An experimental study addressing the mechanisms of passive dissolution of titanium in biologic environments was performed. The effect of solution constituents on dissolution, surface chemistry and oxidation was monitored by electro-thermal atomic absorption spectrometry (EAAS), Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS).
Two experiments were performed: the integral experiment determined the passive dissolution kinetics by sequential determination of the titanium released into a solution of accumulated dissolution products; the differential measured the dissolution rate as a function of exposure to fresh test solution.
Titanium samples, both fibers and thin films, were immersed in the test solutions and maintained at 37°C, 10% O₂,5% CO₂ and 7.2 pH. Upon expiration of the immersion times, up to 5000 hours (208 days), the solutions were collected and analyzed for titanium, and the samples used for surface analysis.
Upon exposure to the ambient two types of hydroxyl (OH) groups were distinguished on the TiO₂ surface. The chemistry of the surface changed as a function of immersion: an increase in OH groups and P (non elemental) were detected at the surface. The P group was presumed to be H2PO4- and HPO42-.
The passive dissolution kinetics of titanium in physiologic environments depended on the constituents of the solution. Additions of both EDTA (ethylenediaminetetraacetic acid) and serum proteins to an electrolyte solution increased the dissolution rate. From the surface analytical data it followed that the enhancement was solution mediated.
The dissolution kinetics obeyed a two phase logarithmic model. The oxidation kinetics did not follow a unique theoretical relationship; nevertheless, the data could be explained on the basis of a limiting oxide thickness (ℓL).
It was proposed that the dissolution kinetics were dependent on surface chemistry, electric field and diffusion mechanisms. These mechanisms can explain the observed dependence of passive dissolution kinetics on solution ligands and oxide physical properties.
The hypothesis of surface chemistry/electric field/diffusion driven passive dissolution and oxidation may have general validity to metallic biomaterials that rely on passivity for their corrosion resistance.