Specific resistivity interface

Therefore one, or a combination, of the following measures can be used to increase the amount to be transferred per unit time (1) Enhancing driving force (2) Increasing interface area and (3) Reducing specific resistance. All three measures are, of course, effective in principle. However, their potential to enhance transfer and the degree of difficulty in carrying them out are quite different in practice. 

Figures 4.3(a) and (b) are sections in the zx-plane showing the distribution of potential (( )) in the solution as cross sections of imaginary surfaces in the solution of equal potential (isopotentials) and the distribution of current as current channels with cross sections defined by traces of the surfaces. ..(n - l),n, (n + 1)... perpendicular to the isopotentials. These traces are located such that each current channel carries the same total current. Figure 4.3(a) applies to an environment of higher resistivity (e.g., water with specific resistivity of 1000 ohm-cm) and Fig. 4.3(b) to an environment of lower resistivity (e.g., salt brine, 50ohm-cm). The figures are representative of anodic and cathodic reactions, which, if uncoupled, would have equilibrium half-cell potentials of E M = -1000 mV and E x = 0 mV and would, therefore, produce a thermodynamic driving force of Ecell = E x - E M = +1000 mV. This positive Ecell indicates that corrosion will occur when the reactions are coupled. For the example of Fig. 4.3(a), the high solution resistivity allows the potential E"m at the anode to approach its equilibrium value (E M = -1000 mV) and, therefore, allows the potential in the solution at the anode interface, < )s a, to approach +1000 mV (recall that (j)s = -E"M). The first isopotential above the anode, 900 mV, approaches this value. The solution isopotentials are observed to decrease progressively and approach 0 mV at the cathode reaction site.

Another governing relationship, however, is Ohm s law, which leads to a dependency of the corrosion current on both the polarization characteristics of the anodic and cathodic reactions and on the total electrical resistance of the system, Rtotal. Rtotal includes the resistance in the metal between anodic and cathodic areas, RM a metal junction resistance if different metals are associated with the two areas, Rac any anode- or cathode-solution interface resistance, Rai and Rci and the solution resistance, Rs. The latter depends on the specific resistivity or conductivity of the solution and the geometry of the anode-solution-cathode system. 

The distribution of potential in the solution along the solution/metal interface is shown in Fig. 4.6. If the anode and cathode areas are not connected, they will exhibit their thermodynamic or open circuit potentials, with the potentials in the solution at the anode and cathode being equal to +1000 mV and 0 mV, respectively. When the anode and cathode areas are in contact, current will pass causing polarization of the interface reactions. With a solution-specific resistivity of 1000 ohm-cm, the solution potential at the center of the anode is decreased... 

FlC 4.6 Solution potentials at the solution/metal interface for environ- mentsof indicated specific resistivities. Refer to Fig. 4.3(a) and (b). 

Carrier liquid and segment liquid consist normally of substances or mixtures with very different electrical properties. For example, the dielectric constants of an oil or a perfluorinated alkane (typical carrier liquids for aqueous segments) are much smaller than those of water. Electrical measurements can use differences in specific resistances of direct or alternating cm-rent. In principle, changes in electrochemical properties like media-dependent electrode potentials or Faraday currents can also be used for the detection of the interfaces. 

The term has the units of area specific resistance, Qcm, and is referred to as the charge transfer resistance, denoted by and is given by R, = An important point to note here is that a linear relationship between anS the current density, i, in the low current density limit does not imply ohmic relationship, since the response time for the process is long, and is determined by whatever is the underlying physical process. In the simplest case, the charge transfer process is describable by a parallel R — C circuit, in which case the time constant is given as RC. Thus, in DC measurements, the capacitive part is not reflected. At the same time, in the current interruption experiment, the voltage drop across the interface is usually not separable from the other time-dependent parts of the impedance. Measurement of frequency response, however, allows one to estimate both R and C. More about this is discussed later. 

Specific resistance (ASR). The high-frequency intercept on the real impedance axis rqtresents the value of ohmic ASR in the cell, which is generated from the ionic resistance in the electrolyte layer, both ionic electronic resistances in the electrodes, and the contact resistance from the interfaces and current collectors [58], From this figure, it is seen that ohmic ASR of the cells were reduced with the decrease in the electrolyte thickness of the cells. Assuming each cell had the same ohmic resistances in their electrodes, interfaces, and current collectors because of their similar material compositions, structures, and fabrication techniques, the ionic resistance in the electrolyte layer was decreased significantly by the reduction in the electrolyte thiek-ness. This is also one of the major reasons for the higher power output of thinner electrolyte layer that is shown in Figure 11.24a. 

The cables designed for use at voltages over 49 kV require that the conductor and insulation shields be firmly bonded to the insulation in order to avoid any possibiUty of generating corona at interfaces strippable insulation shields are not accepted. The A ETC specifications for cables rated for 59—138 kV require a volume resistivity of one order of magnitude lower than for the medium voltage cables. 

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