Corrosion and fretting corrosion behaviour of surface modified titanium and ti6al4v in simulated body fluid

Abstract

In spite of having excellent mechanical and corrosion resistance properties, the poor tribological property limits the acceptance of titanium and its alloys for their use as implant materials. The naturally formed passive oxide layer of titanium (~ 4-5 nm) comprises of either amorphous or poorly crystallized non-stoichiometric TiO2, provides protection against the harmful effects of aggressive environments and i responsible for the higher corrosion resistance. However, it has been reported that under in vivo conditions the stability of the passive layer could be altered. Implant retrieval analysis has shown accumulation of metal ions on tissues adjacent to the implant, which might have originated from the implant, even by rubbing against the soft tissues. The formation of passive layer and subsequent fracture leads to the generation of wear debris that could cause adverse tissue reactions. Fretting and sliding wear conditions can also lead to fracture of the passive layer and under extreme conditions, they can cause loosening and eventual failure of the implant. The above limitations preclude the use of titanium and its alloys for articulating surfaces. The present study aims to modify the surface of the titanium and its alloy without altering their inherent properties. Thermal oxidation (TO) and anodizing techniques were used to develop a hard and thick oxide layer on titanium and its alloy. Thermal oxidation of commercially pure titanium (CP-Ti) and Ti6Al4V alloy was performed at 500, 650 and 800 °C for 8, 16, 24 and 48 h in air. Anodizing of CP-Ti and Ti6Al4V alloy was carried out at a constant applied potential of 20 V for 60 min using 1 M H3PO4 containing 0.3 wt. % HF as an electrolyte solution. The corrosion and fretting corrosion behaviour of untreated, thermally oxidized and anodized CP-Ti and Ti6Al4V alloy samples in Ringer’s solution was evaluated using open circuit potential-time measurement, potentiodynamic polarization and electrochemical impedance spectroscopy. Based on the corrosion and fretting corrosion behaviour of untreated, thermally oxidized and anodized CP-Ti and Ti6Al4V alloy, the most suitable material for implant application is identified. The surface morphology of TO samples reveals the presence of oxide scales (mainly rutile and Ti(O)) throughout the surface. CP-Ti samples do not exhibit any spallation up to 800 °C irrespective of the oxidation time whereas spallation of oxide layer is observed for Ti6Al4V alloy samples when they are subjected to thermal oxidation at 800 °C for 16 and 24 h. TO CP-Ti shows better corrosion protection than TO Ti6Al4V alloy. The morphology of the anodized CP-Ti sample clearly reveals that its surface is homogeneously covered with oxides. In contrast, the anodized Ti6Al4V alloy exhibits a homogeneous distribution of pores throughout its surface. Anodized CP-Ti shows better corrosion protection than anodized Ti6Al4V alloy. The fretting motion causes fracture of the naturally formed passive film on them which results in a cathodic shift in free corrosion potential (FCP). With the onset of fretting, untreated and anodized CP-Ti and Ti6Al4V alloy samples exhibit a sudden cathodic shift in FCP. However, thermally oxidized samples did not show sudden drop in potential. Based on the fretting corrosion resistance, the untreated, anodized and thermally oxidized CP-Ti and Ti6Al4V alloy samples can be ranked as follows: TO Ti6Al4V > TO CP-Ti > Anodized CP-Ti > Anodized Ti6Al4V > Untreated Ti6Al4V ≈ Untreated CP-Ti.