Apparent polarization resistance

Denoting by F i, AE) the left-hand side member of equation (11), by Dini s theorem on implicit functions [54] the apparent polarization resistance, Rpa, is defined by the relation... 

Figure 4.1.28. Error in the apparent polarization resistance due to the relative displacement of WE and CE in a planar cell. The error is calculated for an electrolyte resistivity of 1012 cm and a polarization resistance 0.16 2 cm for both the WE and CE. A positive displacement is defined as a displacement of the WE in the direction of the RE, as in Figure 4.1.27 (for further details see Winkler etal. 1998).

In order to obtain a correct value for /cor from the polarization resistance, it is necessary, on one hand, to measure under steady-state conditions (current stable with time) and, on the other hand, to avoid ohmic drop effects. Indeed, in presence of an ohmic drop between the reference and working electrodes, instead of r oi, one measures an apparent polarization resistance r -p given by ... 

Linear polarization resistance (intrusive). The linear polarization resistance (LPR) technique is an electrochemical method that uses either three or two sensor electrodes. In this technique, a small potential perturbation (typically of the order of 20 mV) is applied to the sensor electrode of interest, and the resulting direct current is measured. The ratio of the potential to current perturbations, known as the polarization resistance, is inversely proportional to the uniform corrosion rate. The accuracy of the technique can be improved by measuring the solution resistance independently and subtracting it from the apparent polarization resistance value. The technique is well known (its theoretical basis had already been developed in the 1950s), and it is widely used under full immersion aqueous conditions. 

As the measurements show, the small heater without an electrical separation (from the boiler) is not detrimental to cathodic protection. However, with the uninsulated built-in Cu heat exchanger without an electrical separation, cathodic protection was not achieved. As expected, the polarization increased with increasing conductivity of the water. It should be pointed out that the Cu tube was tinned and that the tin could act as a weak cathodic component. Apart from the unknown long-term stability of such a coating, the apparent raising of the cathodic polarization resistance of tin is not sufficient to provide cathodic protection with such a large fixture. This applies also to other metal coatings (e.g., nickel). 

Another frequently encountered complication is the need to correct polarization data for errors that arise from the contribution of solution resistance, Rs. Solution resistance contributes to a voltage error as well as a scan rate error (2). Since the applied potential is increased by an ohmic voltage component, an apparent value of polarization resistance is obtained that overestimates / p by an amount equal to Rs. Consequently, the corrosion rate is underestimated. 

Two types of losses can be distinguished in battery cells (i) ohmic losses proportional to the current, that is, with a constant value of ohmic resistance f ohm and (ii) polarization losses because of electrode polarization (change of electrode potential during current drain) with a complex dependence on current, that is, with a variable value of polarization resistance /ipoi. The total of both ohmic and polarization resistances f app =- ohm+ poi called the cell s apparent resistance... 

The design to decrease R is straightforward in that the electronic or ion conductivity of the components has a direct influence on the value of For instance, separators with different porosity and pore tortuosity lead to different values of apparent electrolyte resistance, since these two factors play a critical role in the polarization inside the separator. (In fact, air permeability and the Gurley number are more frequently used in industry to evaluate the degree of penetrability of active species through the separator than the terminologies such as porosity, tortuosity, and polarization [9].) The separators with more porous and less tortuous pore structure result in smaller Ro (or polarization). 

If the WE and CE possess different polarization resistances and time constants defined as t = / p x C (where C is the area-specific capacitance of the electrode/ electrolyte interface), this extension may also introduce artifacts in the impedance data [25, 31, 35]. In other words, when shifting the CE to RE, the equipotential surface probed by the RE approaches the CE/electrolyte interface at co —> 0 and the RE may no longer isolate the potential drop of the WE. In addition to the apparent increase in the electrolyte resistance, this causes a partial introduction of the CE impedance into the measured WE impedance [26, 31]. 

The strange anodic polarization behaviors (negative difference effect, lower apparent valence, low anodic dissolution efficiency, low anodic polarization resistance and poor passivity ) are closely associated with the AHE process which is further related to the onset of localized corrosion or pitting . A comprehensive anodic dissolution model can be employed to understand these. 

The linear polarization resistance (LPR) technique is based upon the measurement of the apparent resistance of a corroding electrode when it is polarized by a small voltage of the order of 10 millivolts. The apparent resistance is determined from the current flowing as a consequence of the small applied voltage and is inversely proportional to the corrosion rate. 

Sc + doping for Ti + can greatly improve the electronic conductivity of Li4Ti50i2. As a result, the doped Li4Ti50i2 has low polarization resistance and a high apparent diffusion coefficient for Li+ ions. The doped Li4Ti50i2 has reversible capacities of 174 and 94 mAh/g at 1 and 40 C rates, respectively [4]. 

As far as conductometry is concerned, there remain a few complications caused by processes at the electrodes, e.g., electrolysis above the decomposition voltage of the electrolyte with some liberation of decomposition products at the electrode, or apparent capacitance and resistance effects as a consequence of polarization of the electrode and exchange of electrons at its surface. In order to reduce these complications the following measures are taken ... 

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