Molten fluoride salts can be used as the fuel and coolant for molten salt reactors (MSR) and electrolytes for spent nuclear fuel (SNF) reprocessing. 46.5%LiF-11.5%NaF-42%KF (FLiNaK) melt is the prospective media for these purposes due to their desirable thermophysical and nuclear properties. Finding construction materials with sufficient corrosion and mechanical resistance is the most challenging task for practical implementation of MSR concept. In the present study the corrosion and mechanical properties of different types of construction materials were investigated. The materials included various low carbon Ni-Cr-Fe-Mo, Ni-Cr-Mo, and Ni-Mo alloys, and metals with relatively positive electrode potentials. The corrosion experiments were performed in FLiNaK melt at different temperatures (from 550 to 750 °C) in the specially designed stainless steel cells under high-purity argon atmosphere. Corrosion properties of studied materials were investigated under static conditions, and the duration of each test was 100 h to enable the comparison of the experimental data. In a special series of experiments fluorides of typical fissile nuclides and fission products were added to the salt electrolyte to estimate the influence of the red-ox potential on the corrosion resistance of the materials. Corrosion rates were determined from the weight loss measurements and chemical analysis of quenched melts. Surface and microstructure of corroded samples was examined by various microscopic techniques. Mechanical properties of investigated materials were also studied at the ambient temperature and at 600 °C. Advantages and limitations of different types of construction materials were evaluated on the basis of data obtained. The effect of temperature on corrosion and mechanical properties of the studied materials were determined. Possible mechanisms of corrosion of various materials in fluoride melts were proposed. The alloys based on Ni-Cr-Mo and Ni-Mo systems, and molybdenum and its alloys were selected for further long-time tests under dynamic conditions to determine the resource of materials in contact with molten fluorides for MSR and SNF recycling technologies.
not Available.
High-temperature spectroelectrochemestry was applied to study corrosion of various types of stainless steel in molten salts. The electronic absorption spectra of products of anodic dissolution of stainless steel major components (iron, chromium, nickel, molybdenum, manganese, titanium) were measured in NaCl-KCl melt at 750 °C. The effectiveness and limitations of applying spectroscopic method for studying alloys corrosion was demonstrated on example of anodic dissolution of AISI 316L, 316Ti and 321 austenitic steels. The major corrosion products of steels are iron, manganese and chromium species. Prolongation of anodic dissolution leads to increasing chromium-to-iron ratio in the melt. Titanium in steels forms very stable carbonitride species that aren't dissolved during anodic oxidation.
No abstract is provided for this article.
The corrosion resistance of AISI 316L, 12Kh18N10T (analogue of AISI 321) austenitic stainless steels and their basic components (metallic iron, nickel, chromium, molybdenum) was investigated in the melts based on the NaCl–KCl equimolar mixture at 750 ºC. Austenitic stainless steels underwent high temperature sensitization, and it was shown that this phenomenon determined the intergranular type of the steel corrosion. It was demonstrated that the presence of uranium ions in the electrolyte intensified corrosion processes. Metallic molybdenum demonstrated the highest corrosion resistance among the construction materials studied.
Molten salts, particularly alkali halides, can be used for developing new methods of rare and refractory metals production and pyrochemical reprocessing spent nuclear fuels. Molten salt nuclear reactor now again attracts attention as one of the prospective power generating systems. Wide scale application of fused salts in various areas of technology is hampered by the problem of finding suitable corrosion-resistant materials. Corrosion of metals in molten salts is electrochemical in nature, and therefore the most suitable constructive materials for these media are electropositive refractory metals, for example, niobium. High cost of these materials and the difficulties associated with the mechanical processing and welding limit their application. A way to overcome this problem is forming a protective layer of a refractory metal on the base material. In the present work a novel method of forming niobium-based coatings by currentless deposition in molten salts is proposed. Nicrofer 6020, Nicrofer 6616, Hasteloy N, and KhN65MVU nickel based alloys and metallic nickel were chosen as the base materials for the present investigation. Corrosion tests were carried out in NaCl–KCl–NbCl n (n=3.5 or 5 wt. % niobium) and NaCl–KCl–UCl 3 (1 wt. % uranium) melts at 750 o C. Duration of the corrosion tests was 30 h. Contacting nickel based alloys with NaCl–KCl–NbCl n melt leads to increase of the sample mass due to the formation of an alloy between nickel (the base constituent of the studied alloys) and niobium on the sample surface, Figure. Nickel-niobium alloy is formed as a result of disproportionation of niobium (III) ions, 4Nb 3+ + 3Ni alloy → Ni 3 Nb + 3Nb 4+ . Nb(IV) ions produced in the same reaction can be reduced to Nb(III) by niobium metal placed in the melt. Formation of a Ni-Nb alloy layer on the sample surface decreases the rate of the base alloy corrosion. This Ni-Nb phase forms a continuous layer on the sample’s surface and, therefore, this phase can protect the studied alloy from further corrosion. Such protection would be very useful for construction materials used for niobium electrorefining in molten chlorides; especially since currently the problem of finding suitable materials is one of the limiting factors in producing high-purity niobium. The samples of Nicrofer 6616 and 6020 alloys with the formed niobium coatings were tested in NaCl–KCl–UCl 3 (around 1 wt.% U) melts (30 h exposure at 750 ºС). Here the weight of the samples covered with Ni 3 Nb phase decreased and the corrosion rates were comparable with those obtained earlier for uncoated samples in uranium-containing melts. Figure. Microstructure of Hastelloy N (a), KhN65MVU (b), Nicrofer 6616 (с) and Nicrofer 6020 (d) alloy samples after 30 h contact with NaCl–KCl–NbCl n melts (n=3.5, 5 wt. % Nb) at 750 ºС. Figure 1
The corrosion of austenitic 12Kh18N10T, 10Kh17N13M2T, and 03Kh17N14M3 steels and their components (Cr, Fe, Ni, Mo) in NaCl-KCl-VCl2 melts with 5 wt % V at
Growing application of niobium and its alloys in electronics, chemical technology, aerospace and nuclear industry leads to increasing world demands for high-purity metallic niobium. Electrolytic refining in molten chlorides is one of the most prospective methods of high-purity niobium production. Prolonged contact of niobium-containing melts with metallic niobium can be employed for stabilizing necessary niobium species in the electrolytic baths. A complex of independent physical and chemical methods of analysis was used to study the mechanism of niobium interaction with niobium chloro-species in fused salts. In the current study, we found that upon contacting metallic niobium with a melt containing higher oxidation state niobium ions, the average oxidation state of niobium in the salt phase, mass of the metal and potential difference between niobium and an indicator electrodes decreases. After a certain period of time, these parameters tend to achieve certain constant values. This moment corresponds to the stationary state of the system. Analysis of the experimental data showed that the mixture of niobium (III) and (IV) ions is the final product of interaction of niobium-containing chloride melts with niobium metal and therefore, a mixture of niobium ions in two oxidation states is present in equilibrium with the metal. The average oxidation state of niobium in the obtained melts (after contacting with Nb metal) increases by increasing the niobium concentration. This observation correlates with the results of cathodic current efficiency measurements during niobium electrorefining. Thermodynamic calculations were performed using niobium red-ox and equilibrium electrode potentials in NaCl-KCl based melts at 700 0C. The results confirmed that equilibrium oxidation state of niobium in the fused chloride can increase by increasing the total concentration of niobium in the melt.
The reaction of uranium dioxide with excess hydrogen chloride in alkali chloride melts (LiCl, 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl) has been studied between 450 and 750 ◦ C, and the reaction products were characterized by electronic absorption and X-ray absorption spectroscopy. Uranium( V), [UO 2 Cl 4 ] 3− , and uranium(IV), [UCl 6 ] 2− , species were formed. They depended upon the temperature and the radius of the alkali cations present. Uranium(V) ions predominated in melts with small cations (LiCl and 3LiCl-2KCl).
Products of anodic dissolution of uranium metal and the cathodic reduction of uranium(IV) ions in a molten eutectic mixture of lithium and potassium chlori
Solubility of several transition metal chlorides (NiCl 2 , CrCl 2 , MoCl 3 , FeCl 2 ) was measured in KCl-AlCl 3 based melts. It was found that the solubility of studied metal chlorides depends on K : Al mole ratio. MoCl 3 solubility decreases with increasing AlCl 3 content. Solubility of CrCl 2 and FeCl 2 reaches maximum at K : Al ratio of 1 and decreases when this ratio either de-creases or increases. The dependence of NiCl 2 solubility on K : Al mole ratio is V-shaped with the maximum near 0.9-0.95. The effect of temperature on solubility of transition metal chlorides in KCl-AlCl3 melts was also investigated. Increasing temperature does not alter the character of «solubility – K : Al mole ratio» dependences.
