THERMODYNAMICS OF TI-H2O-F(-FE) SYSTEMS AT 298K

The thermodynamics of Ti-F-H2O systems are summarized in the form of activity-pH and potential-pH diagrams, calculated from recently published critically assessed standard Gibbs energies of formation of the various species involved; however, there is considerable uncertainty in those data. The main differences compared with previously published potential-pH diagrams for such systems are the exclusion of Ti2+ ions, since there appears to be no evidence for their existence in aqueous solutions, and the inclusion of an area of stability for TiH2, which excludes that for elemental titanium. In electrolytes with fluoride activities > 0.01, TiF2-6 ions are predicted to predominate in moderately acidic solutions and are responsible for the corrosion of titanium metal in such electrolytes which destabilise otherwise passivating oxide films such as TiO2 (c, hydrated). Diagrams are also presented including the non-stoichiometric oxides Ti3O5 and Ti4O7, which have large areas of stability at intermediate potentials, though they are not stable if rutile is also considered. Potential-pH diagrams for Fe-Ti-H2O systems were also calculated, as ilmenite (FeTiO3) is the primary mineral source of titanium, the iron being acid leached to produce TiO2. © 1990.