| Titanium alloy has the properties of low density,good mechanical properties,corrosion resistance and high biocompatibility.They are one of the ideal materials for biomedical implant materials.Therefore,it is very important to explore the corrosion behavior of selective laser melted（SLMed）titanium alloys in flowing simulated body fluids.Therefore,this thesis investigated the in vitro corrosion behavior of SLMed bulk and porous Ti-6Al-4V alloy in static（ST）and dynamic（DT）Hank’s solutions and the characteristics of the passive films formed on SLMed Ti-6Al-4V alloy.First,the electrochemical corrosion behavior of SLMed Ti-6Al-4V alloy in static and dynamic Hank’s solutions was investigated.Electrochemical tests,scanning electron microscopy,X-ray photoelectron spectroscopy and other tests were carried out.It can be found that ST and DT samples show similar potentiodynamic polarization behavior.However,the corrosion current density of ST(6.355±0.167μA·cm-2)is slightly lower than that of DT(6.683±0.526μA·cm-2).Electrochemical impedance spectroscopy（EIS）results show that the passivation film formed on the DT after immersion for 1800 s has a lower charge transfer resistance（0.689±0.11 MΩ·cm2）than the charge transfer resistance of the passivation film formed on the ST（0.774±0.15 MΩ·cm2）.After a 30-day immersion test,it was found that the passivation films formed on the immersed samples were mainly composed of Ti O2,Ti2O3 and Ti O.The oxygen concentration of the passivation film formed on DT is higher than that of ST formed at the same depth.At the same time,after a 30-day immersion test,DT has higher ion release than ST(such as Ti4+and Al3+).These conclusions all indicate that the formation and dissolution rate of the passivation film formed on DT is higher.After the potentiodynamic polarization test and the 30-day immersion test,both ST and DT samples had salt deposits.The salt deposits was mainly caused by the depositions of calcium ions and phosphate ions in Hank’s solutions,and salt deposits were mainly composed of apatite and hydroxyapatite.Dynamic Hank’s solution accelerated the salt deposition,and the amount of salt deposition increased with the increasing of immersion time.At the same time,manufacturing defects on the surface of the sample become the preferred locations for salt depositionSecond,through potentiostatic polarization,electrochemical impedance spectroscopy and the Mott-Schottky tests,the semiconductive properties of the passivation films formed on the sample surface were examined,and it was found that the passivation films showed typical n-type semiconductive characteristics.The results showed that the donor density,oxygen vacancy diffusion coefficient and passivation film thickness of the DT sample were all higher than those of ST under different potentials.The passivation film has a faster growth rate in dynamic Hank’s solution and the dynamic Hank’s solution provides more calcium and phosphate ions to the surface of the passivation film,resulting in more salt deposits.According to the point defect model,the dynamic Hank’s solution led to a higher dissolution rate of metal ions,and therefore led to a continuous transformation of titanium to titanium ions and oxygen vacancies,resulting in a higher density of oxygen vacancies and formed passivation film has a higher oxygen diffusion coefficient of the DT sample The potentiostatic polarization test indicated that the current density value decreased rapidly with the increasing polarization time,specifying that a passivation film was formed.Hence,the passivation film formed on the surface of the ST sample has stronger protection and thereby better corrosion resistance.Finally,the SLMed porous Ti-6Al-4V was immersed in static and dynamic Hank’s solutions at 37°C for 60 days,120 days,and 180 days,respectively.The immersed samples were characterized by scanning electron microscopy,X-ray diffraction spectroscopy,inductively coupled plasma mass spectrometer（ICP-MS）and other tests.It was found that the weight of the immersed sample changed significantly.After 180-day immersion,the weight gain rate of the DT sample reached about 10%,approximately 10 times the weight gain rate of ST samples.At the same time,the samples were analyzed by SEM and energy spectrum,and it was found that the corrosion products were mostly gathered between adjacent metal particles.The corrosion products were consistent with the immersion results of the bulk samples.Corrosion products were mainly caused by calcium ions and phosphate ions in Hank’s solution.The salt deposits on ST and DT samples were mainly composed of apatite and hydroxyapatite.The dynamic Hank’s solution accelerated salt deposition,and the amount of salt deposition increases with increasing the immersion time.A compression test was performed afterward on the immersed samples,and it was found that the mechanical properties of the porous samples decreased with the extension of the immersion time. |
