INFLUENCE OF THE DEFORMATION AND HEAT TREATMENT OF (a+b)-TITANIUM ALLOYS VT6 AND VT22 ON THEIR CORROSION RESISTANCE

Abstract

Relevance. Titanium and its alloys are the most durable and corrosion-resistant metal materials that makes them widely used in mechanical engineering, aircraft and machinery, chemical apparatus construction, medicine. The use of such construction materials with increased resistance in aggressive environments makes it possible to increase the efficiency of technologies for processing natural raw materials (high-pressure reactors, centrifuges, separators, high-speed pumps, heat exchangers, communications), the production of chlorine and alkalis (evaporators, electrolysis equipment), organic synthesis (equipment for the production of halogen derivatives), the production of nitric acid, ammonia and nitrogen fertilizers. The combination of mechanical and thermal treatment of alloys, including the use of severe plastic deformation, allows modifying their structure and obtaining materials with improved physical and mechanical characteristics. Purpose: to determine the effect of various modes of the deformation and heat treatment of titanium alloys VT6 and VT22 on their corrosion resistance in aqueous solutions of acids, alkalis and salts. Objects: samples of VT6 and VT22 alloys with different structures, aqueous solutions of acids, alkalis and salts. Methods: DC voltammetry, gravimetry under conditions of free corrosion, optical microscopy, scanning electron microscopy, X-ray diffractometry, atomic emission spectroscopy. Results. Under conditions of severe plastic deformation, samples of titanium alloys VT6 (triaxial pressing in the temperature range from 550 to 800 °C) and VT22 (radial shear rolling in the temperature range from 750 to 850 °C, additional cold rolling of hot-rolled samples, aging at 550 °C for 3...6 hours) with ultrafine-grained structure have been produced. It was found that VT6 alloy with ultrafine-grained structure has less corrosion resistance under conditions of free corrosion in solutions of sulfuric acid of a high concentration (5 M), as well as in 3,5 % NaCl solution at anodic polarization compared to the initial alloy with coarse-grained structure. The main types of corrosion damage to the surface of the ultrafine-grained samples were ulcers and pitting, whereas for samples with a coarse-grained structure, continuous corrosion was characteristic. The increase in the rate of corrosion and morphological features of the destruction of the surface are explained by structural-phase heterogeneity and segregation of alloying elements as a result of the deformation and thermal effects. The corrosion resistance of VT22 samples with an ultrafine-grained structure correlates with the content of the b-phase and with the features of the interphase distribution of alloying elements resulting from the polymorphic transformation of a-TiDb-Ti under conditions of the deformation and heat treatment. It is shown that VT22 samples after hot rolling are the most stable in acid solutions (0,1 M HCl, H2SO4) and the least stable in alkali solution (0,1 M NaOH) compared to samples treated under other regimes. In a 3,5 % NaCl solution, the ultrafine-grained sample VT22 showed the least resistance after additional cold rolling due to a large concentration of structural defects that contribute to increased corrosion cracking of the material. The interpretation of the experimental results was carried out using theoretical calculations of the equilibrium composition of corrosion products formed in the multicomponent systems under study.

Keywords

• titanium alloys
• severe plastic deformation
• heat treatment
• a- and b-phases
• corrosion resistance
• aqueous solutions of acids
• alkalis and salts