Influence Of Rare-Earth Oxide Particles On The Corrosion Resistance Of Ceramic Coatings Produced On AZ91D Magnesium Alloy By Micro-Arc Oxidation

Magnesium and its alloy have been used in range of application including automobile, aerospace,3C electronic products and medical instruments due to its low density, high specific strength, good electromagnetic shielding properties and biocompatibility etc. However, surface treatments are usually necessary for magnesium alloy because of its high chemical activity and poor corrosion resistance. Micro-arc oxidation(MAO) is one of the most commonly used surface treatments employed by magnesium alloy.Micro-arc oxidation provides magnesium alloy a in-situ growth protective layer, while the corrosion resistance enhancement is limited because of the porous structure of MAO coatings. In this paper, influence of rare-earth oxide particles addition on the MAO coatings were studied by means of SEM, XRD, electrochemical method, immersing test; Particles doping mechanism was also discussed. The experimental results show that:1. MAO coatings were produced on AZ91D magnesium alloy in Nd2O3 and Y2O3 particles containing electrolyte, the results showed that:the doping of Nd2O3 particles increased the porosity of MAO coatings, thus reduced their corrosion resistance properties. However, the addition of Y2O3 particles can decrease the porosity of coatings, so as to improve their corrosion resistance.2. The concentration of rare-earth particles in the electrolyte plays a important role on the corrosion resistance performance of MAO coatings, doping effect was unapparent if the concentration is too low, while the consistency of coatings could be destroyed by excessive particles concentration. In this paper, the best particles doping concentration is lOg/L, which provided the minimum porosity and optimal corrosion resistance for MAO coatings.3. Rare-earth oxide particles were negative charged in the electrolyte, thus migrated to the anode due to electric field force during the process of micro-arc oxidation. When bipolar pulse power employed, the particles were absorbed and desorbed from the sample surface alternatively, particles could not be able to accumulated, particles doping mainly controlled by molten, the concentration of particles has little effect on the corrosion resistance of MAO coatings under bipolar pulse power mode.4. As the oxidation time prolonged, on one hand, corrosion resistance of MAO coatings were improved because of the thickening of coatings; on the other hand, the increase of flaws could weaken corrosion resistance of coatings. The doping process of rare-earth oxide particles happened in the arc oxidation stage, rare-earth oxide content in the MAO coatings increased with the extension of oxidation time,8 minutes was the optimized timing in this paper.