Titanium oxide film fracture and repassivation: The effect of potential, PH and aeration

The oxide fracture and repassivation behavior of titanium alloy (Ti-6Al-4V) was analyzed using a novel scratch test methodology. In this test, a diamond stylus attached to a sensitive load cell and the actuators of a scanning electrochemical microscope were used to impart high speed scratches of 0 to 90 μm in 0.5 to 1 ms while the sample was immersed in physiological saline solutions and potentiostatically held. The resultant current transient was collected using computer acquisition techniques and analyzed to determine the current excursion and time constants for repassivation. The effects of applied load, sample potential, pH and aeration on the current transient response were investigated. The results show that aeration had little effect on the repassivation process. Potential and pH both influenced the results; higher peak currents were associated with lower pH and higher potentials. The time constants for repassivation also varied with pH and potential. A theoretical model of the repassivation process and the mechanical characteristics of the scratch test are presented and used to explain the observed behavior during fracture and repassivation.