Titanium oxo-hydroxo complexes as precursors of protective oxide films on the surface of titanium alloys in high-temperature water

Method of electro-spray mass spectrometry has been applied to identify titanium chemical species presented in aqueous solution after exposure of PT-3V titanium alloy sample to high-temperature water under hydrothermal conditions (240 degrees C) for different experiment durations. Nickel, iron(II) and chromium(III) sulfates as well as hydrazine hydrate were used to simulate the composition of nuclear reactor first circuit coolant. Upon exposure to high-temperature water the surface of alloy corrodes and titanium passes into solution in the form of ions Ti3+ and Ti4+, which are hydrated and hydrolyzed. [Ti(OH)2(N2H4)H2O]+, [Ti(OH)2(H2O)6]2+ and [Ti(OH)2(N2H5)(H2O)5]+ are the principal mononuclear hydroxocomplexes formed. Upon increase of experiment duration hydrolytic polymerization takes place resulting in formation of [Ti2O3(OH)(H2O)4]+ and [Ti2O(OH)3(H2O)7Na]2+ as principal dinuclear species and [Ti4O2(OH)7(H2O)6]+ tetramer as final step. The evidences of heteronuclear titanium-iron hydroxocomplexes [FeTiO(OH)3Na]+, [FeTiO(OH)3Na]2+ presence also have been obtained. The comparison of the results obtained with the data of atomic absorption spectroscopy allows to conclude that polymerization of titanium hydroxocomplexes observed is not associated with an increase in its concentration during the experiment but rather is a consequence of an irreversible hydrolysis process at high temperatures. The results obtained allow us to suggest the next sequence of chemical transformations as a result of corrosion of a titanium alloy: dissolution of titanium with the formation of mononuclear aqua-hydroxocomplexes in solution -> polymerization of the resulting hydroxoforms up to the formation of tetramers -> precipitation of hydroxides -> solid-phase metamorphization into anatase. As for the formation of mixed oxides, the first stage is probably the co-precipitation of hydroxides, and then a solid-phase reaction.