Microstructures And Corrosion Behavior Of Micro-arc Oxidation Coating On Mg-1zn-xmn Alloys

Magnesium alloys show a good application prospect in the field of biological implant materials due to their low elastic modulus, close to human bone density, satisfactory biocompatibility and biodegradation. However, the corrosion rate of pure magnesium and its alloy is too fast in the complex physiological system environment with pH 7.4?7.6 and rich in Cl-. Hydrogen gas accumulation is easy to form and can lead to the inflammatory reaction of the implanted part. At the same time, fast corrosion can cause premature loss of mechanical integrity of magnesium alloy implants, which cannot meet the requirements of service time.Therefore, it is urgent to improve the corrosion resistance of magnesium alloy and investigate its corrosion mechanism. Alloying and surface treatment are two effective ways to improve the corrosion resistance of magnesium alloys.In this work,Zn and Mn were used as the alloying elements. The Mg-1Zn-xMn (x=0.8, 1.0,1.2, wt%) magnesium alloys were fabricated by casting, homogenization treatment, direct extrude, then the alloys was treated by micro-arc oxidation process. The microstructure and its corrosion properties of Mg-1Zn-xMn magnesium alloy with micro-arc oxidation in SBF solution were studied. The relationship between microstructure and corrosion properties of ceramic coating was discussed. The main conclusions are as following:(1) The micro-arc oxidation ceramic coatings is in good bonding with the substrate, and the main constitute phases are MgO and Mg2SiO4. The ceramic coatings are composed of porous loose layer and dense layer. With the increase of micro-arc oxidation time, the defects of ceramic coating increases, large discharge micro pores appeares. Micro cracks appear on the ceramic coatings due to non-uniform cooling of the melt.(2) With the same micro-arc oxidation process parameters, the porosity and corrosion current density of micro-arc oxidation ceramic coating on Mg-1Zn-0.8Mn alloy are the smallest,so the corrosion resistance is the best.(3) The surface ceramic coating can significantly improve the hardness of the alloy. With the increase of the micro-arc oxidation time, the film becomes thicker and the hardness becomes larger. The hardness of Mg-1Zn-1Mn alloy is 178HV when the thickness of ceramic coating is 5?m, and the hardness of alloy is 274HV when the thickness of ceramic coating is 10?m. Micro-arc oxidation ceramic coating can increase the hardness of Mg-1Zn-xMn alloy by 3-4 times.(4) Micro-arc oxidation can significantly improve the corrosion resistance of Mg-1Zn-xMn alloy. Electrochemical test show that the corrosion rate of Mg-1Zn-1Mn alloy is 0.252 mm/a,the corrosion rate of the alloy by micro-arc oxidation for 5 min reduces to 0.0265 mm/a. The corrosion resistance increases nearly 10 times.(5) After immersed in SBF solution for 15 days at 37 ?, the alloys processed by micro-arc oxidation for 5 min have the minimal mass loss and show the best corrosion resistance. The corrosion behavior of the alloys is mainly pitting corrosion and filiform corrosion. The corrosion products on the surface of the alloys are mainly composed of HA(Ca10(OH)2(PO4)6), MgCl2and Mg(OH)2.