Nº 3 (2025)

One of the ways to increase the efficiency of aluminum production is the use of low-temperature electrolytes and the production of demand aluminum master alloys. Earlier it was noted that it is effective to obtain aluminium master alloys via electrolysis of low-temperature electrolytes, allowing to arrange production without the need to obtain individual alloying elements and aluminium. It is relevant both from the practical and scientific point of view is the study of the possibility of obtaining aluminium master alloys with such electronegative elements as scandium, yttrium, strontium and calcium, etc. In this paper, the possibility of obtaining the Al-Y master alloy during electrolysis of a low-temperature electrolyte based on the KF-AlF₃ system with the addition of Y₂O₃ at a temperature of 800°C was studied. For this purpose, the kinetics of the cathode process on a molybdenum and glassy carbon electrode was studied in the melt under study with different oxide content using chronovoltammetry method. It is shown that the addition of Y₂O₃ practically does not affect the voltampere dependences and the mechanism of the process, increasing the cathodic currents of reduction of aluminium and yttrium ions, as well as the anodic currents of oxidation of cathodic reaction products. Based on electrochemical measurements, it was assumed that the co-reduction of aluminum with yttrium is possible at current densities above 0.4-0.5 A/cm². The process of obtaining Al-Y alloys in the KF-NaF-AlF₃ melt with the addition of 1 wt% Y₂O₃ under conditions of aluminothermic synthesis and galvanostatic electrolysis of the melt at a cathodic current density of 0.5 and 1.0 A/cm² was studied. As a result of aluminothermic reduction, an alloy with an yttrium content of no more than 0.07 wt.% was obtained, while during electrolysis, Al-Y master alloys with an yttrium content from 0.75 to 1.28 wt.% were obtained. The obtained values correspond to the extraction of yttrium from its oxide of 4.4; 47.5 and 81.3. It is suggested that the increase of synthesis duration and periodic loading of Y₂O₃ into the melt will allow to obtain Al-Y master alloys with increased yttrium content.

This work presents thermodynamic modeling and experimental study of the oxide-fluoride phase formed during the production of high-entropy refractory, lightweight alloys of the Al–Ti–Zr–V–Nb system by the method of combined aluminothermic reduction from metal oxides of titanium, zirconium, niobium and vanadium. The aim of the work was to determine the optimal conditions for obtaining such alloys and to find the characteristics of phase separation. Modeling showed that the formation of a lightweight and refractory alloy requires temperatures of at least 1600°C and an aluminum content in the range of 15 to 40 atomic percent. In this case, it is recommended to use a small excess of aluminum in the charge to ensure the transition of some aluminum to the metallic phase. Calculations of surface tension and density showed a significant difference between the metallic and oxide-fluoride phases, which contributes to the settling of the metallic melt to the bottom of the crucible and the formation of a clear separation boundary between the two phases. Interfacial tension in the range of 1000–1600 mJ/m² ensures minimal wetting of the slag by the metal and reduces the amount of non-metallic inclusions in the metal, which has a positive effect on the quality of the final product. It is noted that an increase in the number of components in the melt complicates the selection of empirical coefficients, which limits the accuracy of the calculation by the empirical method. Chemical analysis of the oxide-fluoride phase after the experiment confirmed the presence of zirconium and titanium oxides in it, which increase the surface tension and density compared to the calculated values. However, these parameters remain below the corresponding values of the metallic phase, which ensures effective phase separation and the formation of a solid metal ingot without excessive adhesion to the oxide-fluoride phase. The results obtained demonstrate the promise of the selected conditions for the production of high-quality high-entropy alloys and can be used for further evaluation calculations and optimization of technological processes.

One of the basic technological operations of the currently developed pyrochemical technology for reprocessing spent nitride nuclear fuel from fast neutron reactors (SNF RBN) is high-temperature treatment (HTT) in a gas environment. The aim of the work was to study the effect of oxygen-containing gas environments: a dry mixture of Ar-20 vol. % O₂ and a mixture of Ar-20 vol. % O₂, with 60% humidity for the degradation of 10CrNi45Al alloy, a candidate material for the manufacture of the HTT apparatus. Corrosion tests lasting up to 1000 hours were carried out at 500°C. It was established by the X-ray diffraction method that the main corrosion products formed on the surface of samples kept in a dry gas atmosphere are Al₂O₃, Fe₂O₃ and NiFe₂O₄. The presence of moisture in the gas environment contributes to the formation of NiO and NiСrO₄. In a dry gas mixture, an outer layer is observed on the surface of the sample, which is individual fragments of corrosion products: oxide compounds of iron, chromium, nickel. The surface of the material is covered with a continuous film with a thickness of 2 to 5 μm based on aluminum oxide. For samples tested in a wet gas mixture, a violation of the continuity of the internal protective layer was revealed. The outer loosened layer consists of iron oxides, under which a layer with a predominant content of oxygen-containing chromium compounds was revealed.

The paper focuses on the research into impact of hydrocarbonate treatment in the boiling 1 М solution of NaHСО₃ on the corrosion resistance of copper, nickel, and low-carbon chromium-nickel stainless steels during their corrosion testing in the NaOH melt at the range of temperatures from 400 to 500°С. The hydrocarbonate treatment of materials was conducted for two hours following holding them (treatment) in the NaOH melt preliminary dewatered and deaerated by Ar in intervals multiple of 96 h. The total duration of corrosion tests was 288 h. The microstructure and phase composition of surface layers that formed on materials under study during corrosion tests in the NaOH melt coupling with hydrocarbonate treatment were investigated using X-ray phase analysis and electron microscopy methods. It has been found that the hydrocarbonate treatment of materials neither affects the total corrosion rate for the materials under study in the NaOH melt in the given temperature range. It has been discovered that the hydrocarbonate treatment of nickel containing three oxide phases – NiO, Ni(OH)₂ и γ-NiOOH – after being held in NaOH melt impacts the proportions of the oxide phases. Nickel oxyhydroxide (NiOOH) is unstable in aqueous weakly alkaline solutions bound to and is spontaneously reduced to nickel dioxide (NiOOH → Ni(OH)₂) resulting in the formation of a passive film on the nickel surface. The film consists of two oxide phases (NiO и Ni(OH)₂) and has high protective properties. In the course of the hydrocarbonate treatment of copper, which after having been held in the NaOH melt contains a 2-layer film of oxides Cu/Сu₂О/СuО in the surface layer, oxide and carbonate layers with higher protective properties do not form. The hydrocarbonate treatment of stainless steel containing 17.5 and 18.5 % Ni and (6.0 – 6.5) % Mo demonstrating increased corrosion resistance in the NaOH melt at temperatures not exceeding 500°С (as nickel does) does not affect the corrosion resistance of the alloy elements. Doping the steel of the given composition with such alloying elements as copper, manganese, and silicon, which might cause local depassivation of steel under certain conditions, does not affect the protective properties of the passive steel film forming in the NаOH melt. Such film consists of oxides (hydroxides) of predominantly corrosion resistant chromium (Cr₂O₃) and nickel compounds (NiO, Ni(OH)₂) or their mixed oxides NiCr₂O₄ (NiO∙Cr₂O₃), as well as ferrous oxides Fe₃O₄ γ-Fe₂O₃. As the nickel and molybdenum content in the steel decreases (to 13.0 % and 2.0 % respectively), or temperature of the NаOH melt increases up to 600°С, more defective porous oxide layers form on the steel surface. They contain a larger portion of less stable ferrous (II, III) and nickel (II) oxides: FeO, NiO, Fe₂O₃ as well as a small amount of mixed oxides NiCr₂O₄ (NiO∙Cr₂O₃) leading to increase in corrosion rate.