| Magnesium and its alloys have good application prospects in orthopedics,cardiovascular stents and bone tissue engineering as biomedical degradable materials because of their degradability and biosafety.However,its corrosion rate in physiological environment is too fast to meet the clinical requirements.Therefore,improving the corrosion resistance of magnesium and its alloys is of great significance.The chemical conversion method is simple and effective,the composition of the conversion film is controllable,and can have corrosion resistance and biocompatibility at the same time.It is an effective method to improve the corrosion resistance of magnesium alloy.Phytic acid is a natural organic substance harmless to human body.Fluorine can promote bone formation in human body.Fluorine in the film can promote bone formation around the implant material.However,the single conversion film formed on the surface of magnesium alloy has some defects,such as thin film,high porosity,microcracks and so on,which leads to limited protection effect.Therefore,based on the fluorine conversion treatment,a new green magnesium fluoride/phytic acid composite film was prepared on the surface of AZ80magnesium alloy by phytic acid conversion treatment.The morphology,composition,film-forming process and corrosion resistance in Hank's simulated body fluid were studied,and its corrosion process and mechanism were discussed.The conclusions are as follows:The results of microstructure analysis show that the structure of the composite film prepared on the substrate surface is more uniform and compact,and the continuity between the inner and outer films is good.Magnesium fluoride is used as the intermediate film to seal the microcrack in the outer layer of phytic acid.Compared with the single film,the thickness of the composite film increases by about 2.8-3.0?m.In the process of film formation,the formation of magnesium fluoride intermediate layer makes the matrix degrade uniformly in phytic acid conversion solution.At the same time,the dissolution of fluorine conversion layer promotes the chelation of phytic acid,improves the compactness of the composite film,and the overall quality of the film is good.The electrochemical test results show that the corrosion potential(-1.267V)of the composite film sample is the largest and the corrosion current density(7.121×10-7A/cm~2)minimum.Compared with AZ80 bare sample,single magnesium fluoride sample and single phytic acid film sample,the corrosion potential increased by 333mv,129mv and 222mv respectively,and the corrosion current density decreased by 1-2 orders of magnitude respectively.The film resistance and impedance modulus of the composite film increase,and the polarization resistance(8290?·cm~2)of the composite film sample measured after immersion in Hank's simulated body fluid for 120h is still much greater than that of the AZ80bare sample(3382?·cm~2)before immersion.It can be seen that the composite film improves the corrosion resistance of a single film and significantly reduces the corrosion rate of the matrix in Hank's simulated body fluid.The experiment shows that during the whole process of immersion in Hank's simulated body fluid for 120h,the composite film layer effectively slows down the corrosion rate of the substrate at the initial stage of immersion.After soaking for 120h hours,the hydrogen evolution(3.48±0.11m L),weight loss rate(0.34±0.02mg·cm-2·d-1)and corrosion rate(0.72±0.02mm·y-1)of the composite film samples were the smallest,and the corrosion rate(CRw)decreased by about 51%,15%and 22%respectively.The corrosion morphology immersion in Hank's simulated body fluid for 120h shows that the corrosion area of the composite film sample surface is small and the corrosion degree is light.The cross-section corrosion morphology shows that the corrosion product layer formed on the surface of the composite film sample has a uniform and thin structure,which reduces the corrosion rate of the matrix.Therefore,the local corrosion of the matrix has been significantly improved. |
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