Corrosion and Surface Modification of Ti-6Al-4V Implant Alloy Under Various Simulated Biological Environments

Abstract

Ti and its alloys, for instance Ti-6Al-4V, are generally considered as highly corrosion resistant, due to the very stable passivity. These materials are widely used in biomedical applications for permanent implants, and they have an excellent reputation for biocompatibility. Some problems related to the chemical stability of Ti-based implants exist in the practical application, mainly due to the conjoint action of chemical and mechanical attack, i.e. fretting corrosion. This can lead to release and accumulation of Ti in tissue adjacent to titanium implants. However, it is also known that depassivation of Ti can take place under deaerated acidic conditions. Even though the typical chemistry in a "bulk" biological environment is pH-neutral (buffered to pH 7.4), under certain conditions, for instance in crevices, much more aggressive environments can be present. Local acidification is possible not only due to metal cation hydrolysis reactions, but in addition by possible inflammation reactions (such as following a surgery). Inflammation conditions also lead to presence of H2O2 in the surrounding. This is noteworthy in view of corrosion of Ti alloys, as H2O2 is known to complex Ti cations and hence acceleration of dissolution in possible. This presentation will discuss dissolution of the Ti-6Al-4V implant alloy under different simulated biological environments, including simulation of inflammation conditions (pH decrease, presence of H2O2). Electrochemical measurements and metal release indicate drastically increased dissolution rates under simulated inflammation conditions. In addition, selective phase etching is observed. Various surface modification treatments for improved performance are being explored, such as anodization or TiO2 nanoparticle coating. In addition to corrosion studies, bioactivity of the surfaces in view of Ca-phosphate precipitation is being investigated.