| Magnesium and its alloys have attracted much attention as potential biodegradable implant materials due to their excellent biocompatibility, mechanical compatibility and degradability. However, the corrosion resistance of magnesium and its alloys is poor. In the high chloride containing physiological environment, they have high corrosion rate, limiting their applications. Therefore, it is important to develop magnesium alloys with controllable degradation rate.For improving the corrosion resistance of magnesium alloys, chemical transformation method was used to fabricate phytic acid conversion coating. On the basis of previous studies of the mesoporous 45S5 bioglass coatings in our research group, mesoporous 45S5 bioglass/phytic acid converion composite coating was synthesized on the surface of AZ31 magnesium alloy via sol-gel dip-coating method. In this work, the influence of process conditions on the phytic acid coating was studied, the effects of heat treatment in the temperature range of 150 °C- 400 °C on the chemical composition of the coatings and the crack healing were investigated, and the degradation property of the composite coating coated Mg alloy was inspected by the electrochemical and immersion tests.The better corrosion-resistant phytic acid conversion coatings can be obtained after dipping the magnesium alloy into the 0.7 wt% phytic acid solution at 40 °C for 40 min. The conversion coating(~4.8 ?m in thickness) formed in the pH=4.5 solution presented bulk feature and had plenty of distributed meshy cracks(7.5~8.4 ?m in width), and the surface structure of the crack blocks was loose and porous. The tensile adhesion strength with the Mg substrate is 20.4±3.1 MPa. The heat treatment temperature has a significantly effect on the surface morphology and chemical composition of the conversion coatings, which will subsequently affect the corrosion behavior of these coated magnesium alloys. When heat-treated at 300 °C, the coatings change from amorphous magnesium phytate into crystalline Mg2P2O7. Cracks on the surface of the coatings gradually heal up with the increase of the heat treatment temperature and the corrosion resistance properties of the conversion coated magnesium alloys are also improved gradually. The coating(~1.0 ?m in thickness) surface after being heat-treated at 400 °C is integrity, The tensile adhesion strength with the Mg substrate is 37.9±1.5 MPa. The corrosion potential(Ecorr) was-1.37 V, the corrosion current(icorr) value decreased to 1.46×10-5 A/cm2 and the value of impedance was 1249 ?·cm2.The conversion coating(~1.1 ?m in thickness) formed in the pH=7.5 solution was integrity and was used as the middle layer. The mesoporous 45S5 bioglass/phytic acid converion composite coating was about 2 ?m in thickness, the surface structure of which was integrity and compact, having a good adhesion ability with the substrate. Compared with the bare magnesium alloy, the Ecorr of the coated sample increased from-1.70 V to-0.21 V, the icorr decreased by more than two orders of magnitude and the value of impedance was twelve times larger, suggesting a superior protection to the magnesium alloy substrates. During the immersion test in the SBF, the composite coating can inducted the generation of hydroxyapatite and there was no peeling-off of the coating, improving the biological as well as the anti-corrosion properties of the material. |
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