The Metal-Molten Salt Interface

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

In conclusion, it appears that few metal-molten salt systems behave in the ideally polarizable sense generally associated with the mercury/aqueous solution interface at 298 K. Possible exceptions include some noble liquid metal/melt systems such as mercury/molten nitrates and lead/molten halides at low temperatures (<773 K), but only when the molten electrolyte is extensively purified. Otherwise, systems need to be analyzed as complex impedances, using ac or pulse techniques, to determine whether the minimum interfacial capacitance is affected by extensive factors, leading to parallel pseudocapacitances and Faradaic components. The range of potentials and measuring frequencies for which the interface approaches ideally polarizable behavior also needs to be established. It now seems clear that the multilayer ionic model of charge distribution at the metal/melt interface is more pertinent to molten media than the familiar double layer associated with aqueous solutions. However, the quantitative theories derived for the former model will have to be revised if it is confirmed that the interfacial capacitance is, indeed, independent of temperature in the ideally polarizable region. 

Chemical reaction This involves the formation of distinct compounds by reaction between the solid metal and the fused metal or salt. If such compounds form an adherent, continuous layer at the interface they tend to inhibit continuation of the reaction. If, however, they are non-adherent or soluble in the molten phase, no protection will be offered. In some instances, the compounds form in the matrix of the alloy, for example as grain-boundary intermetallic compound, and result in harmful liquid metal embrittlement (LME) although no corrosion loss can be observed. 

With metals, semiconductors, and insulators as possible electrode materials, and solutions, molten salts, and solid electrolytes as ionic conductors, there is a fair number of different classes of electrochemical interfaces. However, not all of these are equally important The majority of contemporary electrochemical investigations is carried out at metal-solution or at semiconductor-solution interfaces. We shall focus on these two cases, and consider some of the others briefly. 

It is possible that this theory can be adapted to explain molten metal-water thermal explosions although many needed data are still unavailable. One might presume that, at the molten metal-wet surface interface, there is some chemical reaction. Possibly that of the metal plus water or metal plus surface to lead to localized formation of salt solutions. These may then superheat until homogeneous nucleation occurs. The local temperature and pressure would then be predicted to be far in excess of the critical point of pure water (220 bar, 647 K) and a sharp, local explosion could then result. Fragmentation or subsequent other superheat explosions would then lead to the full-scale event. 

Electrode surface area — The area of the - interface between the ionically conducting phase (electrolyte solution, molten salt electrolyte etc.) and the electronically conducting phase (metal, semiconductor etc.) ... 

As Schmickler states [3], Electrochemistiy is the study of structures and processes at the interface between an electronic conductor (the electrode) and an ionic conductor (the electrolyte) or at the interface between two electrolytes . The electrode/electrolyte or electrolyte/electrolyte interface is the region whose properties differ from the two adjoining phases, and/or the place where reactant adsorption and electrochemical reactions occur. Commonly, it is recognized as the interface between an electronic conductor (e.g., metals and semiconductors) and an ionic conductor (e.g., electrolyte solutions, molten salts, and solid electrolytes), known as an electrochemical interface. In a narrow region of an electrode/electrolyte interface, an electrical double layer (EDL) exists. The EDL is believed to be extremely thin, and is an important component of the interface. 

Electrochemical reactions involve the transfer of electric charge across an interface consisting of an electrode (metal or semiconductor) in contact with an ionic conductor (electrolyte solution, molten salt, or solid electrolyte). The electrode material—ionic conductor interface exhibits a high electric capacitance. For instance, its value for a spherical gold surface in 1 M NaC104 aqueous solution is on the order of 10 F, in contrast with the capacitance of the same... 

Hot corrosion refers to corrosion between a metal-oxide and a molten salt deposit. It occurs at the solid-gas interface. Molten salts are extremely corrosive and their presence increases the rate of corrosion by two orders of magnitude when compared to high-temperature corrosion at similar temperatures and conditions [27—29]. They act as solvents, preventing the formation of a stable oxide, or they chemically react with the oxide layers. By transporting through, the salts may damage the protective oxide layers. Two different types of hot corrosion exist, namely. Type I and II. 

---