistance of the film was better than uncolored oxide film. Corrosion weight loss was only 0.05~0.06 g·(m2·h)-1, corrosion potential was higher than the matrix. And the corrosion current density had been reduced about three orders of magnitude in the dynamic potential polarization curve.AZ91D magnesium alloy was treated by micro-arc oxidation (MAO) in the optimized composite solution in which Al2O3 powders were added, then a good micro-arc oxidation electrolyte systems were obtained. It was composed of Na2SiO3 + Na4P2O7 12g/L, Al2O3 3g/L, NaF 2g/L, Na2WO4 1g/L, HOCH2CHOHCH2OH 10ml/L. Morphological characteristic and phase constituent of ceramic coatings were analyzed by the scanning electron microscope (SEM) and the Energy Dispersive X-Ray Fluoresence Spectrmeter (EDX) and the x-ray diffraction (XRD).The results showed that micro-pores in ceramic coatings decreased after adding Al2O3 powders and the loose layer became more compact, new phase of Al2O3 appeared. Electrochemical corrosion experiments in 3.5%NaCl solution showed that the corrosion potential increased about 0.349V, and corrosion current density had been reduced about two orders of magnitude in the dynamic potential polarization curve. The corrosion resistance of ceramic coatings was also improved greatly after adding Al2O3 powders.