Effect of anodization parameters and post-anodizing heat treatments on the corrosion resistance of Ti6Al4V and Cp-Ti

Corrosion of metallic biomaterials in the human body is a commonly overlooked phenomenon, even though it constitutes a serious risk for implant failure. Nevertheless, a fundamental understanding of the electrochemical reactions occurring at the interface biomaterial/human body fluid is necessary in order to successfully engineer surfaces on titanium biomedical alloys, the most widely used solution for bone repair and joint replacement. Cp-Ti (ASTM grade 2) and Ti6Al4V (ASTM grade 5) present good mechanical properties, corrosion resistance and biocompatibility without showing toxicity or immune rejection. However, although the inertness of titanium has been considered a favorable characteristic for years, its lack of bioactivity also presents some challenges, namely the inability to actively regulate osteoblast behavior or battle colonization by pathogenic microorganisms. In this regard, the anodization of titanium is a simple and effective surface treatment that has the potential to improve both osteointegration and antimicrobial activity. The aim of this work is to investigate the effect of the anodizing parameters and post anodizing heat treatment of medical grade Cp-Ti and Ti6Al4V, on their corrosion resistance through potentiodynamic polarization measurements in a simulated body fluid solution. Titania nanotubes, obtained by anodization in an ethylene glycol electrolyte containing 0.5wt.% NH4F and 2.5% V H2O at different applied potentials and different anodization times, have been characterized regarding their morphology and structure. The distinct geometry of the nanotubes and their increased surface area are believed to lead to an improvement in osseointegration. Their particular tubular shape makes them, in addition, excellent reservoir for antimicrobial agents. Post anodizing heat-treatments can develop a crystalline microstructure that closely resembles that of the native hydroxyapatite on the human bone. Nevertheless, the anodization and the post anodization heat treatment can influence the corrosion resistance of the Ti alloys in the human body. The aim of this work is to understand how the process parameters can influence the microstructure and thus the corrosion resistance, in order to avoid a decrease of the corrosion properties.