Study On Degradation Mechanism Of Magnesium Alloy With Fluoride Conversion Coating And Properties Of Degradation Product

In recent years,the research of the biodegradable magnesium alloy obtained widespread attention.Fast degradation of magnesium alloy can quickly cause a dramatic rise of p H in the surrounding environment and the material may fail by the corrosion,which seriously restricts their application as the biodegradable implant materials.In order to improve the corrosion resistance of magnesium alloy,surface protective coating is widely studied.At present,the existing literatures reported that the fluorine conversion coating significantly improved the corrosion resistance and compatibility of magnesium alloys.However,few literatures reported about the degradation mechanism of magnesium alloy with fluoride conversion coating.Therefore,this thesis emplyed SEM&EDS/XRD/IC to analyze the compositions of the magnesium alloy samples after immersion,and investigate the degradation behavior and mechanism of magnesium alloy with fluoride conversion coating.In addition,during the degradation magnesium alloy is gradually conversed to the degradation product,thus the composition,property and the subsequent metabolic mechanism of degradation product is taken as a research focus in this thesis.Based on it,the content,solubility and biocompatibility of degradation product were studied by immersion test,hemolysis test and cellular toxicity test.In this study,the results revealed that after the magnesium alloy with fluoride conversion coating was immersed in the Hank's solution,magnesium fluoride in the coating could be dissolved into F ions and Mg ions.With the dissolution of F ions and Mg ions,H₂O and Cl^-penetrated through coating to the magnesium substrate,then p H could rise with the degradation of magnesium alloy substrate.Later Mg ions could react with OH^-to form Mg?OH?₂.Owning to the low solubility of Mg F₂,formation rate of Mg?OH?₂ was slow,giving rise to an even corrosion resistant coating.As H₂O and Cl^-penetrating to the alloy substrate,the alloy began to degrade,forming Mg?OH?₂ and H₂.Above all,the degradation of magnesium alloy with fluoride conversion coating was gentle and went inward layer by layer.This from another angle explains that fluoride conversion coating can effectively reduce the rate of pitting corrosion.Solubility property,blood compatibility and cell compatibility of degradation product were studied by immersion test and cytotoxicity test.The immersion test result indicated that the degradation product of magnesium alloy could be dissolved into a certain amount of negative ions and positive ions(including Mg²⁺,Ca²⁺,OH^-and PO₃^4-).To analyze the influence of blood with the dissolution ions,the hemolysis test was carried out.Hemolysis test result revealed that hemolysis rate was less than5%for the degradation product,so it could not give rise to the hemolysis and met the biological safety standard.In vitro cytotoxicity test result revealed that cytotoxicity of the degradation product of magnesium alloy became lower and lower with the immersion time extended.As a biodegradable material,the evaluation standard for magnesium alloy is different from the traditional inert material.In this study,a minimal 6 times and a maximal 10 times dilution of the degradation product extract of magnesium alloy were recommended,and there was no cytotoxicity,meeting the biological safety standard.After the safety evaluation of the magnesium alloy,the study shifted the focus to the subsequent degradation product,which provided scientific basis for the safety evaluation for magnesium alloy medical device and laid a theoretical foundation for in vivo sorption and metabolic mechanism research of the degradation product of magnesium alloy.