CORROSION-RESISTANT MOLTEN SALT DEVICE

Abstract

Device (10) comprising: - an element (20) configured to contain or circulate a solution, the element (20) having a wall (21) made of steel or a nickel-based alloy comprising chromium, - a solution (30), in contact with the wall (21), comprising a mixture of molten chloride salts, the solution further comprising Ti2+ ions and Ti3+ ions, the Ti3+ / Ti2+ ratio being between 20 / 80 and 70 / 30. Figure for the abstract: 11

Description

Description Title of the invention: RESISTANT SOFT SALT DEVICE TO CORROSION technical field

[0001] — The present invention relates to the general field of nuclear and solar energy. concentration.

[0002] = The invention relates to a device comprising an element having a steel wall or in a nickel-based alloy comprising chromium in contact with a solution of molten salts (molten chlorides).

[0003] — The invention also relates to a method for preparing such a device as follows that a method for measuring the potential of a solution of molten salts in such a device.

[0004] — The invention finds applications in numerous industrial fields, and particularly for molten salt nuclear reactors including their circuits primary and secondary or even for concentrated solar power plants.

[0005] — The invention is particularly interesting since it allows for a considerable reduction in easily the corrosion phenomena occurring at the interface between a wall and metallic alloy and a solution of molten salts, even for functional devices operating at high temperatures (typically above 500°C). PREVIOUS STATE OF THE ART

[0006] = From an industrial point of view, molten salts can be used for the production of metals by electrolysis (Al, Na, Mg), or even as a heat transfer medium for concentrated solar thermal applications and for bare- clear, as fuel or as a heat transfer fluid for example for the nuclear reactors and including actinide converters.

[0007] — For example, in the case of solar applications, thanks to the thermal inertia of the salts Once melted, it is possible to maintain high temperatures for several hours. even after sunset.

[0008] — For applications at very high temperatures (typically above 600°C), Chloride salts (such as KCl, NaCl and MgCl) are preferred over other salts nitrates which decompose at lower temperatures.

[0009] = However, the high chemical reactivity of these media creates very aggressive substances that promote corrosion. This phenomenon is further accentuated when the The temperatures used in solar and nuclear devices are very high. Thus, materials in contact with these environments are heavily corroded, this which poses problems with the operation and longevity of the installations. Furthermore, the The dissolution of metallic materials also alters the composition and the thermophysical properties of the heat transfer fluid by adding CrCl, and FeCl, for example. Although the corrosive nature of molten chlorides is known, the results of corrosion rates or material consumption (thickness per unit time) are disparate and reflect different and poorly controlled experimental conditions. Nevertheless, corrosion in molten chlorides continues to be the subject of numerous academic and industrial studies. Several solutions have been considered, such as: - use steels with good corrosion resistance (nickel-rich steels), - limit the presence of impurities such as hydroxides and oxides, HCl and Cl, which participate in the oxidation or complexation mechanisms of steel constituents; for example by using a controlled inerting atmosphere (argon in particular) to reduce corrosion, - modify the chemical potential of the molten salt solution. While controlling the chemical potential of the molten salt solution is an effective way to limit corrosion, its value can be affected by fission and transmutation reactions and by the introduction of impurities into the system (HO, Oz). This is particularly likely in the case of a nuclear reactor, where the presence of EuCl, for example, has an oxidizing effect on chromium, iron, and also nickel, due to the high potential of the Eu* / Eu** couple. The use of a redox buffer has historically been employed in molten salt reactors to control the chemical potential of the fluoride salt and limit corrosion. For example, the U* / U* couple in molten fluorides has been shown to mitigate corrosion by controlling the proportions of U* and U# dissolved in the salt (Journal of Nuclear Materials 440 (2013) 243-249). A U+ / U* concentration ratio in the salt < 100 is recommended to limit steel corrosion and intergranular tellurium corrosion up to 750°C. Adjusting the oxidation states of uranium can be achieved by using a reducing metal, such as uranium or beryllium. US document 2017 / 0294241 A1 also proposes adding uranium to control the redox potential and mitigate corrosion in a molten salt reactor. Numerous elements that could act as buffers are mentioned without specifying precise experimental conditions. This concept has also been applied to solar salts with NaCl-KCl-MgCl type concentrations, as described in document WO 2019 / 075177 A1. To limit corrosion, it is possible to add metallic magnesium to alkali and alkaline earth chloride salts containing MgCl. The addition of metallic Mg provides the electrochemical couple Mg*+ / Mg° in solution, which, from a thermo- The dynamics are favorable for inhibiting the dissolution of chromium, iron, and nickel. The use of excess metallic magnesium helps maintain the solution potential below the dissolution potential of the major constituents of steels. However, the presence of excess dissolved magnesium in the salt can lead to the formation of Mg-Ni alloys on the surface of materials, such as 316H steel (Corrosion Science 194 (2022) 109914). Furthermore, metallic magnesium is a strong reducing agent capable of reducing actinide chlorides to metallic actinides. This solution therefore seems unsuitable for a molten salt nuclear reactor, as it would lead to the formation of metallic actinides that could potentially render the fuel salt non-homogeneous. Moreover, the metallic actinides could alloy with metallic materials in contact with the molten salt. For example, plutonium leads to the formation of compounds with iron (Fe,Pu and FesPu) that are liquid at low temperatures, depending on their composition. Document WO2017 / 060741 A1 states that more reactive sacrificial metals, such as zirconium, vanadium, or titanium, can be used to reduce corrosion of steels in contact with molten halogen salts (fluorides or chlorides). This document focuses primarily on the Pauling scale of metals. It mentions the following species pairs: ZrF₆ and ZrF₄, TiF₆ and TiF₆, and VF₆ and VF₆. However, the presence of ZrF₆ in molten salts has never been demonstrated, nor has that of TiF₆. Furthermore, this document does not specify the experimental conditions to be implemented. In particular, this document does not provide precise information on the target concentrations or redox potential, on how to find the appropriate buffer pair depending on the molten salts and the material to be protected, or even on the method for controlling and maintaining the stability of the species in the molten salt solution. Description of the invention One aim of the present invention is to provide a device to limit the corrosion phenomena of metallic alloys in contact with molten chlorides. To this end, the present invention proposes a device comprising: - an element configured to contain or circulate a solution, the element having a wall made of steel or a nickel-based alloy comprising chromium, - a solution, in contact with the wall, comprising a mixture of molten chloride salts, the solution further comprising Ti? ions and Ti** ions, the concentration ratio Ti* / Ti2+ being between 20 / 80 and 70 / 30. The invention differs fundamentally from the prior art by the use of titanium to reduce the corrosion of metal parts in contact with molten chloride salts. Titanium is much less reducing than magnesium and therefore better suited for use in a molten salt reactor (MSR), while also being sufficiently negative to prevent corrosion of metallic alloys, such as steel. Furthermore, titanium is fairly transparent to neutrons, limiting its transmutation in the reactor. Furthermore, controlling the concentration of Ti⁻ and Ti²⁻ ions is essential to maintain the potential range where effective redox control is achieved, thus limiting or even preventing the dissolution of metallic alloys. Outside this immunity range, elements such as chromium and iron dissolve in the molten salt solution. Such a device can be used for concentrated solar power applications and for molten salt reactor (MSR) nuclear reactor applications. According to one embodiment, the device is a nuclear device and the component is a primary circuit of a nuclear fission reactor or a secondary circuit of a nuclear fission reactor. The primary circuit of the nuclear fission reactor and the secondary circuit of the nuclear fission reactor allow the circulation of the chloride salt mixture. The reactor can be a molten salt reactor with a fast neutron spectrum. According to another embodiment, the device is a solar device, for example a concentrating solar power plant, and the element is a molten chloride fluidic circuit or a molten chloride reservoir. The solution comprises a mixture of molten chlorides. 1] This could be, for example: - a binary or ternary mixture of alkali and alkaline earth metal chlorides, generally used as a heat transfer fluid, or - a mixture containing one or more actinide chlorides generally used for nuclear applications. According to a first advantageous embodiment, the mixture of molten chloride salts is a binary or ternary mixture obtained from molten chlorides selected from LiCl, NaCl, KCl, MgCl, CaCl, and BaCl. For a binary mixture, one could choose, for example, a NaCl-MgCl mixture, or a KCl-MgCl mixture, while for a ternary mixture, one could choose, for example, a NaCl-KCl-MgCl mixture. These mixtures form a heat transfer fluid that is particularly useful when the component is a secondary circuit of a nuclear fission reactor, a reservoir, or a fluidic circuit of a concentrated solar power plant. According to a second advantageous embodiment, the mixture of molten chloride salts comprises at least one actinide chloride. By at least, it is meant that the mixture of molten chlorides comprises one or more actinide chlorides. Actinide chlorides are compounds of the formula AcCl; where Ac is an actinide. Preferably, Ac is Pu, Am, U, or Cm. Even more preferably, Pu or Am will be chosen. Actinide chloride(s) will preferably be added to one of the binary mixtures described above. Preferably, the mixture of molten chloride salts is a NaCl-MgCl-PuCl or NaCl-MgCl-PuCl-AmCl mixture. These mixtures form the fuel for the nuclear reactor. This embodiment is particularly advantageous when the component is a primary circuit of a nuclear fission reactor. As a chemical analogue of NaCl-MgCl,-PuCl;, it is possible to use the mixture NaCl-MgCl,-CeCl.. Advantageously, the solution is at a temperature between 450°C and 700°C and preferably between 550°C and 650°C. The wall is made of steel or a nickel-based alloy. Advantageously, the wall is made of a material mechanically resistant to high temperatures. Preferably, the wall is made of stainless steel, for example, 316L steel. According to another preferred embodiment, the wall is made of a nickel-based alloy comprising, preferably, at least 40% nickel by mass. For example, this is a nickel-chromium-molybdenum alloy, in particular NiCr22Mo9Nb. Advantageously, the Ti*+ / T1* ratio is between 30 / 70 and 50 / 50. The device offers numerous advantages. For example, it allows for effective potential control at a sufficiently low value to fall within the immunity range. Furthermore, in the case of a molten salt reactor (MSR), such a device prevents the formation of metallic actinides, and in particular the formation of metallic plutonium, unlike metallic magnesium. Moreover, such a device is simple to prepare. The invention also relates to a method for preparing a device as defined above, the method comprising the following steps: - to provide a device comprising an element configured to contain or circulate a solution, the element having a wall made of steel or a nickel-based alloy comprising chromium, and a solution, in contact with the wall, comprising a mixture of molten chloride salts, - Immerse titanium metal in the solution for a sufficient duration to obtain Ti* and Ti* ions, with the Ti*+ / Ti?* ratio being between 20 / 80 and 70 / 30, - possibly, add ions of a metal having a potential lower than the dissolution of the chromium of the iron metal alloy in the solution, thereby accelerating the formation of Ti**+ and Ti** ions by redox reactions. The ions could be iron ions or chromium ions, for example. The invention also relates to a method for measuring the potential of a solution of a device as defined above. Indeed, it is particularly difficult to use conventional electrodes in Molten salts, such as Ag* / Ag° type reference electrodes, have questionable long-term stability in these aggressive environments. Therefore, there is a need to be able to control the potential of a molten salt solution. According to the invention, the method for measuring the potential of a solution in a device as defined above comprises the following successive steps: - provide a device comprising an element configured to contain or circulate a solution, the element having a wall of steel or a nickel-based alloy comprising chromium, and a solution, in contact with the wall, the solution comprising a mixture of molten chloride salts, Ti₂ ions and Ti⁺ ions, the Ti⁺ / Ti⁺ ratio being between 20 / 80 and 70 / 30, the mixture of molten salts comprising at least MgCl₂, and possibly PuCl₂; - immersing an anode, a cathode and a reference electrode in the solution; the anode and cathode being made of the same material, preferably tungsten, and the reference electrode being, for example, made of molybdenum or preferably tungsten, - measure the potential E, between the cathode and the zero or near-zero current reference (typically the potential is less than SOmV, for example on the order of a few mV), - to impose a current between the anode and the cathode, to deposit metallic magnesium on the cathode, or to deposit metallic plutonium on the cathode if necessary, - measure the potential Ez between the cathode and the zero-current reference, thereby obtaining the value of the Mg?+ / Mg° or Pu*+ / Pu° couple, as applicable, - Calculate the potential of the solution Es. According to Es = Es-E;. This measurement method is particularly advantageous because it allows the formation in situ of a so-called dynamic (or ephemeral) reference electrode and thus the reliable measurement of the solution's potential. Other features and advantages of the invention will become apparent from the supplementary description that follows. It goes without saying that this additional description is given only as an illustration of the object of the invention and should in no way be interpreted as a limitation of this object. Brief description of the drawings The present invention will be better understood upon reading the description of exemplary embodiments given by way of illustration only and in no way limiting, with reference to the accompanying drawings in which: [Fig.1] schematically represents, in cross-section, a device according to a mode of a particular implementation of the invention. [Fig.2] is a graph representing the dissolution voltammograms of pure metals, according to particular embodiments of the invention. [Fig.3] is a voltammogram of a NaCl-MgCl-CeCl solution containing a titanium salt at 600°C, according to a particular embodiment of the invention. [Fig.4] is a graph representing the evolution of the equilibrium potential of a NaCl-MgCl₂-CeCl₂ solution at 600°C as a function of the immersion time of a metallic titanium plate. [Fig.5] is a graph representing the equilibrium potential of a NaCl-MgCl--CeC solution], at 600°C after removing a titanium plate, according to a particular embodiment of the invention. [Fig.6] is a graph of a square wave voltammetry in a NaCl-MgCl,-CeCl solution, at 600°C containing Ti** and Ti?*+ ions, according to a particular embodiment of the invention. [Fig.7] represents the potential of a NaCl-MgCl,-CeCl solution at 600°C as a function of the concentration ratio Ti* and Ti**, according to a particular embodiment of the invention. [Fig.8] is a scanning electron microscope image of a 316L steel sample after immersion in a NaCl-MgCl-CeCl solution at 600°C for 168h, without redox control. [Fig.9] is a scanning electron microscope image of a 316L steel sample after immersion in a NaCl-MgCl;-CeCl; solution containing Ti* and Ti?* ions having a potential of about -2V vs CL / Cl, at a temperature of 600°C for 168h, according to a particular embodiment of the invention. - [Fig.10] is an X-ray dispersive X-ray (EDX) analysis performed on a 316L steel sample after immersion in a NaCl-MgCl,-CeCl solution; containing Ti**+ and Ti?* ions having a potential of about -2V vs CI / CI, at a temperature of 600°C for 168h, according to a particular embodiment of the invention. [Fig.11] is a scanning electron microscope image of an Inconel® 625 sample after immersion in a NaCl-MgCl-CeCl solution, containing Ti* and Ti?* onions having a potential of about -2V vs Cl, / Cl, at a temperature of 600°C for 168h, according to a particular embodiment of the invention. [Fig.12] is an X-ray dispersive X-ray (EDX) analysis performed on a sample of Inconel® 625 after immersion in a NaCl-MgCl-CeCl solution; containing Ti** and Ti?* ions having a potential of about -2V vs CI / CI, at a temperature of 600°C for 168h, according to a particular embodiment of the invention. [Fig.13] is a graph representing the potential of a NaCl-MgCl-CeCl solution at 600°C. [Fig.14] is a graph representing the potential of a NaCl-MgCl,-CeCl solution; containing Ti*+ and Ti?+ ions at 600°C. The different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible. DETAILED DESCRIPTION OF SPECIFIC METHODS OF IMPLEMENTATION Although this is by no means limiting, the invention finds particular applications in the field of nuclear energy or even solar energy (concentrated solar thermal). The invention is applicable to any device 10 comprising: - an element 20 configured to be in contact with a solution 30 and having a wall 21 made of steel or a nickel-based alloy comprising chromium, and - a solution 30 of molten chlorides, in contact with wall 21. In particular, element 20 can be used to contain the solution 30 of molten salts. Device 10 can be, for example, a tank, reservoir, or enclosure. Element 20 can also be used to circulate the molten salt solution 30. Element 20 can be, for example, a tube. In particular, element 20 can be a primary circuit of a molten chloride nuclear fission reactor, a secondary circuit of a nuclear fission reactor including molten chlorides, or a molten chloride reservoir or a molten chloride circulation circuit of a concentrated solar power plant. The element 20 has at least one wall 21. It may have several. For example, in the case of a tube, it is a side wall. In the case of a tank, vessel, or enclosure, the element may include a side wall, a bottom wall (or base), and a top wall (or lid). Each of these walls may be in contact with the solution 30. The material forming the wall (steel or nickel-based alloy) contains chromium. It also contains iron. It may also contain one or more other elements. By way of non-limiting choice, the other element(s) may be selected from nickel, molybdenum, tungsten, silicon, cobalt, aluminum, titanium, niobium, and manganese. Chromium is the least stable element in the metallic alloy; in other words, it is the most negatively charged element. Next come iron, nickel, and molybdenum. Tungsten is the most oxidation-resistant material; its anodic limit is the oxidation of chlorides to chlorine gas, and its cathodic limit is the reduction of Mg to magnesium metal. There is a potential range between the reduction potential of Mg to magnesium metal and the oxidation potential of the most negatively charged element. (Here, chrome) for which it is possible to limit the dissolution of steels. According to a first embodiment, the wall 21 is made of stainless steel. It could be, for example, 316L steel. According to another embodiment, wall 21 is made of a superalloy. For example, a nickel-chromium superalloy, and preferably a nickel-chromium-molybdenum superalloy, could be chosen. In particular, the NiCr22Mo9Nb superalloy, marketed under the name Inconel® 625 by Special Metals Corporation, could be selected. The solution 30 of molten salts (also called molten salt bath) is in contact with the wall 21 of the element 20. Solution 30 comprises a mixture of molten chlorides. This may be, for example, a binary or ternary mixture of alkali and alkaline earth metal chlorides, generally used as a heat transfer fluid, or a mixture containing actinide chlorides, generally used for nuclear applications. The molten salts used to form the first type of mixture are preferably chosen from LiCl, NaCl, KCl, MgCl, CaCl, and BaCl. For a binary mixture, one could choose, for example, a NaCl-MgCl mixture, or a KCl-MgCl mixture, while for a ternary mixture, one could choose, for example, a NaCl-KCl-MgCl mixture. In the second case, for application in a refractive index, one or more compounds of the formula AcCl, where Ac is an actinide, will preferably be added to one of the binary mixtures described above. Preferably, Ac is Pu, Am, U, or Cm. Even more preferably, Pu, Cm, or Am will be chosen, or even more preferably, Pu or Am. For example, a mixture of NaCl-MgCl-PuCl; or a mixture of NaCl-MgCl-AmCl; or a mixture of NaCl-MgCl-PuCl-AmCl. As a chemical analogue of NaCl-MgCl,-PuCl;, it is possible to use the mixture NaCl-MgCl,-CeCl. The compound AcCl; preferably represents between 4.5% and 20% molar of the mixture of molten salts. For example, a mixture of NaCl-MgCl-CeCl (57 / 30 / 13 or 61 / 34 / 5) can be used as a simulant for the actinide converter (NaCl-MgCl-PuCl). The ratios are molar ratios given relative to the solution. Solution 30 also includes T1?* ions and Ti** ions. The choice of the ratio of the concentrations of Ti” ions and Ti**+ ions is essential to protect the wall material 21 in contact with solution 30. The Ti** / Ti?* ratio is between 20 / 80 and 70 / 30, for example between 30 / 70 and 70 / 30 or between 20 / 80 and 50 / 50, and preferably between 30 / 70 and 50 / 50. The potential of solution 30 depends on both the ratio T1*+ / T17* and the temp- Temperature according to Nernst's law (equation (1)): 1 rich (1) TiCk, D ' ErP+ 4.724 = ETP* 4uT2+ + RFnan eu With : E is the potential in volts. E° the standard potential of the couple in Volts, R is the constancy of ideal gases, T is the temperature in Kelvin. n is the number of electrons exchanged, F is the Faraday constant, aTiCl, the chemical activity of the oxidant, aTiCl, the chemical activity of the reducing agent. Preferably, in the solution, the concentration of titanium is 0.1 to 3 molar percent relative to the total concentration of the solution. During the operation of device 10, solution 30 is preferably at a temperature between 450°C and 700°C and preferably between 550°C and 650°C. For example, at a temperature of 600°C, with a Ti** / Ti2+ ratio of 20 / 80, the potential of solution 30 is approximately -2V vs. Cl, / Cl. Such a potential is sufficiently negative to prevent the dissolution of chromium observed at around -1.75V vs. Cl, / Cl. This potential also prevents the dissolution of other elements such as iron, nickel, and molybdenum, which have potentials higher than that of chromium. For a temperature of 600°C, with a Ti* / T17* ratio of 50 / 50, the potential of solution 30 is approximately -1.9V vs CL / CI, which is also sufficient to prevent the dissolution of the chromium present in the metal alloy of the wall. With a ratio of 70 / 30, the calculated potential of the solution is -1.83V vs CL / CI at 600°C, which is also sufficient to protect the chromium. Thus, for higher temperatures (for example, for a temperature of 700°C), the potentials will be higher. It is possible to use this device 10 in the presence of an inert atmosphere (argon or helium in particular). It is also advantageous to control the purity of the inputs associated with the sealing of the heat transfer fluid circuit. Solution 30, and in particular the mixture of molten salts, are purified beforehand before being used. The process for preparing a device 10 as defined above comprises the following steps: - provide a device 10 comprising a wall 21 made of steel or a base alloy nickel containing chromium, the wall 21 being in contact with a solution 30 comprising a mixture of molten chloride salts, - Immerse a piece of titanium metal in the solution for a sufficient time to obtain Ti₂ ions and Ti₂** ions, with the Ti₂** / Ti₂* ratio being between 20 / 80 and 70 / 30, - possibly, add to the solution ions of a metal having a potential lower than the dissolution of the chromium of the metal alloy, thereby accelerating the formation of Ti?*+ ions and Ti*+ ions. The addition of titanium metal to a solution containing a mixture of molten chloride salts leads to the oxidation of titanium and the formation of Ti⁺ and Ti⁺ ions. For example, a molten chloride solution (NaCl-MgCl⁻,-CeCl⁻) or NaCl-MgCl⁻,-PuCl⁻, in the absence of titanium, has an equilibrium potential between -1.2V and -1V vs. Cl / Cl. Therefore, when titanium is added, it is oxidized, and the equilibrium potential of the solution decreases until it stabilizes around -2V vs. Cl / Cl for a Ti⁺ / Ti⁺ ratio of 20 / 80. Adding ions to the solution accelerates the formation of Ti** and Ti# ions via redox reactions. Any element with a potential lower than the dissolution rate of chromium in the metal alloy can be used. Preferably, chromium, aluminum, or zirconium are suitable. The duration for which the titanium part is immersed in the solution depends on several parameters, including the surface area of ​​the immersed portion of the titanium part, the volume of the solution, agitation, circulation speeds, etc. A person skilled in the art will choose the duration based on the device's dimensions and in order to obtain the desired Ti* / Ti?+ ratio. For example, a sufficient duration might be from 1 to 10 hours. We will now describe in more detail a method for measuring the ratio Ti** / Ti?+ in a device 10 as defined previously. The measurement process includes the following steps: - provide a device 10 comprising a wall 21 made of steel or a nickel-based alloy comprising chromium, the wall 21 being in contact with a solution 30 comprising a mixture of molten chloride salts, Ti₂*+ ions and Ti*+ ions, - perform square wave voltammetry in the solution to obtain reduction and oxidation curves representing the differential current as a function of the potential, - determine the proportion of T1** ions and Ti** ions, by integrating the area under the reduction and oxidation curves. Knowing the ratio T1*+ / Ti°*, and using Nernst's law, it is possible to calculate the potential of solution 30. The apparent standard potential incorporating the coefficients- activity fructs were evaluated at -1.89V vs CI / CI. This measurement method is particularly advantageous because the shape of the curves allows for a quick and easy determination of the specific case. Indeed, when the oxidation curve shows an almost complete Gaussian distribution, the Ti₂O₅ content is well above 50%. At a 50 / 50 ratio, the oxidation curve begins at the peak and only shows the right-hand side of the Gaussian. The same applies to the reduction side. We will now describe a method for measuring the potential of a 30 solution of molten salts. The measurement process comprises the following successive steps: a) immerse an anode, a cathode and a reference electrode in solution 30 comprising a mixture of molten chloride salts, and optionally Ti₂O₇²⁺ ions and Ti₂O₇²⁺ ions, the mixture of molten salts comprising at least MgCl₂, b) measure the potential E, between the cathode and the zero or near-zero current reference, c) to impose a current between the anode and the cathode, in order to deposit metallic magnesium on the cathode, d) measure the potential Ez between the reference and the cathode at zero current, thereby obtaining the value of the couple Mg? / Mg°, e) calculate the potential of solution E, according to Eso = E2-E1. The electrodes are advantageously made of the same material that does not oxidize in the solution. The electrodes (anodes and cathodes) are advantageously made of tungsten. The reference electrode is advantageously a metal wire that does not oxidize in the solution: this electrode is called a dynamic or ephemeral electrode. Preferably, a molybdenum reference electrode is chosen, or even more preferably, a tungsten electrode. In step b), the potential between the cathode and the reference is measured in solution 30 with zero current. The potential is almost zero (typically less than 50 mV, in particular less than 5 mV) since the electrodes are in the same metallic conductor. Before step c), it is also possible to measure the potential between the anode and the zero-current reference. The potential will also be almost zero. In step c), a constant current pulse is applied, for example for a duration of between 5 s and 20 s, for example 10 s. During this step, the cathode potential becomes fixed at the reduction potential of Mg* to magnesium metal, while the anode potential evolves towards the oxidation potential of chlorides to chlorine gas. Preferably, the anode / cathode surface area ratio is between 1 and 3, ideally 2, and the Jcu is between 0.05 A / cm² and 0.8 A / cm². In step d), the potentials of the anode and cathode are measured in the solution 30 at zero current relative to the reference. The cathode potential temporarily takes the value of the electrochemical couple Mg?+ / Mg®. The cathode potential after the pulse does not depend on the applied current. According to the Nernst equation, it depends only on the Mg*+ concentration. However, due to the high Mg⁺ concentration in the solution (logarithmic term), the cathode potential is also relatively independent of variations in the Mg* concentration. The time during which the potential of the Mg** / Mg° couple can be observed is dependent on the current applied: the greater the current, the greater the quantity of Mg metal deposited and the longer the potential of the Mg* / Mg® couple can be used as a reference value. In step e), the potential of solution 30 is obtained by comparing the potential values ​​between step b) and step d) when magnesium is still present on the cathode. In the case of a ternary mixture of molten salts including PuCl₂, plutonium will be formed instead of magnesium. In step c), metallic plutonium will be deposited on the cathode and in step d), the value of the Pu* / Pu° couple is obtained. Examples | if: limiting of implementation: Study of the anodic dissolution of chromium, iron and nickel: The anodic dissolution of chromium, iron, and nickel was first studied in a NaCl-MgCl-CeC solution (57 / 30 / 13) at 600°C. This salt is used as a simulant for the actinide converter (NaCl-MgCl-PuCl). The voltammograms obtained for these metals are shown in [Fig. 2]. Several observations can be made by comparing the current-potential plots obtained with the different metals: - Tungsten is the material that best resists oxidation; its anodic limit is the oxidation of chlorides to dichlorine, and its cathodic limit is the reduction of Mg to magnesium metal. - The dissolution potentials of chromium, iron, and nickel confirm that chromium is the least stable element (most negative potential), followed by iron and nickel, which is consistent with the sequence predicted by the thermodynamics of pure substances. - there is a wide potential range (noted Z on [Fig.2]) between -1.75 and -2.75 V vs E(CL / CT) where effective redox control can be operated to limit the dissolution of steels. The difficulty, therefore, lies not only in obtaining this potential range in a highly corrosive solution that generally operates at high temperatures (typically above 450°C, or even above 550°C, and more preferably between 600°C and 700°C), but also in being able to control and... maintain. Study of the Ti* / Ti+ buffer electrochemical couple: Titanium was studied in a solution of molten salts NaCl-MgCl--CeCl; at 600°C. Depending on the potential of the solution, titanium can be metallic, or dissolved in the salt in the form of divalent, trivalent and tetravalent complexes ([Fig.3]). The wave observed before the dichlorine wall corresponds to the oxidation of Ti** to Ti*. The reduction wave around -1.6 V corresponds to the reduction of Ti* to Ti?+. The reduction peak of Ti** in metallic titanium is observed around -2.35 V, it is associated with its fine oxidation peak characteristic of the dissolution of the electro-deposited metal. The equilibrium potential of a solution of molten alkali and alkaline earth chlorides in the absence of titanium is between -1 and -1.2 V. In the presence of metallic Ti, Ti* is reduced to Ti?+, The Ti** and Ti* ions are stable and therefore coexist in the solution according to the equilibrium: 2 Ti**+ + Ti € 3 Ti*+ In this case, potential control is ensured by the couple T1*#T12+, The presence of metallic titanium leads to a decrease in the potential of the solution to a stable value around -2 V ([Fig.4]). Once the reduction of Ti** to Ti** has been obtained, the titanium metal plate is removed from the solution and the equilibrium potential of the solution is measured as a function of time: It rises very slowly and linearly until it reaches its original value before the reduction phase after several hundred hours ([Fig.5]). The Nernst equation was then verified using the electro-analytical technique of square wave voltammetry (SWV) to determine the proportions of Ti* and Ti⁻* as a function of time in the molten salt solution. The proportion of the two species was estimated by integrating the areas under the curves of Ti* reduction to Ti⁻* and Ti⁻ oxidation to Ti* measured regularly over time after the removal of the titanium plate ([Fig. 6]). The curve with crosses in [Fig. 6] represents a solution where titanium is present in more than 90% of the form Ti*⁺, while the other curve shows a 50 / 50% mixture of Ti⁻* and Ti₃⁻. The evolution of the potential as a function of the logarithm of the ratio of Ti** and Ti* concentrations obtained by SWV is linear, in accordance with the Nernst equation ([Fig. 7]). The experimental slope (RT / nF) is 0.078, corresponding to an electron count of 0.97, which is consistent with the Ti* / Ti** electronic transition. Verification of the Nernst equation allows us to conclude that the solution potential is indeed controlled by the presence in the solution of molten divalent and trivalent titanium salts. The most negative potential corresponds to a Ti*+ / T1* ratio of 20% / 80%, the potential the highest a ratio of 70% / 30%. Immersion of 316L steel in NaCl-MgCl-CeCl; without redox control for one week: A 316L steel plate was immersed in NaCl-MgCl₂-CeCl₂ solution for one week (168 hours) at 600°C, without redox potential control. The plate was held in the solution by a molybdenum wire, with the molybdenum-material contact kept outside the molten salt. The tests were performed in an inert glove box under argon. The molten salt was in an alumina crucible, itself placed inside a sealed quartz cell swept by high-purity argon (Alphagaz₂). After immersion for one week, the submerged portion of the sample is sectioned and analyzed by SEM-EDX ([Fig. 8]). Analysis of the section shows a classic corrosion profile in these environments: - Intergranular corrosion, - Attacked depth of approximately 100µm, - Beyond this depth, the mass composition of 316 steel is found. The corresponding EDX analysis confirms the loss of Cr and Fe. Immersion of 316L steel and Inconel® 625 in NaCl-MgCl-CeCl with redox control by the Ti** / Ti couple for one week: Immersion of 316L steel or Inconel® 625 plates was carried out in NaCl-MgCl-CeCl₂, containing the Ti⁺ / Ti₂⁺ pair, at 600°C for one week (168 hours) and one month (720 hours). The solution potential was first adjusted by immersing a metallic titanium plate to obtain titanium in solution predominantly in the form of Ti₂ (-80% Ti₂⁺ and 20% Ti). The resulting potential of approximately -2 V vs. Cl / Cl₂ is sufficiently negative to prevent the dissolution of chromium observed at approximately -1.75 V vs. Cl / Cl₂. The SEM image of the 316L sample after immersion for 168 hours with redox control reveals that the 316L steel was protected during this test ([Fig. 9]). Deposits are observed on the surface of the sample. EDX analysis indicates material attack over a 1 to 2 µm layer at the molten salt / metal interface ([Fig. 10]). This layer is rich in Ti. Molybdenum is also present, likely originating from contamination of the measurement electrodes. The observations of the Inconel® 625 sample after immersion are similar to those made for 316L steel. The coupon is not corroded or only slightly corroded ([Fig.11]). EDX analysis ([Fig. 12]) also shows a deposit of titanium and aluminum (contamination from the alumina crucible) alloyed with nickel. The layer thickness is slightly greater (-3-4 µm) than that observed on 316 steel (-2 µm). Under the reducing conditions imposed by the T1* / T17* redox buffer, the deposition of Metals on the surface appear to form more easily with Inconel®. Indeed, this material, being richer in nickel, can form numerous alloys within these temperature ranges. Immersion of 316L and Inconel® 625 in NaCl-MgCl,-CeCl; with redox control by the Ti* / Ti pair for one month (720 h): A 316L steel plate was immersed at 600°C for one month. The sample was cut to allow for the characterization of a submerged portion and a portion positioned above the salt. As before, the sample showed minimal corrosion. A thin layer of Ti was observed at the extreme periphery of the sample. Comparison of the submerged and non-submerged portions of the sample did not show more pronounced corrosion in the submerged portion and ruled out uniform corrosion of the sample in the portion in contact with the solution. EDX analysis of the sample's periphery confirms that, as before, the sample is attacked to a depth of -2 µm, resulting in a loss of chromium, iron, and nickel, and an enrichment in titanium. This corrosion thickness, identical for immersions of one week and one month, suggests that the corrosion layer does not increase over time. After immersing the Inconel® 625 plate at 600°C for one month, the sample shows minimal corrosion, and a layer of titanium alloyed with nickel forms around the periphery of the Inconel® 625 samples. The corrosion layer thickness is approximately 3 µm, as in the one-week test, suggesting that corrosion progresses slowly over time, similar to the case with 316 steel. Measurement of the solution potential: Corrosion control using a suitable buffer couple is effective but requires a method for measuring the chemical potential of the solution. An electrochemical technique has been developed to monitor the chemical potential of the solution. It is based on the principle of a so-called dynamic reference electrode. The method consists of creating a metallic deposit on an inert electrical conductor (tungsten or molybdenum, for example) by applying a current pulse. In a salt containing a high proportion of MgCl, such as NaCl-MgCl, KCl-MgCl, or NaCl-KCl-MgCl, the passage of current leads to the production of metallic magnesium at the cathode. When the current is stopped, the potential of the cathode takes the Nernst equation for the Mg*+ / ME° couple and thus constitutes a reference electrode generated in situ in the saline environment.The potential value of this reference electrode is stable long enough to serve as a reference electrode; the value is also particularly reproducible due to the high MgCl content. To perform the measurement, a three-electrode setup is used. The cathode and anode are made of tungsten, and the quasi-reference electrode is made of molybdenum or... tungsten is preferred. The method comprises three distinct phases: - Phase 1: Measurement of the zero-current potential between the two electrodes (anode vs. reference and cathode vs. reference), - Phase 2: Application of a current over a short period (< 10 s) - phase 3: Measurement of the zero current potential between the two electrodes (anode vs reference and cathode vs reference). These three phases are illustrated in [Fig. 13]. The solution used is a NaCl-MgCl-CeCl solution at 600 °C. Ea represents the anodic potential and Ec represents the cathodic potential. During phase 1 (labeled 1 on the graph), the anode and cathode potentials are measured with zero current relative to the reference; the values ​​are close to zero (< 50 mV). During phase 2 (labeled 2 on the graph), a 10-second constant current pulse is applied. The cathode potential becomes fixed at the reduction point of Mg²⁺ to magnesium metal, while the anode potential becomes fixed at the oxidation point of chlorides to chlorine gas. Several currents (20 mA, 40 mA, and 60 mA) were tested. During phase 3 (labeled 3 on the graph), the anode and cathode potentials are measured with zero current. The potential of the cathode temporarily takes the value of the electrochemical couple Mg** / Mg° before returning to its original value.The anode potential, after the current is stopped, quickly returns to its initial potential. The cathode potential after the pulse does not depend on the applied current, as shown in [Fig. 13]. According to the Nernst equation, it depends only on the Mg concentration. It is also relatively independent of variations in Mg concentration due to its high concentration in the solution (logarithmic term). The time during which the potential of the Mg₂ / Mg₂ couple can be observed depends on the applied current. The higher the current, the greater the amount of Mg metal deposited, and the longer the potential of the Mg₂ / Mg₂ couple can be used as a reference value. The potential of the solution is measured by comparing the potential values ​​between phase 1 (close to 0) and phase 2 (when the Mg* / Mg° couple is stable). In the case shown in [Fig. 13], the potential measurement is approximately -1.5 V, indicating that the potential of the tungsten electrode in the molten salt is 1.5 V higher than that of the dynamic reference electrode Mg* / Mg°. This method was repeated in a NaCl-MgCl-CeCl solution, including the Ti**+ / Ti?+ buffer pair at 600 °C ([Fig. 14]). The characteristics of the curves obtained are identical to those of [Fig. 13], the major difference being the value of the measured potential of the molten salt solution: the potential of the molten salts containing titanium is now only about 0.5 V higher than that of the electrode. dynamic Mg? / Mg°. The presence of titanium in the solution therefore lowers its potential by about 1 V, thus bringing the potential of the solution and therefore of the electrically conductive materials in it into an immunity range which strongly limits the corrosion of the metals. The oxidizing nature of molten salt baths tends to convert the reduced species of the redox buffer Ti₂ to Ti⁺ over time. Consequently, the Nernst potential of the salt tends to increase over time. The proposed measurement allows this potential increase to be detected. The solution potential can be lowered within a few hours by immersing metallic titanium in the solution to convert the Ti⁺ to Ti⁺.

Claims

Demands

1. Device (10) comprising: - an element (20) configured to contain or circulate a solution, the element (20) having a wall (21) made of steel or an alloy based nickel containing chromium - a solution (30), in contact with the wall (21), comprising a mixture of molten chloride salts, characterized in that the solution further comprises Ti?* ions and Ti** ions, the Ti* / Ti+ concentration ratio being between 20 / 80 and 70 / 30.

2. Device according to claim 1, characterized in that the element (20) is a primary circuit of a nuclear fission reactor or a se- circuit secondary of a nuclear fission reactor.

3. Device according to claim 1, characterized in that the element (20) is a reservoir or fluidic circuit of a solar power plant concentration.

4. Device according to any one of claims 1 to 3, characterized in that element (20) is a secondary circuit of a reactor of nuclear fission, a tank or a fluid circuit of a power plant concentrated solar power and in that the mixture of chloride salts molten is a binary or ternary mixture obtained from chlorides molten materials selected from LiCl, NaCl, KCl, MgCl, CaCl, and BaCl...

5. Device according to claim 4, characterized in that the mixture of Molten chloride salts is a mixture of NaCl-MgCl, KCl-MgCl; or NaCl-KCl-MgCl.

6. Device according to claim 2, characterized in that the element (20) is a primary circuit of a nuclear fission reactor and in that the a mixture of molten chloride salts comprises at least one chloride of actinide.

7. Device according to claim 6, characterized in that the mixture of Molten chloride salts is a mixture of NaCl, MgCl, and PuCl; or NaCl- MgCl,-PuCl;-AmCl,.

8. Device according to any one of the preceding claims, characterized in that that solution (30) is at a temperature between 450°C and '700°C and preferably between 550°C and 650°C.

9. Device according to any one of the preceding claims, ca- characterized in that the wall (21) is made of stainless steel, for example in 316L steel, or a nickel-chromium-molybdenum alloy, for example NiCr22Mo9Nb.

10. Device according to any one of the preceding claims, ca- characterized in that the Ti** / T17* ratio is between 30 / 70 and 50 / 50.

11. | Method for preparing a device (10) as defined in one of the Claims 1 to 10, comprising the following steps: - provide a device (10) comprising an element (20) configured to to contain or circulate a solution, the element (21) having a wall (21) of steel or of a nickel-based alloy comprising chromium, and a solution (30), in contact with the wall (21), comprising a mixture of molten chloride salts, - immerse titanium metal in solution (30) for a duration sufficient to have Ti*+ ions and Ti** ions, the ratio Ti**+ / Ti?+ being between 20 / 80 and 70 / 30, - optionally, add to solution (30) ions of a metal having a potential lower than the dissolution of chromium from the metallic alloy, thereby accelerating the formation of Ti⁺ and Ti⁺ ions.

12. Method for measuring the potential of a solution (30) of a device (10) as defined in any one of claims 1 to 10, the measurement method including the following successive steps: - provide a device (10) comprising an element (20) configured to to contain or circulate a solution (30), the element (21) having a wall (21) made of steel or a nickel-based alloy comprising chromium, and a solution (30), in contact with the wall (21), the solution (30) comprising a mixture of molten chloride salts, Ti** ions and Ti** ions, the Ti* / Ti7* ratio being between 20 / 80 and 70 / 30, the mixture of molten salts comprising at least MgCl₂, and possibly PuCl;, - immerse an anode, a cathode and a reference electrode in the solution (30), the anode, cathode and reference electrode being in one same material, advantageously tungsten, - measure the potential E, between the cathode and the zero-current reference or virtually zero, - to impose a current between the anode and the cathode, in order to deposit metallic magnesium on the cathode, or to deposit plutonium metallic particles on the cathode, if applicable. - measure the potential E between the cathode and the zero-current reference, whereby we obtain the value of the Mg°* / Mg° or Pu*+ / Pu° couple, the if applicable, - calculate the solution potential E,, according to E,0 =