Effect of Thermomechanical Treatment of α + β Titanium Alloys VT6 and VT22 on Their Corrosion Resistance

Abstract

Titanium and its alloys are among the most durable and corrosion-resistant metallic materials, which determines their wide application in mechanical engineering, aircraft and engine building, chemical apparatus, and medicine. The use of such structural materials with high resistance in aggressive media makes it possible to increase the efficiency of technologies for processing natural raw materials (high-pressure reactors, centrifuges, generators, high-speed pumps, heat exchangers, communications) and the production of chlorine and alkalis (evaporators, electrolysis equipment), organic synthesis (equipment for the production of halogen derivatives), nitric acid, ammonia, and nitrogen fertilizers. A combination of mechanical and thermal treatment of alloys, including the use of severe plastic deformation, can be used to modify their structure and to produce materials with improved physical and mechanical characteristics. The purpose of this work is to determine the effect of various thermomechanical treatment conditions for VT6 and VT22 titanium alloys on their corrosion resistance in aqueous solutions of acids, alkalis, and salts. VT6 and VT22 alloy samples with various structures and aqueous solutions of acids, alkalis, and salts are used for experiments. DC voltammetry, gravimetry under free corrosion conditions, optical microscopy, scanning electron microscopy, X-ray diffraction analysis, and atomic emission spectroscopy are used to analysis. Samples of titanium alloys VT6 (uniform pressing in the temperature range 800–550°C) and VT22 (radial-shear rolling in the temperature range 850–750°C, additional cold rolling of hot-rolled samples, aging at 550°C for 3–6 h) with an ultrafine-grained (UFG) structure are fabricated by severe plastic deformation. The VT6 alloy with an UFG structure is found to have lower corrosion resistance under free corrosion in high-concentration sulfuric acid solutions (5 M) and a 3.5% NaCl solution during anodic polarization in comparison with the initial alloy with a coarse-grained (CG) structure. The main types of corrosion damage to the UFG sample surface are corrosion pits and pitting, and samples with a CG structure are characterized by continuous corrosion. The increase in the rate of corrosion and the morphological features of surface destruction are explained by structural and phase heterogeneity and the segregation of alloying elements as a result of deformation and thermal actions. The corrosion resistance of UFG VT22 samples correlates with the β-phase content and the interphase distribution of alloying elements during the α-Ti \( \rightleftarrows \) β-Ti polymorphic transformation during thermomechanical treatment of the material. VT22 samples after hot rolling are shown to be most stable in acid solutions (0.1 M HCl, H₂SO₄) and least stable in an alkali solution (0.1 M NaOH) compared to samples treated under other conditions. In a 3.5% NaCl solution, an UFG VT22 sample exhibits the lowest stability after additional cold rolling due to a high concentration of structural defects, which enhance the corrosion cracking of the material. The experimental results have been interpreted upon calculating the equilibrium composition of the corrosion products having formed in the multicomponent systems under study.