Effects of fluoride concentration and elastic tensile strain on the corrosion resistance of commercially pure titanium

The corrosion resistance of commercially pure (CP) titanium in 1% NaCl+0 1% NaF solution (pH=6) under different elastic tensile strains was investigated by using electrochemical impedance spectroscopy (EIS) measurement technique. The polarization resistance (Rp), which is inversely proportional to the corrosion rate, of CP titanium in the test solution was obtained from the EIS data. Different elastic tensile strains, namely 0%, 1%, 2%, and 4%, were applied on the test specimens by using a tensile test machine during the corrosion tests. Corrosion morphology was characterized by using a scanning electron microscope. Surface chemical analyses were performed by using energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The results showed that the NaF concentration and the elastic tensile strain had a statistically significant influence on the Rp (P<0.001). The Rp decreased on increasing the NaF concentration and the elastic tensile strain. When the NaF concentration was lower than 0.01%, the Rp value (>3.4×105 Ω cm2) was mainly ascribed to the formation of a protective titanium dioxide (TiO2) on the metal surface, regardless of the elastic tensile strain applied. However, when the NaF concentration was higher than 0.1%, the protectiveness of TiO2 was destroyed by fluoride ions, leading to severe corrosion of CP titanium.