Studies Of Microstraucture And Characteristics Of Microarc Oxidation Coating On Magnesium Alloys

Magnesium and its alloys have a lot of excellent physical and mechanical properties, such as low density, high ratio strength, high vibration strength, bending toughness and easy processing, as well as good cast and wielding performance. However, magnesium is easily corroded due to its active chemical property, especially in wet environment and in the case of existing chlorine ions. Therefore, it is necessary to improve corrosion resistance property of magnesium and its alloys. Except industrial applications, magnesium and its alloys are also considered to be a promising biodegradable material because their characteristics, such as density, strength and elasticity quantum, are close to human bones. Moreover, magnesium planting materials can form non-toxic oxides once they are corroded in human environment, so they are ideal and low density biodegradable hard issue repair materials.Micro-arc oxidation is an advanced and recently developed technique that used to preparing ceramic coatings on colored metals. The process of this technique is simple and electrolyte components are adjustable, which are beneficial to form high quality coatings. In addition, electrolytes don't contain any harmful metal ions and chemical components, which make sure the safety of technology.In this paper, ZK60 and ZK61 magnesium alloys were processed under both constant current mode and constant voltage mode. Phase compositions, surface morphology and corrosion resistance characteristics of micro-arc oxidation coatings were detected by XRD, SEM and electrochemistry workstation. Besides, three kinds of phosphates including Na(PO₃)₆, Ca(H₂PO₄)₂ and Na₃PO₄ were added into electrolytes respectively, and their influence on resulting coatings characteristics, such as thickness, phase compositions, surface morphology, hardness and corrosion resistance, were compared.Based on voltage-time response under constant current mode, micro-arc oxidation process can be divided to be three phase, which are normal anode oxidation, micro-arc oxidation and local spark discharge. Coatings composed of MgO, MgSiO₃ and Mg₃(PO₄)₂, and new phase Mg₂SiO₄ appeared as the increase of current density. Prolonging processing time and increasing current density lead to the increase of thickness and hardness of coatings, and surface morphologies of coatings are also changed from flatness to roughness, the number of micro-cracks is increased. Among them, the corrosion resistance of micro-arc oxidation coating which is processed for 20 min is the best. For three kinds of electrolyte, the coatings thickness are all increased clearly as increasing of voltage, in contrast, the influence of frequency and duty ratio is less than the influence of voltage. Under the same electrical parameters, coatings obtained in electrolyte containing Na(PO₃)₆ are thickest, followed by Na₃PO₄, and thinnest in electrolyte containing Ca(H₂PO₄)₂. Comparing two kinds of substrates, coatings formed on ZK60 alloy are thicker than that on ZK61 alloy. XRD results showed that all coating comprised of MgO, MgSiO₃, Mg₂SiO₄ and Mg₃(PO₄)₂. Voltage and concentration of electrolyte don't change phase composition of coatings, but influence on the content of phases. SEM results show that all coatings are typically porous. For three kinds of electrolyte, surface morphologies of coatings became rougher as the increase of voltage. Micro-pores on the coating obtained in 300 V have tiny size and distribute evenly, while micro-pores on the coating obtained in higher voltages have larger size and distribute unevenly. The number of micro-cracks also increases as increasing voltages and the coating obtained under 500V was roughest. Similarly, micro-hardness of coatings increases continually as the increase of voltage, while the influence of frequency and duty ratio is not clear. Under 400V, coatings obtained in electrolyte containing Na(PO₃)₆ has the highest micro-hardness, which is 147.85HV_0.5, and the micro-hardness of coatings formed in the other electrolytes have no big difference, which are 127.71 HV_0.5 and 128.21 HV_0.5.Based on potentiodynamic polarization, polarization resistance (Rp) change frequently instead of increased continually. In addition, in the electrolyte containing Na(PO₃)₆ and Na₃PO₄, increasing duty ratio and frequency lead to the increase of corrosion resistance of coatings, while it is opposite in electrolyte containing Ca(H₂PO₄)₂. Increasing the concentration of Na₃PO₄, the corrosion resistance of resulting coating is also improved. Comparing two kinds of substrate, polarization resistance of coatings formed on ZK61 are basically higher than that of coatings formed on ZK60. Comparing three kinds of phosphates adding to electrolytes, under the same processing parameters, micro-arc oxidation coatings obtained in electrolyte containing Na₃PO₄ has the highest polarization resistance, which means the best corrosion resistance.