Limiting current densities for

The limiting current density /) for the transport of species i to the reacting site is given from Fick s law by ... 

On the other hand, it can be assumed for the oxygen corrosion of steel in aqueous solutions and soils that there is a constant minimum protection current density, 4, in the protective range, U limiting current density for oxygen reduction according to Eq. (4-5) (see Section 2.2.3.2). Then it follows, with V = +1,1 = 2nr, S = 27crs and d = dU from Eq. (24-54), instead of Eq. (24-58) [12-14] ... 

Another important point regarding the fabrication process of MPLs is the fact that, typically, when carbon fiber paper is used as the DL, the MPL is coated just on one surface of the CLP. However, when a carbon cloth is used, a homogeneous water suspension of carbon powder and PTFE is filtered under vacuum onto both faces of the carbon cloth material to form the MPLs [153,158,161,171], followed by drying and sintering as mentioned earlier. Antolini et al. [161] were able to demonstrate that carbon cloth with double MPLs, for both the anode and the cathode sides, showed better performance than when a CFP was used as the cathode DL with one MPL. At low current densities, the difference between the two DLs was not as obvious, but it became more evident at higher current densities because the limiting current densities for each case are quite different ( 1.6 A cm for CFP vs. 2.7 A cm for CC) (see Figure 4.20 for more details). 

Example 6.2. Calculate the diffusion limiting current density for the deposition of a metal ion at a cathode in a quiescent (unstirred) solution assuming a diffusion layer thickness 8 of 0.05 cm. The concentration of ions in the bulk (cj,) is 10 moEL (10 moEcm ), the same as in Example 6.1. The diffusion coefficient D of in the unstirred solution is 2 X lO cm /s. Using Eq. (6.83), we calculate that the limiting diffusion current density for this case is... 

Zone III the E vs ln(l — j/ji ) logarithmic curve corresponds to concentration polarization, which results from the limiting value ji of the mass transfer limiting current density for the reactive species and reaction products to and/or from the electrode active sites an increase inji from 1.4 to 2.2 Acm leads to a further... 

Influence of Rotating Disk Electrode Condition (Stationary or Rotating) on the Diffusion-Layer Thickness and the Limiting Current Density for the Reaction... 

The concept of a limiting current density is of great practical importance. Table 7.11 shows some typical experimental limiting current densities for four energy producers employing oxygen (where the calculation of limiting current density is more complicated than that of (7.206). 

How many times higher than the exchange current density does the limiting current density for the electrochemical reaction ox(soln) + e = red(soln) have to be to assume that the reaction is activation controlled at an overpotential of -300 mV (Take p = 0.5 and T= 298 K.) (Gokjovic)... 

The limiting current density for an electrodialysis system operated at the same feed solution flow rate is a function of the feed solution salt concentration, as shown in Equation (10.17). As the salt concentration in the solution increases, more ions are available to transport current in the boundary layer, so... 

In their work, Andrieux and Saveant analyse the behaviour in terms of the characteristic current densities for each process. This has the advantage that, in general, the expressions for these characteristic current densities are fairly straightforward and obvious and, therefore, easily derived. In the case where the different processes involved are strictly first-order, the reciprocal of total current density can be obtained by combining the reciprocals of the current densities for the individual steps. For example, the limiting current density, //., for a reaction at the surface of a rotating disc coated by a permeable membrane is given by... 

Figure 9 Polarization curve of carbon steel in deaerated, pH 13.5 solution at 65°C. Sample was initially held potentiostatically at —1.2 V(SCE) for 30 min before initiation of the potentiodynamic scan in the anodic direction at 0.5 m V/s. The cathodic loop results from the fact that the passive current density is only 1 pA/cm2, which is less than the diffusion-limited current density for oxygen reduction for the 0.5 ppm of dissolved oxygen present. (From Ref. 8.)...

The corrosion engineer concerned with mass transport controlled corrosion is interested in determining the limiting current density for a variety of geometries... 

Figure 16 Limiting current densities for oxygen reduction on rotating disks for pure Cu, Al2oCu2(Mn,Fe)3, and pure A1 in 0.1 M Na2S04 + 0.005 M NaCl under ambient aeration. The Levich prediction assumes a dissolved oxygen concentration of 6 ppm (1.8 X 10 4 M). (From G. O. Ilevbare, J. R. Scully. Corrosion 57, 134 (2001).)...

Figure 39 Experimental and calculated diffusion-limited current densities for RDE experiment.

You should be able to calculate a theoretical predicted limiting current density for each rpm selected the results are shown in Fig. 39. 

The consequence of this last assumption is that the rate of the mineralization reaction is independent on the chemical nature of the organic compound present in the electrolyte. Under these conditions, the limiting current density for the electrochemical mineralization of an organic compound (or a mixture of organics) under given hydrodynamic conditions can be written as (1.23)... 

In general, current efficiency [tj in (8.3)] has to be determined by laboratory experiments. However, for such compounds as phenol which are susceptible of a fast reaction with OH radicals, the model proposed by Iniesta et al. (2001) may be used. Following this model, when the galvanostatic electrolysis is performed in a batch reactor and the process is only limited by the mass transfer of the reactant to the electrode surface, r] is equal to 1 when the applied current density is lower than the initial limiting current density for mineralization of the reactant ... 

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