Preparation And Corrosion Resistance Of Hydrophobic Bioactive Coating On Magnesium Alloy

Biodegradable magnesium alloys are the most promising orthopedic implants due to its non-toxicity,biodegradability,elastic modulus similar with human bone and could avoid stress-shielding effect.However,magnesium alloys have poor corrosion resistance in physiological body environment,the implants will lose its mechanical integrity before the bone tissue totally heal and results in surgical failure,thus limit the application of clinic.Therefore,how to enhance the corrosion resistance of magnesium alloys have become a hot research topic in recent 10 years.In this paper,two kinds of hydrophobic coatings were synthesized on AZ31 magnesium alloy.The one was obtained by micro-arc oxidation(MAO)and further modification of stearic acid,the other was obtained by stearic acid modification of bioactive composite coating which was synthesized by electro-depositing hydroxyapatite on MAO coating.The influence of surface modification of stearic acid on both MAO coating and bioactive composite coating was studied by SEM,EDS,XRD and FTIR.The corrosion resistance and degradation behavior of different samples were characterized by electrochemical test and vitro simulated body fluid(SBF)immersion.SEM morphology and XRD analysis indicated that MAO coating was consist of Mg3(PO4)2 and MgO,with a porous surface morphology and a thickness of about 6 ?m.Stearic acid modification had no obvious influence on MAO coating's morphology,while changed its wettability and a hydrophobic surface was obtained.The SBF contact angle of MAO coating was 39.58°,while that of samples after modification of stearic acid achieved more than 100°,and the contact angle increased with the treat time of modification.The electrochemical test shows that the corrosion potential of bare magnesium alloy in SBF was-1.70 V,and that of MAO sample increased to-1.58 V,while that of MAO sample after modified for 60 min increased to-1.49 V.Hydrophobic MAO coating inhibited charge transfer process,enhanced the impedance of both inner barrier layer and outer layer,improved the protection of MAO coating for magnesium substrate.Vitro SBF immersion test indicated that magnesium alloy coated with hydrophobic MAO coating not only had a lower mass loss than that of MAO treated magnesium alloy,but also induced precipitation of octacalcium phosphate.The calcium phosphate deposited on MAO coating had a dual structure,namely,inner compact layer which combine closely to MAO coating and belt-like calcium phosphate arrays.The as-prepared calcium phosphate was consisted of Ca3H2P4O14,Ca9.74(PO4)6(OH)2.08,Ca4H(PO4)3·2.5H2O and CaHPO4,and further transformed into hydroxyapatite after alkali treatment.The hydrophobicity of bioactive composite coating increased with the modification time,achieving a SBF contact angle of 135.92° after 60 min of modification.The corrosion potential and corrosion current density of magnesium alloy with bioactive composite coating were-1.55 V and 6.34×10-5 A·cm-2,respectively,while that of sample after 60 min of modification were-1.39 V and 4.30×10-7 A·cm-2.The results of vitro SBF immersion test demonstrated that hydrophobic bioactive composite coating could provide better protection than bioactive composite coating.