Influence of fluoride concentration and pH on corrosion behavior of Ti-6Al-4V and Ti-23Ta alloys in artificial saliva

Titanium alloys exhibit an excellent corrosion resistance in most aqueous media due to the formation of a stable oxide film and some of these alloys (particularly Ti-6Al-4V) were chosen for surgical and odontological implants for this resistance and their biocompatibility. Treatments with fluorides (F-) are known as the main method to prevent plaque formation and dental caries. Toothpastes, mouthwashes and prophylactic gels can contain from 200 to 20 000 ppm F- and can present neutral to acidic character, which can affect the corrosion behavior of titanium alloys devices present in the oral cavity. In this work, the behavior of Ti-6Al-4V and the new experimental Ti-23Ta has been evaluated in artificial saliva of pH 2, 5 and 7 and different F- concentrations (0, 1000, 5000 and 10 000 ppm), through open-circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy. A defined correlation between pH and F- concentration settled the active or passive character of the materials. For both alloys, an active behavior was observed for pH 2 and 1000 to 10 000 ppm F- and for pH 5 and 5000 and 10 000 ppm F-. The passive behavior was observed for the other investigated conditions. The F- concentration increase and pH decrease reduced the corrosion resistance of the alloys and decreased the stability of their passive film. The corrosion behavior of both alloys was very similar, but the Ti-23Ta alloy generally presented slightly higher corrosion resistance.