Tertiary current distribution

Tertiay Current Distribution. The current distribution is again impacted when the overpotential influence is that of concentration. As the limiting current density takes effect, this impact occurs. The result is that the higher current density is distorted toward the entrance of the cell. Because of the nonuniform electrolyte resistance, secondary and tertiary current distribution are further compHcated when there is gas evolution along the cell track. Examples of iavestigations ia this area are available (50—52). 

Current distribution Distribution of reaction rates on an electrode surface. Primary current distribution is calculated by considering only electric field effects both overpotential and concentration gradients are neglected. Secondary current distribution takes both field effects and surface overpotential into account. Tertiary current distribution takes field effects, surface overpotential, and concentration gradients into account. 

Tertiary current distribution. Field effects, kinetic limitations, and mass-transfer limitations are all considered. 

The complexity of a model increases as we proceed from the primary to the tertiary distribution and as the number of spatial dimensions that are considered increases. Essentially all published solutions have been reduced to one or two dimensions, and most include only simulations of the primary and secondary current distributions. For the special case in which only mass transport is limiting, a large number of correlations for the current distribution are available. 

Channel flow between plane parallel electrodes is shown in Fig. 11. This geometry is similar to that of the disk in that an electrode and an insulator intersect in the same plane. Because of many geometric similarities, the general characteristics of the primary and secondary current distributions are similar. At the edges the local current density is infinite for the primary current distribution (Fig. 12). Increasing the kinetic limitations tends to even out the current distribution. The significant contrasts appear in a comparison of the tertiary current distributions. In channel flow, the fluid flows across the electrode rather than normal to it. Consequently, the electrode is no... 

A dimensionless parameter known as the Wagner number is useful for qualitatively predicting whether a current distribution will be uniform or nonuniform (2,40,41). This parameter helps to answer the question, Which current distribution applies to my cell primary, secondary or tertiary ... 

The tertiary current distribution Ohmic factors, charge transfer controlled overpotential effects, and mass transport are considered. Concentration gradients can produce concentration overpotentials. The potential across the electrochemical interface can vary with position on the electrode. 

Primary, secondary, and tertiary current distributions have been defined for selected geometries that are of practical interest to the corrosion engineer (2-6). 

The rotating disk electrode will have a uniform tertiary current distribution but an extremely nonuniform primary current distribution with the current density at the electrode edge approaching infinity (8-12). For a disk electrode of radius r0, embedded in an infinite insulating plane with the counterelectrode far away, the primary current distribution is given by... 

Therefore let us instead consider the more practical case of the tertiary current distribution. Based on the dependency of the Wagner number on polarization slope, we would predict that a pipe cathodically protected to a current density near its mass transport limited cathodic current density would have a more uniform current distribution than a pipe operating under charge transfer control. Of course the cathodic current density cannot exceed the mass transport limited value at any location on the pipe, as said in Chapter 4. Consider a tube that is cathodically protected at its entrance with a zinc anode in neutral seawater (4). Since the oxygen reduction reaction is mass transport limited, the Wagner number is large for the cathodically protected pipe (Fig. 12a), and a relatively uniform current distribution is predicted. However, if the solution conductivity is lowered, the current distribution will become less uniform. Finite element calculations and experimental confirmations (Fig. 12b) confirm the qualitative results of the Wagner number (4). 

Figure 7 (a) Rotating disk electrode, flush mounted into an insulating plane, (b) Current distributions for primary, secondary, and tertiary cases as a function of radial position r/ r . (c) Variation of as a function of rlr0 for various values of the reciprocal Wagner... 

Fig. 5 The effect of primary, secondary, and tertiary current distribution on deposit thickness.

When the current density is so high that local concentration of the depositing metal changes, tertiary current distribution begins to affect the deposit thickness. The effect of concentration polarization depends on the ratio of surface concentration to bulk electrolyte concentration of the depositing metal. The surface concentration depends on the local mass-transfer... 

Tertiary current distribution. This method of analysis applies to those systems where there is significant mass transport and electrode polarization effects. Electrode kinetics is considered, with electrode surface concentrations of reactant and/or products that are no longer equal to those in the bulk electrolyte due to finite mass transfer resistance. The analysis of tertiary current distributions is complex, involving the solution of coupled... 

When the mass transport process is rate determining, and in the presence of a supporting electrolyte, a convective-diffusion equation for this tertiary current distribution must be solved ... 

FIGURE 13.6 Current distribution on a plane electrode catalyst for primary distribution (dashed line), secondary distribution (dotted line), and tertiary distribution (continuous line). 

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