Diffusion fused salts

The obvious method of reducing corrosion in fused salts is to choose a system in which either the metal can come to equilibrium with the melt, or else truly protective passivity can be attained. In most cases in industry neither of these alternatives is used. In fact, fused salt baths are usually operated in air atmosphere, and the problem is the prevention of excessive corrosion. This can be done in two ways, (a) by reducing rates of ingress of oxidising species (mainly O2 and H2O) from the atmosphere, and rates of their diffusion in the melts, and (b) by keeping the oxidising power (redox potential) of the melt low by making periodic additions to the bath. 

A few cases have been recorded (Dreyer, Zeit Phys, Ghem, XLVlii. 487, 1904) where the effect of the addition agent on the crystallisation velocity is to be attributed to an alteration in the diffusion constant of the solute in the solution, thus the velocity of crystallisation of formanilide is increased and not decreased by the addition of methyl or ethyl alcohol to the fused salt. 

A more mechanistic approach, Instantaneous Normal Mode (INM) theory [122], can be used to characterize the collective modes of a liquid. Ribeiro and Madden [123] applied this theory to a series of fused salts, including both noncoordinating and coordinating species. They found that the INM analysis provided a good estimate of the diffusion constants for noncoordinating fused salts. For coordinating ions, however, the situation was complicated by the existence of transient, quasimolecular species. While a more detailed analysis is possible [124], the spectrum becomes sufficiently complicated that it would be difficult to characterize specific motions in the system. 

The second problem concerns an understanding of the sharing of transport duties (e.g., the carrying of current) in pure liquid electrolytes. In aqueous solutions, it was possible to comprehend the relative movements of ions in the sense that one ionic species could drift under an electric field with greater agility and therefore transport more electricity than the other until a concentration gradient was set up and the resulting diffusion flux equalized the movements when the electrodes were reached. In fused salts, this comprehension of the transport situation is less easy to acquire. At first, it is even difficult to see how one can retain the concept of transport numbers at all when there is no reference medium (such as the water in aqueous solutions) in which ions can drift. 

Thus, there is no concentration variable to be taken into account in the consideration of transport phenomena in a pure liquid electrolyte. Hence, there cannot be a concentration gradient in a pure fused salt, and [because of Pick s first law see Eq. (4.16)] without a concentration gradient there cannot be pure diffusion. In an aqueous... 

Results of Self-Diffusion Experiments. Self-diffusion coefficient studies with fused salts really began to gather momentum after radioisotopes became... 

The activation energy for self-diffusion is usually a constant, independent of temperature. It is, however, characteristic of the particular liquid electrolyte. The dependence of the activation energy for self-diffusion on the nature of the fused salt was experimentally found by Nanis and Bockris in 1963 to be expressible in the simple relation of Fig. 5.28, given by the equation... 

This equation was deduced in Section 4.4.8. It is of interest to inquire here about its degree of appiicabiiity to ionic liquids, i.e., fused salts. To make a test, the experimental values of the self-diffusion coefficient D and the viscosity tj are used in conjunction with the known crystal radii of the ions. The product D r//T has been tabulated in Table 5.22, and the plot of D tj/T versus 1/r is presented in Fig. 5.31, where the line of slope k/6n corresponds to exact agreement with the Stokes-Einstein relation. ... 

Radiotracer Method of Calculating Transport Numbers in Molten Salts. In the discussion of the appiicabiiity of the Nernst-Einstein equation to fused salts, it was pointed out that the deviations could be ascribed to the pairedjump of ions resuiting in a currentiess diffusion. With fused NaCI as an example, it has been shown that there is a simpie reiation between the experimentally determined equivaient... 

The assumed identity of the rates of hole diffusion and ionic diffusion is recalled. Thus, the final expression for the diffusion coefficient of ions in a fused salt is the same as that for holes, i.e., Eq. (5.98). 

Calculate the transport numbers of the cation and the anion in molten CsCl at 943 K. The experimental equivalent conductivity of the fused salt is 67.7 ohms cm equiv. The observed diffusion coefficients of Cs" and Cl ions in molten CsCl are 3.5 x 10 - and 3.8 x 10" cm s", respectively. (Contractor)... 

There are several commercial companies producing alkali flame detectors. In one version, two flames are stacked, one above the other. The lower, plain hydrogen-air flame bums the sample the combustion products are swept into the second flame which is doped with a sodium salt deposited on an electrically heated wire (31). The upper detector functions as alkali flame detector. Another modification uses a detector jet tip formed from fused salt the flame bums in contact with the salt surface (15, 32, 33, 34, 35). A third form uses an alkali-doped porous metal (36) or a platinum capillary filled with potassium hydroxide and carbon (37). When the capillary is heated to 900°—1000°, the carbon causes the grain boimdaries in the platinum to become enlarged, allowing alkali to diffuse slowly through it. The above remarks should be considered illustrative of detectors on the market and by no means comprehensive. Commercial firms, of course, must add the limitations of the patent situation to the diflBculties encountered in constructing these detectors. In fact, some detectors seem to be constructed more for the patent lawyer than for the analyst (38). 

Hot corrosion is the accelerated oxidation of a material at elevated temperatures induced by a thin film of fused salt deposit [36]. It is called hot corrosion because, being caused by a thin electrolyte film, it shares some similarities with aqueous atmospheric corrosion, in which corrosion is commonly controlled by the diffusion of oxygen to the metal surface. In hot corrosion, the soluble oxidant is SO3 (8207") in the fused salt. 

Fused-salt electroplating, which is commonly referred to as metal-liding, is a process for surface modification and surface hardening by electrodeposition fiom fused-salt electrolytes. Two unique aspects of this electrodeposition process are (1) elements that cannot be plated by conventional processes may plate by fused-salt electrodeposition and (2) if the deposition rate is controlled to match the diffusion rate of the... 

Both aqueous and fused-salt electrol3rtes have been used for plating the platinum group elements. Platinum has been used as a diffusion-barrier layer in aluminiding nickel-base alloys and MCrAlY coatings. Platinum... 

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