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

• O.N. Pestova¹, N.A. Glukhoedov¹, S.N. Orlov^1,2,3, A.S. Mereshchenko¹, N.A. Bogachev¹ and M.Yu. Skripkin¹

¹ Institute of Chemistry, Saint-Petersburg State University, 7/9 Universitetskaya Embankment, 199034 St. Petersburg, Russian Federation
² Institute of Nuclear Industry, Peter the Great St. Petersburg Polytechnic University (SPbPU), 29, Polytechnicheskaya Street, 195251 St. Petersburg, Russian Federation
³ “Gazprom-Neft” LTD, 75-79 Moika embankment, Saint-Petersburg, Russian Federation

Abstract: 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°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(H₂О)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.

Keywords: titanium alloy, titanium oxo-hydroxocomplexes, protective oxide film, primary circuit, pressurized water reactor

Int. J. Corros. Scale Inhib., 2023, 12, no. 4, 2066-2079
doi: 10.17675/2305-6894-2023-12-4-33

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