Molten salts Redox potential

Figure 5.1 Behavior of the molten salt redox potential measured relative to dynamic Be reference electrode versus specimen exposure time in Li,Na,Be/F corrosion loop NC 1 [59].

Results of a 1200h loop corrosion experiment [59] with online redox potential measurement demonstrated that high-temperature operations with molten 15 LiF-58 NaF-27 BeF2 (mol%) salt are feasible using carefully purified molten salts and loop internals. In an estabhshed interval of salt redox potential 1.25—1.33 V relative to Be reference electrode, corrosion is characterized by uniform loss of weight from a surface of samples with low rate. Under such exposure salt contained respectively less than (in mass%) Ni—0.004 Fe—0.002 Cr—0.002. Specimens of HN80M-V1 and HN80MTY alloys from hot leg of the loop exposed at temperatures from 620°C to... 

The structural materials retained for the MSR container were special Ni-Mo alloys with a low concentration of Cr. The composition of the reference Hastelloy N was optimized by ORNL researchers for corrosion resistance (both in a low-oxygen gas atmosphere and in molten fluorides), irradiation resistance, and high-temperature mechanical properties. It has been also demonstrated that the salt redox potential is a key parameter in the corrosion phenomena of stmctural materials of MSRs. The chemical corrosion can be controlled by a redox buffers (e.g., UF4/UF3), which control the potential of the fuel and coolant salts. 

Under certain conditions, it will be impossible for the metal and the melt to come to equilibrium and continuous corrosion will occur (case 2) this is often the case when metals are in contact with molten salts in practice. There are two main possibilities first, the redox potential of the melt may be prevented from falling, either because it is in contact with an external oxidising environment (such as an air atmosphere) or because the conditions cause the products of its reduction to be continually removed (e.g. distillation of metallic sodium and condensation on to a colder part of the system) second, the electrode potential of the metal may be prevented from rising (for instance, if the corrosion product of the metal is volatile). In addition, equilibrium may not be possible when there is a temperature gradient in the system or when alloys are involved, but these cases will be considered in detail later. Rates of corrosion under conditions where equilibrium cannot be reached are controlled by diffusion and interphase mass transfer of oxidising species and/or corrosion products geometry of the system will be a determining factor. 

A positive standard cell potential tells you that the cathode is at a higher potential than the anode, and the reaction is therefore spontaneous. What do you do with a cell that has a negative " gii Electrochemical cells that rely on such nonspontaneous reactions cire called electrolytic cells. The redox reactions in electroljdic cells rely on a process called electrolysis. These reactions require that a current be passed through the solution, forcing it to split into components that then fuel the redox reaction. Such cells are created by applying a current source, such as a battery, to electrodes placed in a solution of molten salt, or salt heated until it melts. This splits the ions that make up the salt. 

Chlorostannate and chloroferrate [110] systems have been characterized but these metals are of little use for electrodeposition and hence no concerted studies have been made of their electrochemical properties. The electrochemical windows of the Lewis acidic mixtures of FeCh and SnCh have been characterized with ChCl (both in a 2 1 molar ratio) and it was found that the potential windows were similar to those predicted from the standard aqueous reduction potentials [110]. The ferric chloride system was studied by Katayama et al. for battery application [111], The redox reaction between divalent and trivalent iron species in binary and ternary molten salt systems consisting of 1-ethyl-3-methylimidazolium chloride ([EMIMJC1) with iron chlorides, FeCb and FeCl j, was investigated as possible half-cell reactions for novel rechargeable redox batteries. A reversible one-electron redox reaction was observed on a platinum electrode at 130 °C. 

Solubility of Cases The possible change in the melt character by variation of the gas phase in contact with the molten salt strongly depends on the solubility of gas species. The solubility of gases also affects the corrosive nature of the melt by changing its redox potential and oxidizing character. 

G. D. Del Cul, D. F. Williams, L. M. Toth, and J. Caja, Redox Potential of Novel Electrochemical Buffers Usefirl for Corrosion Prevention in Molten Fluorides, published in the Proc. 13" International Symposium on Molten Salts, 20 F Meeting of the Electrochemical Society, Philadelphia, PA, May 12 17, 2002 (2002). 

In the concentration range of metals with mole fraction N less than (3-5) 10 the activity of coefficients in the molten salts remains constant and they can lead to the values of the conventional formal redox potentials [6] ... 

Power sources with liquid organic and inorganic aprotic electrolytes, solid electrolytes and molten salt electrolytes are gaining in significance because by this way it is possible to use very active electrode materials, e.g. lithium with a very negative redox potential. Thus power sources of high energy density are realizable. 

The significance of the redox control to molten salt transmuter systems with uranium-free fuel is that in some cases where the fuel is, e.g., PUF3, the Pu(III)/ Pu(IV) redox couple is too oxidizing to present satisfactory redox buffered system. In this case as it was proposed by ORNL redox control could be accomplished by including an HF/H2 mixture to the inert cover gas sparge which will not only set the redox potential, but will also serve as the redox indicator if the exit HF/H2 stream is analyzed relative to inlet [44]. 

In corrosion studies [59—61] the central focus was placed on the compatibihty of Ni-based alloys with molten Li,Na,Be/F salt system as applied to primary circuit of MOSART design fueled with different compositions of actinide trilluorides from LWR spent fuel without U-Th support. These studies (see Table 5.5) included (1) compatibility test between Ni-Mo alloys and molten 15 LiF-58 NaF-27 BeF2 (mol%) salt in natural convection loop with measurement of redox potential (2) study on PuFs addition effect in molten 15 LiF-58 NaF-27 BeF2 (mol%) salt on compatibility with Ni-Mo alloys and (3) Te corrosion study for molten 15 LiF-58 NaF-27 BeF2 (mol%) salt and Ni-Mo alloys in stressed and unloaded conditions with measurement of the redox potential. Three Hastelloy N type modified alloys, particularly, HN80M-VI with 1.5% of Nb, HN80MTY with 1% of Al, and MONICR [59] with about 2% of Fe, were chosen for our study in corrosion facilities (see Tables 5.3 and 5.6). 

Thus, in the modified Lewis (or Lux-Flood) concept, pure alkali halides represent the highest degree of basicity as the solvent composition changes from alkali halide-rich to alkali halide-deficient melts, the solvent becomes acidic. Acid-base properties of molten halides may be used to explain stabilization of unusually low (or high) oxidation states, the differences in stability of the same oxidation state in related melts, and the effects on coordination observed spectrally for certain metal ions. Or, restating the idea in other terms, the redox potentials depend on melt basicity. Thus, the systematic variation of melt composition is a useful technique in the arsenal of the molten salt electrochemist who is interested in the chemistry of solute species in molten salt solvents. In this respect, it is important to note that variation of temperature may be used to serve the same purpose for example, it has been shown that in neutral chloro-aluminates C1- decreases with temperature. 

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