| Magnesium (Mg) alloys, a novel class of degradable metallic biomaterials, have attracted growing interest due to their excellent mechanical and physiological properties. However, magnesium alloy will lose the mechanical properties quickly when implanted in human body due to the poor corrosion resistance, which does not match with the bone healing period. Therefore, it is necessary for magnesium alloy to improve its corrosion resistance to be used as implant materials. In order to improve the corrosion resistance of magnesium alloy, hydroxyapatite (HA) as well as fluorine apatite (FHA) coatings were prepared on AZ91D magnesium alloy by hydrothermal method in this study. The surface morphology, phase composition and corrosion resistance of coatings prepared with two kinds of chelating agents have been studied. The morphology, phase composition and the corrosion performance of coatings were systematically characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Infrared spectroscopy, the electrochemical test and in vitro test. The formation mechanism of FHA coating was also analysed. The main conclusions of the paper were summarized as following:(1) The HA coating has been successfully fabricated on magnesium alloy by hydrothermal method through using polyaspartic acid (PASP) as a chelating agent. The as-prepared coating is mainly composed of regular plush-like structure approximately 2μm in diameter. The thickness of the coating is about 10 μm, and increased with the extension of time. It was found that the corrosion potential of HA-magnesium alloy improved by 0.08 V, and the corrosion current density decreased more than an order of magnitude compared with that of blank sample. The transfer resistance (Rt) of HA-magnesium alloy is 2-3 times compared with that of blank sample. After being immersed in SBF for 3 days, the surface of the HA-magnesium alloy was filled by calcium apatite, Ca/P=1.41 close to the stoichiometric ratio of Ca/P in HA. Compared with Mg element, the content of Ca and P are increased. The calcium phosphate mineralization was occurred in the HA coating. However, the blank sample was broke seriously when it immersed in SBF solution for 3days. Therefore, the corrosion resistance and the biological compatibility of magnesium alloy can be improved because pf the existence of HA coating, which is beneficial for magnesium alloy to be widely used as bone implant materials.(2) The HA coating has been successfully fabricated on magnesium alloy by hydrothermal method through using two sodium ethylene diamine tetraacetic acid (Na2-EDTA (EDTA)) as a chelating agent. When the concentration of EDTA is 10g/L and the pH value is 5.5, the coating with a relatively better quality coating can be obtained. The morphology of the coatings was petal-like, divergenceing from the center to the surrounding, and the magnesium alloy substrate is completely covered. The corrosion current density of HA-magnesium alloy (3.0329*E-6 A/cm2) in SBF solution was reduced by two orders of magnitude than that of blank sample, the corrosion potential of HA-magnesium alloy improve by 0.27 V than that of blank sample. The ratio of transfer resistance (Rt) between HA-magnesium alloy and magnesium alloy blank sample was reached 1:3. The corrosion resistance of AZ91D magnesium alloy was improved when prepated a HA coating. In vitro corrosion test, corrosion behavior of HA-magnesium alloy was started from the edge, keeping lesser degree compared with blank sample. The existence of HA coating on magnesium alloy was profitable to prevent corrosive ion to breake the matrix, which can reduce the degradation rate of the magnesium alloy substrate.(3) In this paper, the NaF solution was used as the source of F-. FHA coating on the surface of magnesium alloy was prepared by hydrothermal method. The morphology of coating is composed of plush-like, strip-like, plate-like structures. The coating was about 2-3 μm. The corrosion potential of FHA-magnesium alloy in SBF solution is higher than that of blank sample. The corrosion current density was 3.91*E-5 A/cm2, which was reduced by one order of magnitude compared with that of blank sample. Two impedance responses were happened on FHA-magnesium alloy surface, and the transfer resistance (Rt) was increased for the existence of FHA coating on magnesium alloy. In vitro test, the corrosion rate of blank sample was fast than coated sample in SBF solution. After 6 days, the pH value of solution gradually increased. The FHA coating can improve the corrosion resistance of magnesium alloy in SBF. |
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