CORROSION STUDY OF SOLID AND POROUS TITANIUM FOR SURGICAL IMPLANT APPLICATIONS

Abstract

Porous titanium is used in biomedical implant applications as it encourages osseointegration and has a low elastic modulus. However, pores are inherent defects that will decrease the corrosion resistance. This thesis focuses on the comprehensive in-vitro study of solid and porous titanium using open-circuit potential (OCP) measurements, potentiodynamic measurements, electrochemical impedance spectroscopy (EIS) and capacitance study. The modelling of the experimental results is also performed for analysis and prediction purposes. OCP measurements revealed that the presence of pores seems to encourage mass transport of species in and out of the pore channels. Next, the inconsistent trend observed in the OCP graphs for porous titanium suggests that these specimens have oxide films with different initial thickness, which are often hard to predict. The current densities obtained from potentiodynamic measurements for porous titanium compacted at various pressures, showed that the more porous compacts have higher corrosion rates due to their larger true surface area. The observed trend of the (Ecorr,back-Ecorr,forw) values, suggests that the degree of corrosion protection offered by the anodic film decreases as the specimens become more compact. The EIS results gave further insight into the corrosion processes of porous titanium. It was found that for porous titanium, charge transfer reactions dominate at high and mid frequencies while diffusion-controlled process is dominant at low frequencies. From the capacitance study, the observation in the reciprocal capacitance versus potential plot for porous titanium suggests that there are two kinds of oxide layers formed on the surfaces- an inner compact layer and an outer "needle-like" layer. The effects of the addition of hydrogen peroxide (H2O2) were also studied using all the above electrochemical methods. It was found that generally, the corrosion rates at corrosion potential were higher; however, many of the trends described in the above paragraph were not present due to the instability of the concentration of H2O2 in the early stages of testing. From the modelling of the experimental EIS data, the Randles equivalent circuit was found to be a suitable model for porous titanium at all the compaction pressures. Relevant quantitative parameters were obtained from this model. These values were studied and were found to be in close agreement with theoretical values as well as values obtained from potentiodynamic measurements. An empirical model for polarisation curves was also obtained, thus making available a prediction tool for corrosion behaviour. It is hoped that with these comprehensive experimental results and the derived corrosion models, clinicians will be able to make a more deliberate decision in the selection of bone implants.