| Magnesium and its alloys have great application prospects in orthopedic implants,cardiovascular stents and other fields because of their good biocompatibility,mechanical compatibility,biodegradability and bone induction ability.However,the excessively rapid degradation rate of magnesium and its alloys in the physiological environment has become a key factor limiting their clinical application.Therefore,it is of great significance to improve the degradation rate of magnesium and its alloys.Because tannic acid and phytic acid are both extracted from natural plants,they are non-toxic to humans,and both can be successfully prepared conversion coating on magnesium alloys to control the degradation rate of magnesium alloys.However,most of the studies are from the industrial point of view,and few in the field of biomedical research.And the effect of two conversion coatings on the degradability of AZ80 magnesium alloy has not been reported.Therefore,in this paper,tannic acid conversion coating and phytic acid conversion coating were prepared on the surface of AZ80 magnesium alloy,and the effect of the two conversion coatings on the degradability of AZ80 magnesium alloy was studied.Specific research contents include:(1)Based on the results of previous studies,the effects of pH,conversion time and temperature on the formation of tannic acid conversion coating were studied by SEM,polarization curve and impedance.The results show that the optimal process is that the pH of the conversion solution is 4,the conversion time is 10 min,and the conversion temperature is 25 ℃.Under this condition,the surface of the tannic acid conversion coating prepared is relatively complete and dense.(2)The tannic acid conversion coating was prepared on AZ80 magnesium alloy by chemical conversion.After immersed in Hank’s simulated body fluid for 120 hours,the morphologies of the samples before and after cleaning the corrosion products were analyzed by SEM and macro photos,and they were found that local corrosion occurred on the alloy surface.The corrosion pits on the surface of the conversion coating were shallow,which mainly degraded within the crystal.Compared with AZ80 magnesium alloy substrate,the amount of hydrogen evolution with the conversion coating was reduced by 32.7% and the degradation rate was reduced by 34.3%.Electrochemical tests showed that the corrosion potential of the tannic acid conversion coating was increased by 219 mV compared to the alloy substrate,the corrosion current density was reduced by 95.7%,and the coating resistance and charge transfer resistance were also increased.So the tannic acid conversion coating has a good effect.(3)The coating forming process of phytic acid conversion coating is as follows: the concentration of phytic acid is 5g/L,the pH value is 5,the conversion time is 20 min,and the conversion temperature is 25 ℃.The coating was prepared on AZ80 magnesium alloy by chemical conversion.After immersed in Hank’s simulated body fluid for 120 hours,the morphologies of the samples before and after cleaning the corrosion products were analyzed by SEM and macro photos,and they were found that local corrosion occurred on the alloy surface.The surface with the phytic acid conversion coating was occurred obvious pitting corrosion.Compared with the magnesium alloy substrate,the corrosion pits were shallower and the degradation area was less.The amount of hydrogen evolution with the conversion coating was reduced by 20.4% and the degradation rate was reduced by 25.7% compared with the AZ80 magnesium alloy substrate.Electrochemical tests showed that the corrosion potential of the phytic acid conversion coating was increased by 141 mV compared to the alloy substrate,the corrosion current density was reduced by 85.7%,and the coating resistance and charge transfer resistance were also increased.Therefore,the phytic acid conversion coating effect is relatively good.Compared with phytic acid conversion coating,tannic acid conversion coating has a better protective effect on the alloy. |
PDF Full Text Request