Transport in Concentrated Electrolyte Solutions

Nearly all pit models have been based on transport equations which strictly apply in solutions much more dilute than those usually found in pits, which exceed 1 M and often approach saturation in the metal chloride salt. The fundamental shortcoming of dilute solution transport theory is that it accounts only for interactions between ions and solvent molecules, and not between pairs of ions. Ion-ion interactions are manifested, for example, by deviations of the solution conductivity from values predicted by dilute solution theory, which become appreciable at concentrations as low as 0.01 M. This section will examine specific inaccuracies resulting from the dilute solution approximation, and point out cases where the use of concentrated solution transport models is tractable. Dilute and concentrated solution approaches will be compared in the context of a simple example of a one-dimensional pit with passive sidewalls. The metal and electrolyte solution were taken to be aluminum in 0.1 M NaCl. There are no cathodic reactions or homogeneous reactions in the pit, and the solution composition at the pit mouth is that of the bulk solution. This example was described in more detail in an earlier publication. This example is chosen because of its simplicity and since the behavior of the dilute solution model may be familiar to readers. 

The general multicomponent flux equations in concentrated solutions appear in the form  

Considerable simplification results if the electrolyte solution may be approximated as a binary electrolyte. The use of this approximation in pitting models was shown to be widely applicable by Engelhard et al., in the context of dilute solution transport theory. In the example, it may be recognized that since electrical migration drives Na+ ions out of the pit, these ions would be present only very close to the pit mouth, and hence the pit solution could be approximated as an AICI3 electrolyte. The inverted flux equations for water, Al +, and Cl ions may be combined with the species conservation equations, i.e., (1), to obtain a diffusion equation in terms of the electrolyte concentration c. 

The electrolyte concentration profile c(x) is used to determine two types of potential. First, the potential which would be measured using reference electrodes equilibrating with the local solution is calculated. We take the reference electrode to be of Ag/AgCl type, which equilibrates with Cl ions in solution according to 

The potential of a Ag/AgCl reference electrode in the bulk solution, relative to a hypothetical electrode of the same type positioned at the dissolving surface (tc = Z,) is 

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Dufreche J-F, Bernard O, Durand-Vidal S, Turq P (2005) Analytical theories of transport in concentrated electrolyte solutions from the MSA. J Phys ChemB 109 9873-9884. doi 10.1021/jp050387y... 

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