Oxidation kinetics of titanium thin films in model physiologic environments

The effects of a model physiologic solution on the surface chemistry and oxidation kinetics of titanium were determined. Auger electron (AES) and X-ray photoelectron (XPS) spectroscopies were used to identify changes in oxide stoichiometry, adsorbed surface species, and oxide thickness as a function of exposure to a balanced electrolyte with 8.0 mM ethylenediaminetetraacetic acid (EDTA). Prior to immersion, the oxide on the processed films was identified as TiO2 with two types of hydroxyl (OH) groups adsorbed. The chemistry of the mean surface region changed as a function of immersion: an increase in OH groups and P (nonelemental) was detected. The oxidation kinetics did not follow a unique theoretical relationship. However, the data can be explained on the basis of a limiting oxide thickness (lL). It is proposed that electric-field-assisted transport of metal ions into the oxide lattice is the rate-determining step for oxide growth. The model predicts lL ∼ 7.5 nm with subsequent oxide growth of less than 0.02 nm/day, which compares well with the oxide growth of samples exposed to human serum and retrieved titanium implants.