Polarization resistance definition

When the rate of the electrode reaction is measurable, being characterized by a definite polarization resistance RP (Eq. 5.2.31), the electrode system can be characterized by the equivalent circuit shown in Fig. 5.22. 

From a mathematical point of view, this problem can be readily solved by introducing the concept of polarization resistance, R, [5] associated with the implicit function defined by the equation (11). For this purpose, it may be useful to remember that, after Bonhoeffer and Jena s definition [53], the differential polarization resistance is related to the first derivative of the function i(AF), calculated at the point AE=Q, by the relation... 

The practical polarization-resistance technique involves one more assumption than those listed in Section 11.1. It is assumed that the polarization equation can be approximated by a linear relation between the measuring current density and the polarization for small polarization values. Equation (7) can be obtained by expanding the exponentials in Eq. (1) into Taylor series and ignoring all terms higher than first power. There has been a certain amount of controversy about the criteria of validity for this approach, both for the corrosion case and for the electrode kinetics case (see Ref. 116 and references therein). By now, this controversy seems to have been cleared up with the following conclusions. While the polarization curve is never absolutely linear at finite polarization values, the assumption of linearity often introduces only a small error. Furthermore, this error approaches zero, by mathematical definition, as A approaches zero, but the rate of approach depends strongly on the values of and b. The error... 

To determine actual cell performance, three losses must be deducted from the Nernst potential activation polarization, ohmic polarization, and concentration polarization. Definition of the ohmic polarization is simply the product of cell current and cell resistance. Both activation polarization and concentration polarization required additional description for basic understanding. 

The method of field-induced thermally stimulated current (FITSC) consists of measuring— according to a definite (usually linear) heating scheme—the currents generated by the buildup and release of a polarized state in a high-resistivity solid... 

These definitions, formally different with respect to those reported in eqn (14.10), approach each other in the case of negligible inhibition. This approach is very useful because, with this choice, CPC is directly related to the flux reduction due only to external mass transfer resistance and not to the inhibition phenomenon. Moreover, the polarization and inhibition effect are able to be separately identified and split into their own different contributions, which can thus be analyzed to provide a better understanding of the coupled influence of these two phenomena. This is done by defining another coefficient, i.e. the inhibition coefficient, IC, which is on the other hand a quantitative indicator of the inhibition phenomenon only (see next section). 

Definition L A metal is passive if it substantially resists corrosion in a given environment resulting from marked anodic polarization. 

Advice in this section is usually given on the chemical resistance to strong and weak acids, to alkalis and to solvents. A strong acid, or a strong alkali, remains strong by definition, even when it is used diluted a weak acid, by the same token, is still weak even when it is used concentrated. Solvents may be polar or non-polar. The term means that the material is not balanced and so charge separation oh a molecular scale exists. Examples of polar solvents include water, alcohols, ketones, esters, and partially halogenated solvents. Non-polar solvents include carbon tetrachloride, benzene and hydrocarbons. 

It was also stated that a theoretical definition of effective cathodic protection is to depress the potential of the mo.st cathodic areas below that of the mo.st anodic areas. However, as time passes and negatively charged chloride ions move away from the negatively charged steel, the most anodic areas will move. Variation. in the resistance of the concrete will mean that current flow from the anode is more likely to reach the anodic areas of steel and polarize them, rather then the passive, cathodic, areas (e.specially if they are new patch repairs with higher resistance concrete). 

A comparison of eqs. 4.17 and 4. 9 shows that ohmic losses (f n) are separate from losses due to electrode overpotentials, but the internal resistance of an MFC (/ , ,) includes overpotentials that vary with current. Thus, while these two terms for ohmic and internal resistances are often used interchangeably, it is important to note they are in fact different. Our inability to so far separate out the overpotential and ohmic losses that contribute to the internal resistance of an MFC make it convenient to use to classify the MFC. However, as power production increases, and polarization curves become more non-linear, we will need to be careful to separate our definitions of these resistances. 

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