Study On Corrosion Resistance Of Self-sacrificial Ca-P Coating On Biomedical Magnesium Alloy

Magnesium and its alloys have attracted more and more researchers' attentions because of their good biocompatibility,biodegradability and elastic modulus closed to human bone.The-good properties of magnesium alloys make it an alternative to traditional medical implants such as stainless steels,titanium alloys.However,based on the chemical nature of magnesium alloy itself,it is highly susceptible to corrosion in a Cl-riched environment(i.e.human physiological fluids).And the corrosion products including H2 and Mg(OH)2,generated during corrosion,lead to local inflammation and the phenomenon of alkalosis,which limits the further biological application of magnesium and its alloys.Calcium phosphates(Ca-P)coating is a bio-friendly coating material that meets the requirements for improved corrosion resistance and biocompatibility of magnesium alloys.We successfully prepared Ca-P coating with difference inducers on AZ31 magnesium alloy using hydrothermal method.The optimal preparation conditions of Ca-P coating in the presence of inducers were explored,including hydrothermal temperature and hydrothermal time and solution pH.And the effect of inducers,including lithium hydroxide(LiOH·H2O),deoxyribonucleic acid(DNA)and glutamic acid.(L-Glutamic)was to explore on the film formation of Ca-P coating?In addition,the film formation mechanism of calcium phosphate coating have been discussed.Field emission scanning electron microscopy(FE-SEM),X-ray diffractometry(XRD),energy dispersive spectroscopy(EDS),Fourier infrared spectroscopy(FTIR),and X-ray photoelectron spectroscopy(XPS)were used to characterize the properties of the coating,including surface morphology,phase composition,elemental composition,functional groups.The corrosion resistance of the samples was measured through the electrochemical polarization,electrochemical impedence spectroscopy(EIS),immersion and hydrogen evolution experiments.The results showed that the Ca-P coatings on AZ31 magnesium alloy improved the corrosion resistance of magnesium alloy substrate.However,the prepared Ca-P coating exist some differences in appearance,phase and corrosion resistance due to the different inducing agents and preparation conditions.Following conclusions are made:1.The Ca-P coating with self-healing function was successfully prepared on the surface of AZ31 magnesium alloy by hydrothermal method in presence of LiOH·H2O.The optimal preparation parameters are:pH 3.5.hydrothermal temperature 100?.hydrothermal time 4 h.The main components of the coating are CaHPO4·2H2O and Ca10(OH)2(PO4)6·5H2O.Electrochemical experiments show that the corrosion current density of the coating is one order of magnitude lower than that of the substrate,indicating an improved corrosion resistance.The scratch immersion test and the Kelvin scanning probe experiment designated that the coating po-ssessed self-healing ability.The self-healing mechanism of th e coating was proposed.The reason for the re-dissolution and recrystallization of insoluble salts was ascribed to the occurrence of the attack on Mg substrate.2.Ca-P coating was fabricated on the surface of magnesium alloy AZ31 by hydrothermal method in the presence of DNA at 150? for 4 h.According to the dynamic potential polarization curve of the coating,the degree of corrosion resistance improvement is not very high as expected,but it still maintained a relatively complete and dense crystal structure after serving for 250 h in simulated body fluids,indicating that the prepared coating may have a long-term service ability.The formation mechanism of DNA-induced Ca-P coating was put forward.The phosphate groups of DNA could adsorb calcium ions to form a dense and uniform CDHA,DCPA and TCP coating.And the introduction of DNA refined the coating grains and strengthened bonding between the coating and its substrate.3.Ca-P coating was successfully induced by adding biocompatible L-glutamic acid via hydrothermal method at hydrothermal temperature of 90? after 4 h.The coating is mainly characterized by Ca10(OH)2(PO4)6·5H2O and CaHPO4.The corrosion resistance of the L-glutamic acid-induced Ca-P coating was enhanced form the viewpoint of polarization curves and EIS.The presence of L-glutamic acid lowered the corrosion resistance and free corrosion potential of the Ca-P coating relative to the absence of L-glutamic acid.The introduction of L-glutamic acid made the Ca-P coating changed from a cathodic coating into an anodic coating relative to Mg substrate.In addition,the film formation mechanism was proposed for L-glutamic acid-induced Ca-P coating.