Electrolyte stagnant

This is a case for illustrating the effects of nomnetallic materials (gasket, mbber, concrete, wood, plastic and the like) in contact with a surface metal or aUoy exposed to an electrolyte (stagnant water). For instance, as Fontana [5] pointed out, crevice attack can cut a stainless steel sheet by placing a stretched mbber band around it in seawater. Thus, metal dissolution occurs in the area of contact between the alloy and mbber band. 

In contrast to external protection, the anodes in internal protection are usually more heavily covered with corrosion products and oil residues because the electrolyte is stagnant and contaminated. The impression can be given that the anodes are no longer functional. Usually the surface films are porous and spongy and can be removed easily. This is achieved by spraying during tank cleaning. In their unaltered state they have in practice little effect on the current output in ballast seawater. In water low in salt, the anodes can passivate and are then inactive. 

It remains to be determined whether the previous experiments , which have been interpreted as confirming the cathodic protection of aluminium by zinc, can be truly interpreted in this fashion or whether they are due to the accumulation of Zn in the electrolyte. Under laboratory conditions, and under some practical conditions in stagnant solutions or in recirculating systems, the latter explanation is quite likely. 

Effective diffusivities for these ions in equimolar concentration ratio and with various inert electrolytes, have been determined by several methods (see Table III). The mobility products obtained from capillary cell (stagnant diffusion) and rotating-disk measurements are in fairly good agreement. 

Though there is fluid flow in the bulk of the electrolyte, it is found that there is a layer adjacent to the electrode in which the electrolyte is stationary, or stagnant. Thus the electron acceptors travel by convection from the bulk up to the stagnant layer and then cross the remaining boundary layer by diffusion. This transport by a convection-with-diffusion mechanism has not been taken into account so far. The equations for the time and space variation of concentration [i.e., Eq. (7.178)], for the transition time [Eq. (7.190)], and for the time variation of potential [Eq. (7.192)] have been derived for convection-free conditions, and they break down when convection becomes significant. The first approximation theory given above, therefore, deviates from experiment if the constant current is applied sufficiently long (times on the order of seconds) for convection to be important. 

When transport is not able to do its job adequately and there is a change in the interfacial concentrations of electron acceptors and donors from the bulk values, there is a variation of concentration with distance from the interface toward the bulk of the solution. What matters, however, as far as the charge-transfer reaction is concerned, is the gradient of concentration at the interface because it is this gradient that drives the diffusion flux Jjy Even when there is convection with a laminar flow of electrolyte, the transport in the (assumed) stagnant layer adjacent to the electrode is by diffusion... 

For the case where diffusion of the corrosive ions is the rate controlling reaction, it has been found that P = po (1 + Ac/Aa) where p is the penetration that is proportional to the corrosion rate and p0 is the corrosion rate of the less noble uncoupled metal Ac and Aa are the areas of the more noble and active metal respectively (Uhlig and Revie, pp. 101-103).7 If a galvanic cell is not avoidable, a large anode and a limited size of cathode are recommended. Stagnant conditions and weak electrolytes may lead to pitting in spite of the large area of the exposed active metal. 

Electrophoretic effect — A moving ion driven by an electric field in a viscous medium (e.g., an electrolyte solution) is influenced in its movement by the - relaxation effect and the electrophoretic effect. The latter effect is caused by the countermovement of ions of opposite charge and their solvation clouds. Thus an ion is not moving through a stagnant medium but through a medium which is moving opposite to its own direction. This slows down ionic movement. See also -> Debye-... 

Sand equation — Consideration of the concentration c(x=o,t) at a planar working electrode in contact with a stagnant (unstirred) electrolyte solution for a reaction, where the oxidized species in the bulk is present at a concentration c and the reduced species is initially absent with an applied constant current 7, yields... 

For a vesicle of thickness 6, Immersed in a stagnant (l.e. no convection) dilute electrolyte solution the transport of species across the bllayer can be formulated by the following set of equations (29) ... 

Menon and Landau [52] developed a model to describe transient diffusion and migration in stagnant binary electrolytes. Nonuniformity at a partially masked cathode was found to increase during electrolysis as the diffusion resistance develops. The calculations were done using an alternating-direction implicit (ADI) finite difference method. 

A variable transformation for treating coupled diffusion and migration of multiple ionic species in stagnant solution was recently described by Baker et al. [53]. The problem is redefined in terms of a pseudo-potential which obeys the Laplace equation, enabling solution by a number of available methods. Although it has not yet been applied to electrochemical microfabrication problems, this transformation could be useful to treat cases that do not involve an excess of supporting electrolyte. 

The maximum useful current density through the membrane is normally limited by a phenomenon known as polarization. Concentration polarization is caused due to the depletion of the transported ion at the membrane surface, because of its faster electrolytic transport through membrane phase and its comparatively slower rate of transport through the solution phase. This causes excessive resistance at the stagnant layer near the membrane-solution interface. It is therefore necessary to avoid stagnant layers at the membrane-solution interfaces by operating at high Reynolds number or with turbulence promoters. 

Hence rjo becomes zero for j = 0, but —00 for j —y ju - For stagnant electrolytes, L becomes time-dependent according to Pick s second law ... 

On the basis of the dichotomous representation of the solution near the electrode surface, Eq. (133) simplifies in the two domains. Within the stagnant layer and in the presence of an excess of supporting electrolyte, the two first terms are negligible and one obtains Eq. (134). Conversely, when x > 5conv the solution is macroscopically homogeneous the diffusional contribution then vanishes, and the flux is given in Eq. (135). 

H. Vogt, R.J. Balzer, The bubble coverage of gas-evolving electrodes in stagnant electrolytes, Electrochimica Acta, Volume 50, Issue 10 (2005) pp2073-2079... 

For a voltammetric sensor, the current or potential peak shift that may relate to the concentration of the sensing species is an important measurement. In a dynamic situation in which polarization characteristics are obtained, it is essential that the mass transfer characteristics are reproducible for both calibration and actual measurements. In the case of a stationary planar sensor, stagnant solution or steady flow conditions in a flow cell provides good reproducibility. Or in another case, a sufficiently high concentration of an electrolyte is used to maintain a constant ohmic drop in the cell, regardless of the concentration of the pertinent sensing component. Under these conditions, the mass transfer can be purely diffusional and adequately described by Pick s law of diffusion. 

This treatment is based on the assumption that there is no convection in the electrolyte. This is far from the situation during industrial electrolysis. However, the major contribution from electronic conduction originates in the diffusion layer near the cathode, which can be assumed to be stagnant. 

We saw above that the concentration gradient at an electrode will be linear with respect to the spatial coordinate perpendicular to the electrode surface if the anode/cathode cell were operated at a constant current density and if the fluid velocity were zero. In actuality, there will always be some bulk liquid electrolyte stirring during current flow, either an imposed forced convection velocity or a natural convection fluid motion due to changes in the reacting species concentration and fluid density near the electrode surface. In electrochemical systems with fluid flow, the mass transfer and hydrodynamic fluid flow equations are coupled and the solution of the relevant differential equations is often a formidable task, involving complex mathematical and/or numerical solution techniques. The concept of a stagnant diffusion layer or Nemst layer parallel and adjacent to the electrode surface is often used to simplify the analysis of convective mass transfer in... 

This work is aimed at EQCM studies allowing the instantaneous rate of either the deposition or the dissolution of copper to be measured in parallel to cyclic voltammetry, and a partial current of either Co(II) oxidation to Co(III) or Co(III) reduction to Co(II) to be extracted from the EQCM data. The effect of halide additives on partial reaction rates was studied. A wall-jet EQCM cell [39] was used to ensure a continuous mass transport to/ffom the electrode diffusion limitations [ 39—41]. Comparative EQCM measurements were also performed in a stagnant electrolyte. 

The work of Bard and Goldberg clearly showed how an in-situ electrochemical ESR cell with stagnant electrolyte can be used in the determination of reaction mechanisms and rate constants. 

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