Corrosion Resistance of Titanium Alloys with Oxynitride Coatings in Concentrated Inorganic Acids
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CORROSION RESISTANCE OF TITANIUM ALLOYS WITH OXYNITRIDE COATINGS IN CONCENTRATED INORGANIC ACIDS I. M. Pohrelyuk,1,2 V. M. Fedirko,1 O. V. Tkachuk,1 and R. V. Proskurnyak1
UDC 621.78:669.295
We study the corrosion behavior of VT1-0, VT6s, and VT14 titanium alloys after thermodiffusion oxynitriding in 20 and 30% aqueous solutions of hydrochloric acid and in 40, 80, and 85% solutions of sulfuric acid. It is shown that the oxynitride coatings better protect titanium alloys than oxide and nitride coatings. This is explained by the specific features of the structure and chemical bonds between the atoms in the crystal lattice of titanium oxynitride. It is also shown that the formed oxynitride layer guarantees high corrosion resistances of titanium alloys in hydrochloric acid, whereas the layered structure containing the oxide and oxynitride components better protects titanium alloys in sulfuric acid. Keywords: titanium alloys, oxynitride coatings, corrosion resistance, hydrochloric and sulfuric acids.
Titanium and its alloys are not stable in corrosive media [1–3]. The thin natural oxide films inevitably formed on the titanium surface dissociate in inorganic acids with the formation of weakly soluble salts. As a
result of corrosion, titanium passes into the solution in the form of Ti 3+ ions, and the higher the aggressiveness of the medium, the higher the rate of this process.
To improve the serviceability of products made of titanium alloys under the action of corrosive media, they are subjected to special kinds of treatment, in particular, to nitriding and oxidation [4–7]. This leads to the formations of coatings based on titanium nitrides and oxides (binary compounds). However, in some media and, in particular, in concentrated solutions of inorganic acids, the efficiency of binary compounds is insufficient. To enhance the functionality of titanium alloys under the indicated conditions, it is customary to form protective coatings based on ternary compounds and, in particular, on titanium oxynitrides whose physicochemical properties, including corrosion resistance, are higher as compared with the binary compounds [8–12]. As shown in [8], they are practically insoluble in concentrated and dilute nitric acid and difficultly soluble in aqua regia and in mixtures of nitric and sulfuric acids.
Moreover, the solubility of oxynitride in inorganic acids depends on its composition. Thus, in particular, in hydrofluoric acid, it decreases as the nitrogen content of oxynitride increases. The corrosion resistance of titanium oxynitride in nitric and sulfuric acids is maximal for the compositions corresponding to the minimum of the electron-density distribution, which guarantees the elevated energy of bonding between the atoms in the cubic lattice. The aim of the present work is to estimate the corrosion resistance of titanium alloys with oxynitride coatings in concentrated solutions of inorganic acids.
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Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. Corresponding author; e-mail:
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