Borated graphite cathodes for low-temperature aluminum electrolysis Chimica Techno Acta ARTICLE published by Ural Federal University 2022, vol. 9(2), No. 20229208 eISSN 2411-1414; chimicatechnoacta.ru DOI: 10.15826/chimtech.2022.9.2.08 1 of 9 Borated graphite cathodes for low-temperature aluminum electrolysis Rudenko A.V. a* , Kataev A.A. a , Neupokoeva M.M. ab, Tkacheva O.Yu. ab a: Institute of High Temperature Electrochemistry, Ural Brunch of Russian Academy of Sciences, Ekaterinburg 620137, Russia b: Ural Federal University named after the first President of Russia B.N. Yeltsin, Ekaterinburg 620002, Russia * Corresponding author: a.rudenko@ihte.uran.ru This paper belongs to a Regular Issue. © 2022, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract Electrochemical boriding of the graphite plates in the potassium cry- olite based electrolytes was studied. The boriding were carried out in a cell with vertical electrodes. The procedure included 2 stages: 1) electrolysis in the KF–AlF3–KBF4 melt (CR=1.3) at low current density (0.01–0.02 A/cm2), required for the boron reduction, at 700 and 750 °C; 2) electrolysis in the KF–AlF3–Al2O3 melt at higher current density (0.2 A/cm2), required for the aluminum reduction. The opti- mal conditions of electrodeposition for obtaining the borated wetta- ble cathodes were determined. According to the SEM data, a continu- ous AlB2 layer with a thickness of 7–10 μm was formed on the graph- ite surface. The borated graphite was tested as a wetted cathode dur- ing the low-temperature aluminum electrolysis. Prolonged electroly- sis in a vertical cell with the graphite anode and the borated graphite cathode was carried out in the KF–NaF(10 wt.%)–AlF3–Al2O3 electro- lyte (CR=1.5) at 830 °C. After 100 h of electrolysis, the thickness of the AlB2 layer on the graphite surface was 5 μm, while the cathode surface was completely wetted with aluminum. Thus, we demon- strated the feasibility of using the borated graphite cathode as a wet- table dripping cathode in the low-temperature aluminum electrolysis in the vertical cell. Keywords graphite cathode boriding cryolite electrolysis Received: 04.04.22 Revised: 17.05.22 Accepted: 17.05.22 Available online: 26.05.22 1. Introduction Nowadays, almost all primary aluminum is obtained by the Heroult-Hall process, which has not undergone any funda- mental changes during more than 100 years [1]. The tech- nology consists in the electrolytic decomposition of alumina dissolved in sodium cryolite (Na3AlF6 with additions of AlF3, CaF2, etc.) at 940–970 °C. It is worth noting that aluminum production is an extremely energy-consumption process: the cost of electricity is more than 30%. The process is ac- companied by the consumping of carbon anodes and the emission of a significant amount of harmful gases (CO, CO2 and freons) that create a greenhouse effect [2, 3]. A radical modification of the Heroult-Hall process is possible by re- placing consumable carbon anodes with non-consumable inert anodes [4]. However, as of now, there is no infor- mation regarding the successful application of inert anodes in conventional technology. The main reason for this is the aggressiveness of the cryolite-alumina melt at high temper- atures. To prevent fast corrosive destruction of structural materials, it is necessary to reduce the operating tempera- ture of the process. In this regard, it becomes necessary to create and develop a new technology for low-temperature aluminum electrolysis. Its main advantages would be ener- gy saving, a significant reduction in greenhouse gas emis- sions and an increase of the cell life. In addition, the solubil- ity of metallic aluminum in the molten electrolyte decreases with reducing temperature [5], which will lead to an in- crease in the current efficiency. Attempts to develop a low-temperature process for producing aluminum were focused on modifying the con- ventional electrolyte based on sodium cryolite: lowering the temperature was achieved mainly by increasing the aluminum fluoride content. However, due to the low solu- bility of alumina in such melts, they have not found appli- cation in the industry. http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2022.9.2.08 mailto:a.rudenko@ihte.uran.ru http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0003-0439-5516 https://orcid.org/0000-0002-2157-2484 https://orcid.org/0000-0001-5451-2915 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2022.9.2.08&domain=pdf&date_stamp=2022-5-26 Chimica Techno Acta 2022, vol. 9(2), No. 20229208 ARTICLE 2 of 9 Another way of solving the problem is using an electro- lyte based on potassium cryolite. The physicochemical properties of such melts are well studied [6–8]. The KF–AlF3 mixtures with cryolite (molar) ratio (CR) (CR=X(KF)/X(AlF3)) from 1.3 to 1.5 have a liquidus tem- perature below 800 °C and the solubility of alumina that is higher than in the sodium system. The low-temperature electrolysis in electrolytes, the main component of which is potassium cryolite, was demonstrated by several re- searchers [9–13]. In the low-temperature aluminum electrolysis inert anodes are used instead of graphite anodes. The inert an- odes must meet certain requirements: low solubility in cryolite melts, high electrical conductivity, easy machin- ing, and low cost. The metal anodes fulfill all requirements and are, therefore, the subject of special attention. As a rule, the composition of the material for inert metal an- odes is based on the Cu–Al or Cu–Fe–Ni alloys [14–17]. The low-temperature electrolysis with inert anodes re- quires a new operation system of the electrolytic process and a new cell design, specifically, the use of cells with ver- tically arranged electrodes. Such design, according to pre- liminary calculations, will reduce energy consumption by 25%, increase the current efficiency, while maintaining a high density current, and reduce the cell size [17, 18]. Beck [9, 18] carried out the long-term (300 h) electrol- ysis in the KF–AlF3 molten eutectic and the KF–AlF3 and NaF–AlF3; the temperature varied from 700 to 850 °C. Although Al2O3 was added in an amount of 10 wt.%, the actual concentration of dissolved alumina was about 4 wt.% (as follows from the figures of report [18]). The 10, 200 and 300 A cells were equipped with the Cu–Ni–Fe anodes and the TiB2 cathodes. It was reported that current efficiency reached 95%, and the purity of aluminum was 99.5% in 200-ampere cell. The electrolysis in the (45wt%)KF–AlF3–(5wt%)Al2O3 melt with CR=1.3 was successfully performed at 700 and 750 °С in the 10, 20 and 100 and 1000 A vertical cells [10, 11]. Aluminum bronze was served as the anode, and the TiB2–C composite was used as the cathode. The same elec- trolyte composition was used in electrolysis tests in a cell with the Cu–Ni–Fe inert anodes at 700 °C [15]. However, in addition to the issues concerning corro- sion-resistant material for inert anodes, the composition of low-melting electrolyte, etc., the necessity in the mate- rial of vertical cathodes with good wettability and corro- sion resistance in fluoride melts arises. In addition, sever- al researchers have noted the problem of cathode pas- sivation during low-temperature electrolysis [19–21]. The cathode material should be thermally and chemically sta- ble under electrolysis conditions, but, perhaps, the most important requirement is a good wettability of the cathode surface by the liquid aluminum [22]. It is known that the titanium diboride is the best mate- rial for a wetted cathode. Cathodes made of TiB2 or TiB2–C composite are well suited for a vertical cell design. Brown [23] investigated materials TiB2–B4C, TiB2–BN, TiB2–TiC under electrolysis conditions. The cathodes were prepared using different techniques. The performance of the cath- ode material was evaluated by the values of current effi- ciency compared to commercial HP TiB2. It was found that no one material has performed better than TiB2. However, it should be noted that the TiB2 is very difficult to ma- chine, and its cost is also high. Therefore, the search for new materials for wettable cathodes for the low- temperature aluminum electrolysis remains an urgent task. The electrolytic methods of applying a boride coating on cathodes for the electrolytic aluminum production, in- cluding the electrodeposition of boron and titanium in the form of titanium diboride on graphite and other materials, were described in [24, 25]. The electrodeposition was per- formed in fluoride and chloride-fluoride melts at 650–710 °С, using the K2TiF6 and KBF4 salts as sources of titanium and boron. The resulting coatings were charac- terized by a good adhesion to the cathode and relatively high erosion and chemical resistance in molten salts. The electrochemical boriding of steel in a borax-based molten electrolyte at 950 °C was considered in works [26, 27]. It was described that this process yielded a very dense, uniform, and hard boride layer. The authors [28] studied the electrochemical behavior of boron in a melt KBF4–LiF–NaF–KF at 700 °C and found that boron interacts with the surface of the working elec- trode, which was glassy carbon, platinum, or silver. Ray [29] discovered that small additions of TiO2 and B2O3 to the cryolite-based melt (Na3AlF6–Al2O3–CaF2) at about 1000 °С increase the wettability of the graphite electrode by molten aluminum due to formation of Al–Ti or Al–B alloys on the graphite surface. Moreover, the Al–B alloy improves the wettability of graphite more efficiently. The authors [30] compared the wettability of borated steel and hot pressed TiB2 in terms of the wetting angle. A sample of borated steel was obtained in two stages: first, by boriding steel in a borax melt; then aluminizing was carried out by the thermal diffusion saturation. They found that the borated steel was wetted better by the liq- uid Al than the TiB2. However, it was noticed that the prop- erties of TiB2 largely depend on the preparation technique. Kataev [31] found that during electrolysis in the KF– AlF3–B2O3 melt at 700 °C in cells with graphite electrodes a layer of the AlB2 intermetallic compound was formed on the surface of graphite cathode, which significantly im- proved the wettability of C-cathode with liquid aluminum. Thus, it can be concluded that borated graphite, proba- bly, surpasses even titanium diboride in wettability with the liquid aluminum, and the boron coatings on graphite can be obtained by electrolysis of molten salts. The objectives of this work were: (i) to determine the op- erating parameters for obtaining a boride coating on graphite cathode by electrolysis in the KF–AlF3–KBF4 melt; (ii) to test a wetted cathode in a cell with vertically arranged electrodes during the low-temperature aluminum electrolysis. Chimica Techno Acta 2022, vol. 9(2), No. 20229208 ARTICLE 3 of 9 2. Experimental 2.1. Chemicals The melts were prepared from the reagent-grade NaF, KF·HF, AlF3(99%), KBF4 (99%), Al2O3 supplied by Vekton Ltd (Russia). The KF–AlF3–KBF4 melt was used as a medium for the electrochemical boriding. The KF–AlF3 electrolyte with a cryolite ratio (CR=X(KF)/X(AlF3)) of 1.3 was obtained by mixing the KF·HF and AlF3 salts (in a glassy carbon con- tainer). The mixture was exposed to 750 °C over 4 h in order to remove the HF from the melt due to the thermal decomposition of the KF·HF. The electrolyte preparation process is described in detail elsewhere [8]. The amount of “O” in the prepared electrolyte was determined by the carbothermal reduction technique using an oxygen analyz- er LECO TC836. It was found that the original electrolyte contained 0.6 wt.% “O”, which corresponds to 1.25 wt.% Al2O3. The molten mixture KF–NaF(10 wt.%)–AlF3 with CR=1.5 (CR=(XKF + XNaF)/XAlF3) was used for prolonged 100 h electrolysis in order to test efficiency of the borated graphite cathode. The KF–AlF3–KBF4 electrolyte was chosen for boriding because it has a significantly lower liquidus temperature than the KF–NaF(10 wt.%) –AlF3 molten mixture [7]. This provided a low operating temperature and, consequently, a lower degree of the thermal decomposition of KBF4 with the release of gaseous BF3. In addition, it is known [32, 33] that NaBF4 is more susceptible to the thermal decomposi- tion than KBF4; that is, the presence of NaF in the electro- lyte is undesirable. On the other hand, the electrolyte KF–NaF(10 wt.%)– AlF3 was recommended for the low-temperature electroly- sis because it has a higher alumina solubility [7]. It should be noted that the operating temperature of electrolysis in this electrolyte is higher than 800 °C; however there is no so-called “sodium problem” associated with the accumula- tion of NaF, which enters the electrolyte as an alumina impurity, and is accompanied by a significant increase in the operating temperature [7]. 2.2. Electrochemical cell Boriding the graphite cathode as well as testing a wettable cathode under electrolysis conditions was carried out in the vertical cell. The procedures for these processes have some differences, as will be indicated below. However, the design of the cell remained the same. A schematic diagram of the electrochemical cell is presented in Figure 1. Dense graphite served as the anode. Graphite or borated graphite was used as the cathode. The anode-cathode distance was 20 mm. The dimensions of the electrodes were 20x70x8 mm. The electrodes were not completely immersed in the melt, but to a depth of 75 mm. The current density was calculated on the surface area facing the anode. For relia- ble electrical contact the current leads (steel) were screwed into the threaded electrodes. The current leads were protected from interaction with the atmosphere by alumina cases. The salt mixture (about 400 g) was placed into the alumina crucible (V=600 ml). The alumina powder was filled between the crucible and the container. The ex- periments were carried out in an open cell in ambient air. Figure 1 Schematic diagram of an electrochemical cell: 1 – protec- tive alumina container; 2 – alumina backfill; 3 – alumina crucible; 4 – cathode;5 – electrolyte; 6 – liquid aluminum; 7 – Pt/Pt–Rh thermocouple; 8 – carbon anode; 9 – two tungsten quasi- reference electrodes in alumina case. The temperature of the cell was measured by Pt–Pt/Rh thermocouple using an APPA 109N multimeter. The poten- tial between the anode, the cathode and W quasi-reference electrodes was measured and recorded using an APPA 109N multimeter. The electrolysis was performed with the help of an Autolab PGSTAT 302 with a BSTR20A booster. 2.3. Boriding A boride coating on a graphite cathode was obtained by electrolysis in the KF–AlF3–KBF4 melt. It was evaluated in [33] that in the potassium cryolite melt with the addition of 15 mol.% KBF4 the loss of boron due to the thermal decomposition does not exceed 3% in the temperature range of 400–800 °C. However, taking into account the possible low boron current efficiency, the addi- tion of KBF4 was made in excess. The KBF4 component was added to the KF–AlF3 electrolyte in amount of 0.5 wt.%, that ten times exceeded the calculated value for the for- mation of AlB2 layer (10 m) at the boron current efficiency of 100%. It should be noted that the boron concentration in the electrolyte during boriding was not analyzed. The current density was 0.01 and 0.02 A/cm2, and temperature was varied from 700 to 750 °C. The choice of such a low current density was based on the results of the work [31]. The authors determined the operating parame- Chimica Techno Acta 2022, vol. 9(2), No. 20229208 ARTICLE 4 of 9 ters for the production of Al–B alloy by electrolytic reduc- tion of B2O3 in the KF–AlF3–Al2O3 melt: the initial current density was 0.02 A/cm2 at 700 °C. In order to identify the boride layer by scanning elec- tron microscope (SEM) the electrolysis process was con- tinued at a higher current density. The alumina in amount of 3 wt.% was added to the electrolyte. An aluminum layer on top of the boron facilitated the determination of the presence and thickness of the boron on graphite against the background of aluminum. Cross-section of the borated cathode was analyzed using the SEM with nitrogen-free energy dispersive detector X-act ADD+JSM- 5900LV sup- plied with a wave dispersive micro analyzer, sluice cham- ber and a device for suppression of electromagnetic inter- ference INCA Energy 250 and INCA Wave 500. 2.4. Electrolysis test The borated cathodes were tested under the conditions of the low-temperature aluminum electrolysis in the KF–NaF(10 wt.%)–AlF3–Al2O3 melt with CR = 1.5. The an- ode was graphite. As it was consumed, the anode was re- placed with a new one. Electrolysis parameters were as follows: current density i = 0.2 A/cm2, temperature 830 °C. The current density was chosen based on the re- sults of work [34], in which it was recommended not to increase the cathode current density above 0.35–0.5 A/cm2 in order to avoid the formation of solid deposits on the cathode. As indicated by the authors, at the current densi- ty of 0.2 A/cm2, the current efficiency in the vertical cell was about 67%. Thus, in the present work the amount of added Al2O3 during electrolysis was calculated assuming 60% current efficiency in order to maintain the Al2O3 con- centration in the electrolyte at 2–3 wt.%. During electrol- ysis, liquid aluminum drips from the cathode and forms a pool at the bottom of the cell (Figure 1). The potential differences between the cathode and the W quasi- reference electrode and between the anode and the W quasi-reference electrode were measured during electroly- sis. They were used only to identify possible malfunctions of either the anode or the cathode. 3. Results and Discussion 3.1. Boriding the graphite cathode A wettable coating on the graphite cathode was obtained by electrolysis in the KF–AlF3–KBF4 melt. The current den- sity and operating temperature were varied. According to the data [35], the boron may be electro- chemically deposited from the cryolite-based electrolyte (Na3AlF6 + 1 wt.% Al2O3 + 0.2 wt.% B2O3, 1050 °C) at low- er voltage that that required for the aluminum reduction. The difference between the reduction potentials is report- ed to be about 0.6 V. The authors [36] recommend per- forming the boron electrodeposition in the KF–AlF3–Al2O3– B2O3 (CR=1.3) melt at potentials 0.6 V more positive than the aluminum reduction potential (at 750 °С). Since there is a small amount of oxygen ions in the original electrolyte (as stated above), the oxidation pro- cess of the oxygen-containing ions occurs at the graphite anode with the formation of CO2. Considering that boron is in the ionic melt in the form of a complex anion BF4– [37], the cathode process can be described by the following equation: BF4– + 3e = B + 4F–– (1) During the deposition of aluminum, the interaction of Al and B with the formation of intermetallic compounds (AlB2, AlB12) can occur [38]. According to the Al–B phase diagram, at a temperature of about 750 °С, it is possible to obtain Al–B alloys containing at least 0.09% boron. Thus, the process of obtaining a wettable cathode took place in 2 stages. Stage 1: The objective of the first stage was to obtain a boron coating on the graphite cathode. The electrolysis in the KF–AlF3–KBF4 melt with CR=1.3 was carried out in a galvanostatic mode at low current densities required for boron reduction at 700 and 750 °С. The electrolysis time was varied, while the charge in all experiments remained constant and equal to 0.6 C. Stage 2: The goal of the second stage was to deposit the aluminum at the cathode in order to obtain an aluminum- boron coating. The electrolysis in the KF–AlF3–Al2O3 CR=1.3 melt was carried out in a galvanostatic mode at higher current densities required for the production of aluminum. The duration and current density in all exper- iments were the same. The parameters of two-staged electrolysis are summa- rized in Table 1. Four series of experiments were carried out. In each series, the electrolysis proceeded stably, the voltage was constant both at the first and at the second stages of electrolysis (Figure 2). The images of the coating obtained on graphite in exp.1 with i = 0.02 A/cm2 at 700 °С and the boron distribution map in the aluminum matrix are given in Figure 3. The boron is represented in red. The thickness of the Al layer was 800 μm. A continuous boron layer, 7–10 μm thick, was formed on the graphite surface. The enlarged spec- trum shown in Figure 3b indicates that in the dark areas (spectrum 1, 3, 7, 9) the B content was about 63 at.% and the Al content was about 33 at.%, which corresponds to AlB2. The grain size of AlB2 was 3–5 μm. Table 1 Parameters of two-stage electrolysis for obtaining boride coating on graphite. Parameters Stage 1 Stage 2 exp.1 exp.2 exp. 3 exp.4 Electrolyte KF–AlF3–KBF4, CR=1.3 KF–AlF3–Al2O3, CR=1.3 Addition KBF4 0.5 wt.% Al2O3 3 wt.% Tempera- ture, °C 700 700 750 750 700–750 Current density, A/cm2 0.02 0.01 0.02 0.01 0.2 Duration, h 3 6 3 6 3 Chimica Techno Acta 2022, vol. 9(2), No. 20229208 ARTICLE 5 of 9 Figure 2 Voltage in two-stage electrolysis: 1 – exp.1; 2 – exp.2; 3 – exp.3; 4 – exp. 4 (Table 1). Figure 3 Image of the aluminum layer and the boron distribution map obtained in exp. 1 (a); enlarged area of the image (a) (yellow square). In exp.2 under the same conditions, but with a de- crease in current density and an increase in duration by a factor of 2, the boron distribution changed (Figure 4), while the thickness of the Al layer remained the same (800 µm). The boron was distributed uniformly over the thickness of the aluminum layer; however, no continuous uniform boron layer covering the graphite surface was found. With an increase in the electrolysis temperature to 750 °C and at i = 0.02 and 0.01 A/cm2 (exps. 3 and 4), the boron is almost absent from the aluminum layer (Figure 5). The thickness of the aluminum layer on graphite is 600 and 370 μm, respectively. For testing the coating cathodes under conditions of prolonged electrolysis, the cathode obtained in exp. 1 was chosen. It had the thickest aluminum layer and uniform boride coating on the graphite surface. Figure 4 Image of the aluminum layer and the boron distribution map obtained in exp. 2. Figure 5 Images of the aluminum layer and the boron distribution map obtained at 750 °C at the current density of 0.02 (a) and 0.01 (b) A/cm2. Thus, a twofold decrease in the current density at the same temperature leads to a more uniform distribu- tion of the intermetallic compound AlB2 over the thick- ness of the aluminum coating. An increase in tempera- ture by 50 °C under the given electrolysis conditions prevents the formation of boride coatings on the graph- ite surface. It should be noted that for all the parameters of the two-stage electrolysis, the aluminum formed in the sec- ond stage wetted well and was evenly distributed over the surface of the graphite cathode. The only differ- ences were that, depending on the electrolysis parame- ters applied, the thickness of the aluminum layer, the uniformity of the boron distribution in the aluminum layer and its amount changed. For comparison, uncoat- ed graphite was used as the cathode in the vertical cell. The electrolysis was carried out for 50 h. The photos of the graphite cathode before and after electrolysis are shown in Figure 6. The cathode was covered with alu- minum droplets, indicating that the graphite surface was not wetted by the Al metal. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 2 4 6 8 10 V o lt a g e , V Duration, h 21 43 (a) (b) (a) (b) Chimica Techno Acta 2022, vol. 9(2), No. 20229208 ARTICLE 6 of 9 Figure 6 Uncoated graphite cathode before and after aluminum electrolysis in vertical cell. 3.2. Test of wettable cathode during the aluminum electrolysis The test of the borated graphite cathode were carried out in the KF–NaF(10 wt.%)–AlF3–Al2O3 melt with CR=1.5 at 830 °C for 100 h (in total). The electrolysis with the same cathode was carried out in two steps: the duration of the first step was 75 h and the second step lasted for 25 h. The voltage and potentials of the cathode and the an- ode during electrolysis are given in Figure 7. The electrolysis was sustained, the voltage at step 1 was ~2.1 V. The cathode potential was stable. The in- crease in the anode potential by 38th, 49th, and 62nd h of electrolysis is associated with the consumption and re- placement of the graphite anodes. At the second step, the voltage systematically increased from 2.1 to 2.3 V. It ap- pears from the symbatic nature of curves trend of the voltage and the anode potential that the increase in volt- age is associated precisely with the operation of the inert anode. The increase in voltage after 21st h of electrolysis is explained by the breakdown in the contact of the cur- rent lead with the anode. After each step of electrolysis, the lower part of the cathode was cut off and a cross-section was prepared for a metallographic examination. An image of the cathode sec- tion after 75 h of electrolysis is shown in Figure 8a. The dark gray areas in the image are AlB2, the lighter areas contain less boron. The boride coating layer AlB2 is pre- served, its thickness is 7 μm. The grains are distributed uniformly over the thickness of the Al layer. An image of the cathode cross-section after 100 h of electrolysis is presented in Figure 8b. The thickness of the AlB2 coating on the graphite surface after 100 h (Fig- ure 8b) is about 5 μm. It can be assumed that boron dis- solves in molten aluminum. Based on the SEM data, it was calculated that about 0.0028 g of boron was transferred from the cathode surface to the volume of molten alumi- num during 25 h of electrolysis (Figure 8b). An external examination of the cathode after the exper- iment did not reveal any damage. An image of the cross- section of the graphite cathode with the diboride coating after electrolysis is shown in Figure 9. The cathode surface was completely wetted with alu- minum. The thickness of the aluminum layer was 2 mm. Detachment of the aluminum from the cathode surface is not observed, which indicates good wetting of the graphite surface with aluminum. The current efficiency was 62 %. Thus, the prolonged 100-h electrolysis tests of the graphite cathode, previously coated with the aluminum diboride, in the vertical cell proved its efficiency. Undoubtedly, tests of wetted cathodes should be con- tinued under conditions of longer electrolysis duration. However, it should be noted that, if necessary, an addi- tional boriding of the cathode can be carried out during low-temperature aluminum electrolysis without changing its parameters. Figure 7 Voltage during electrolysis in the vertical cell with wetted borated graphite cathode. Chimica Techno Acta 2022, vol. 9(2), No. 20229208 ARTICLE 7 of 9 Figure 8 Images of the borated cathode after 75 h (a) and 100 h (b) of electrolysis. a b Figure 9 Image of cross-section (a) and general view of the hot wetted cathode after 100h electrolysis (b). 4. Conclusions Boriding the graphite plates was accomplished by electrol- ysis in the KF–AlF3–KBF4 melt (CR=1.3) in two stages. At the first stage, the boron was reduced at the graphite cathode at the current density of 0.01 and 0.02 A/cm2 and temperature 700 and 750 °C, and at the second, the alu- minum was deposited at the current density of 0.2 A/cm2. The second stage was necessary in order to improve the identification of boron against the background of the alu- minum by SEM. The operating parameters for the electro- chemical deposition of continuous boride coatings on the graphite surface were found. Nevertheless, despite the different values of the parameters of two-stage electroly- sis, in all cases, aluminum, being reduced in the second stage of the process, wetted the graphite surface well. The difference was found only in the thickness of the alumi- num layer and the amount and distribution of boron in the aluminum layer. The test of the borated graphite cathode under condi- tions of the low-temperature electrolysis in the KF– NaF(10 wt.%)–AlF3–Al2O3 (CR=1.5) melt at 0.2 A/cm2 and 830 °C for 100 h confirmed the possibility of its use as a wetted dripping cathode in the electrolytic production of aluminum in low-melting cryolite. Supplementary materials No supplementary materials are available. Funding This research had no external funding. Acknowledgment The authors are grateful to the staff of the Shared Access Centre “Composition of Compounds” of the Institute of High Temperature Electrochemistry for their help in metallographic studies. Author contributions Conceptualization: K.A.A., T.O.Yu. Data curation: T.O.Yu. Formal Analysis: R.A.V., K.A.A., T.O.Yu. Funding acquisition: R.A.V., T.O.Yu. Investigation: R.A.V., K.A.A., N.M.M. Methodology: R.A.V., K.A.A. Project administration: R.A.V. Resources: R.A.V., K.A.A. Software: R.A.V., K.A.A. Supervision: T.O.Yu. Validation: R.A.V., T.O.Yu. Visualization: R.A.V. Writing – original draft: R.A.V., T.O.Yu. Writing – review & editing: R.A.V., T.O.Yu. Conflict of interest The authors declare no conflict of interest. Additional information Author ID’s: Rudenko A.V., Scopus ID 57197500558; Kataev A.A., Scopus ID 12241606800; Tkacheva O.Yu., Scopus ID 6602941818. (a) (b) https://www.scopus.com/authid/detail.uri?authorId=57197500558 https://www.scopus.com/authid/detail.uri?authorId=12241606800 https://www.scopus.com/authid/detail.uri?authorId=6602941818 Chimica Techno Acta 2022, vol. 9(2), No. 20229208 ARTICLE 8 of 9 Websites: Institute of High Temperature Electrochemistry, UB RAS, http://www.ihte.uran.ru/?page_id=3118; Ural Federal University, https://urfu.ru/en. References 1. Haupin WE. Principles of aluminum electrolysis. Essent Read Light Met. 2016;2(3):3–11. doi:10.1007/978-3-319-48156-2_1 2. Evans J, Kvande H. 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