Migration electroneutrality condition

As electrolyte species are available in the system considered, the driving forces caused by the electrical potential differences must be taken into account [16]. The migration is described through the Nernst-Planck equation (Eq. (24)). This implies that the electroneutrality condition, Eq. (25), is satisfied. 

According to the electroneutrality condition (4.120), the electrolyte in a pore must contain an equal number of positive and negative charges. Metal dissolution at the bottom of a pore yields ferrous ions, the positive charge of which is compensated by anions, especially sulfates and chlorides. To maintain electroneutrality, the anions migrate towards the base of the pore (Figure 8.19(b)) where they accumulate. This... 

Ions can be transported through an electrochemical solution by three mechanisms. These are migration, diffusion, and convection. Electroneutrality must be maintained. The movement of ions in a solution gives rise to the flow of charge, or an ionic current. Migration is the movement of ions under the influence of an electric field. Diffusion is the movement of ions as driven by a concentration gradient, and convection is the movement due to fluid flow. In combination these terms produce differential equations with nonlinear boundary conditions (1). 

Once the working electrode (W) is covered by the ionic conducting product or the entire solid electrolyte (E) is covered by the electronic conducting product, no electrically shorted surface exists. Thus, further growth in thickness has to involve diffusion of both the ionic species and electrons to the surface to react with the gas phase. Practically, diffusion of one species is much faster than the other. However, electroneutrality must be maintained under this open circuit condition. The growth rate is determined by either migration of electrons or mobile ionic reactants in the deposit (D). In both cases, the increase in thickness should follow the parabolic law. ... 

Equation (3.32) can be used to describe the flux of cations, anions, or electrons through the oxide layer. Due to their different mobilities, different species would tend to move at different rates, however, this would set up electric fields tending to oppose this independence. In fact, the three species migrate at rates that are defined by the necessity of maintaining electroneutrality throughout the scale, i.e, such that there is no net charge across the oxide scale. This condition is usually achieved due to the very high mobility of electrons or electronic defects. 

The charge percolation by electron hopping is necessarily coupled to ion migration inside the coating, as required by fulfillment of electroneutrality. Some important conclusions concerning the conductivity of the coating can be drawn either fi om impedance spectra or by d.c. conductivity measurements (two-band electrode). Necessary condition for the material to be conductive is that it is polarized at potentials close to E° of the redox couple inside the RP. At similar potentials, in fact, meaningful concentration values of both redox forms are present in the RP. This restriction has been experimentally verified for a modified electrode... 

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