Temperature Effects on Oxide Film Properties of Grade-7 Titanium

The passive corrosion properties of Grade-7 titanium (Ti-7) have been studied at elevated temperatures up to 150°C and as a function of time at a constant temperature of 150°C in 0.27 mol/L sodium chloride (NaCl) solution using a combination of open-circuit potential and electrochemical impedance spectroscopy (EIS) measurements. It was found that the influence of temperature on the passive behavior of Ti-7 depended on the manner in which the temperature was changed. If the temperature was raised slowly in 10°C steps, passivity was maintained up to 150°C, but metastable breakdown events commenced once the temperature was increased to ≥70°C. These events could be associated with the active dissolution of Fe-stabilized β-phase and/or TixFe particles in grain boundaries. Impedance analyses showed that the changes in film resistance and capacitance were observable but minor. If the temperature was raised very rapidly to 150°C, an observable film breakdown occurred, leading to a drop of >150 mV in corrosion potential (Ecorr) and the onset of metastable events. Over the subsequent heating period at 150°C, Ecorr increased substantially, and the events disappeared. The EIS analysis indicated the introduction of flaws in the oxide, leading to a substantial drop in film resistance. Over the course of several days, these flaws repassivated.