Corrosion and Tribocorrosion Performance of Thermally Oxidized Commercially Pure Titanium in a 0.9% NaCl Solution

In the present work, the corrosion and tribocorrosion characteristics of thermally oxidized commercially pure titanium in a 0.9% NaCl solution have been investigated. Thermal oxidation (TO) of CP-Ti was carried out at a temperature of 625 °C for 20 h. This treatment results in a multi-layered structure consisting of a 1 µm rutile (TiO2) film and a 9 µm α-titanium oxygen diffusion zone (ODZ) (α-Ti(O)). Electrochemical tests were carried out on surfaces created at various depths from the TO-Ti original surface. It was found that the rutile film generated through TO offers excellent corrosion resistance over that of untreated Ti. Testing also provided evidence that oxygen content in the upper part of the ODZ (depths <5 µm from the surface) helps accelerate passive film formation and thus reduce the corrosion of CP-Ti. Tribo-electrochemical testing of TO-Ti was carried out against an alumina counter face under a load of 2 N and various anodic and cathodic potentials. It is initially shown that the rutile oxide layer offers both low friction and much better resistance to material removal during tribocorrosion than untreated CP-Ti. During sliding wear at open circuit potential, four frictional zones can be identified in a typical friction curve, each having its own characteristics corresponding to the oxide layer, the gradual or partial removal of the oxide layer, the diffusion zone, and the substrate. An unusual anodic protection behavior of the oxide film has also been observed. When the TO-Ti is polarized anodically during sliding, the durability of the oxide layer is prolonged, resulting in low friction and much reduced material loss. When cathodically charged to −1500 mVSCE during sliding, both the TO-Ti and untreated CP-Ti experience a reduction in material loss. This is believed to be related to hydrogen evolution and titanium hydride formation.