Ohmic drop potential evaluation

A further example, which confirms the necessity of evaluating the resistivity of the medium very carefully, concerns the corrosion of rebars in reinforced concrete. In this caae the intensity of the current flowing between the anodic and cathodic zones of a macrocell depends on the resistivity of the concrete and the extent of the region involved. To determine the concrete resistivity various methods have been developed, which can be applied in the laboratory [14] as well as in the field [15]. It should be noted, however, that in the latter case most researchers have pursued the approach suggested by Wenner [16] for the evaluation of the resistivity of soils. The contribution of the ohmic drop to the electrode overvoltage cannot be neglected when the values of the corrosion rate of the rebars are appreciable, even if the current intensity is small within a given polarization potential interval, because under such conditions the interpretation of experimental results could be completely distorted. 

A numerical study of the influence of the ohmic drop on the evaluation of electrochemical quantities has been conducted, for example, over the AE interval [-20, 20] mV by means of the IRCOM program, which makes use of a polynomial of the sixth degree, considering some experimental polarization curves and taking the values of the electrochemical parameters obtained by the NOLI method. The examples examined have shown that the representation of experimental data by a polynomial of the sixth degree is very good and that the evaluation of the correct order of magnitude of the corrosion current density, in the presence of an ohmic contribution to the electrode potential, requires that the actual values of the Tafel slopes be known. 

To characterize potentiomebic probes, one must evaluate their response function, selectivity coefficient, response time, and ohmic drop. The response function of a potentiometric probe is a calibration curve of the measured membrane potential with the log of the concentration (or activity) of the primary ion must be acquired. To use concentration values, the activity coefficients must be known and many of than are tabulated. Another option is to use (when possible) dilute solutions such that the activity coefficients maybe neglected. 

Small galvanostatic transients superimposed to the longer pulse permit determination of the ohmic drop AUq in front of the electrode in close connection to the changes of its surface environment with time (Strehblow and Wenners, 1977). AUq is small in the plateau region and increases when the potential step is observed at r = T (Fig. 1-37). It has been interpreted as the ohmic drop within the salt layer. The major part of the potential step AE is interpreted as the potential drop across a poreless part of the salt film at the electrode surface. A discussion based on the evaluation of these galvanostatic transients leads to a bilayer... 

The term (o — o,c) represents the potential drop through the electrolyte. As defined here, the numerical evaluation of the potential drop requires accounting for the variation in electrolyte conductivity within the diffusion layer. An alternative approach is to define the Ohmic potential drop as being that calculated using Laplace s equation with a uniform solution conductivity. In this case, an additional term is required to accotmt for the influence of the conductivity variation within the diffusion layer on the measured potential. This is incorporated into a concentration overpotential, discussed in Section 5.7.3. 

In this process, a reference electrode is attached to the supply cable of the ROV. There are two further electrodes on the ROV. As it travels along the pipeline, the potential differences between the electrodes provide information on the effectiveness of the galvanic anodes, on the location and current uptake at defect areas and on the potentials. Section 3.6.2 provides information on the evaluation of the potential differences. From the known potential at the initial point on the platform, the potentials given for the method described in Section 3.7.2 can be determined if ohmic voltage drops in the pipeline do not have to be considered. 

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