Metal-Fluoride Complex Stability Constant in Acid Solution as an Indicator to Identify the Corrosion Behavior of Metal Cations on Titanium

The Ti was immersed in 0.058 mol/L NaF containing 1.68 mol/L H₂SO₄ solution with different metal cations (Ca²⁺, Cu²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Zn²⁺, Al³⁺, and Fe³⁺) for 7 d to identify the role of metal cations on corrosion behavior of titanium. To elucidate the role of metal cations on the corrosion behavior of Ti in acid solution, the standard electrode potential, E⁰, hardness of metal cations, X, and metal-fluoride complex stability constant, K_n, were chosen as the potential indicator. It is found that the mass loss fraction obtained from the immersion tests as a function of the metal-fluoride complex stability constant, K_n, has the highest correlation coefficient (0.92). The metal cations (Al³⁺ and Fe³⁺) with higher stability constant of the metal-fluoride complex in acid solution, (K_n > 3.27), show corrosion inhibition effect on Ti in acid solution. The K_n could be a suitable indicator to understand the role of metal cations on the Ti corrosion mechanism.