The bifunctional role played by thiocyanate anions on the active dissolution and the passive film of titanium in hydrochloric acid

Abstract

The electrochemical behaviour of titanium dissolution and passivation in HCl was examined with and without the SCN⁻ anions using potentiodynamic polarisation, electrochemical impedance spectroscopy, and current transients techniques. The nature of the Ti oxide film was characterised by utilising XPS and SEM tools. The corrosion current i _corr, the critical passivation current i _cc, and the passive current i _pass values increased significantly when the HCl concentration, scan rate, and temperature were raised. On the contrary, the addition of SCN⁻ anions decreases the i _corr value, indicating that the Ti dissolution was inhibited as a result of SCN⁻ adsorption on the Ti surface. The inhibition efficiency increased with increasing the concentration of SCN⁻. However, SCN⁻ anions lead to destabilised passivity in the passive zone. The formation of the Ti oxide film is a diffusion-controlled operation, as shown by the i–t curves. According to the XPS measurements, the passive oxide coating consists mainly of TiO₂ and less of Ti₂O₃.