Polarization curves complications

The polarization relations found in the region of high polarization are usually plotted semilogarithmically as AE vs. log i (Eig. 6.1). These plots are straight lines, called Tafel lines (curve 1 in Eig. 6.1), when relation (6.3) holds. More complicated polarization functions are found at many real electrodes in the region of high polarization. Sometimes several Tafel sections can be distinguished in an actual polarization curve (curve 2 of Eig. 6.1) each of these sections has its own characteristic values of parameters a and b). 

EIS has the ability to distinguish between influences from different processes, especially when the system involves multiple-step reactions, parallel reactions, or additional processes such as adsorption. Generally speaking, the measurements and analysis of the EIS for a PEMFC are complicated compared with those of the polarization curve. However, the results from both methods are not insular, and some relationships exist between the complicated impedance spectrum and the simple polarization curve [22],... 

As I mention in another paper (p. loi), two outstanding types of polarization curve are obtained for polar substances dissolved in non-polar solvents. In the first type of curve Pg always diminishes as Cg increases. The second type is less simple it exhibits a maximum for Pg, usually when the concentration is low (such curves are chiefly exhibited by the lower alcohols) see figs, i and 2, p. 104. For the explanation we refer the reader to Debye s article in the Handbuch der Radiologic, More complicated Pg-curves, e.g. those with several maxima, will not be discussed here. 

In the case where the molecules of both media are polar, the polarization curves give us no information, as the relationships are too complicated. 

For a given N-shaped current-potential characteristic, there are two parameters that determine the bistable region. Re and U. In the U/Rg parameter diagram, this region becomes broader while shifting toward larger values of U for increasing, irrespective of the electrochemical reaction [Fig. 2(c)]. Below we will see that this feature is also encountered in all more complicated electrical models that describe simple or complex oscillatory behavior since all of them require an N-shaped polarization curve. 

The processes in real corroding systems are obviously more complicated than represented by this model. Useful quantitative calculation of the distribution of current density, and hence corrosion rate along the surface, based on the polarization curves for the anodic and cathodic reactions and on the geometry of the anodic and cathodic sites is very complex. In principle, computer-based techniques can be used if exact polarization curves and the geometry of the anodic and cathodic areas are available. For most industrially important situations, this information is not available. Also, time-dependent factors, such as film formation, make quantitative calculations of long-time corrosion rates very uncertain. The theory underlying these calculations, however, has been useful in interpreting observations in research and in industrial situations. 

Polarization curves. The rate at which the anodic or the cathodic process takes place depends on the potential ( ). The corrosion behaviour of the reinforcement can be described by means of polarization curves that relate the potential and the anodic or cathodic current density. Unfortunately, determination of polarization curves is much more complicated for metals (steel) in concrete than in aqueous solutions, and often curves can only be determined indirectly, using solutions that simulate the solution in the pores of cement paste. This is only partly due to the difficulty encountered in inserting reference electrodes into the concrete and positioning them in such a way as to minimize errors of measurement. The main problem is that diffusion phenomena in the cement paste are slow (Chapter 2). So when determining polarization curves, pH and ionic composition of the electrolyte near the surface of the reinforcement may actually be altered. 

Platinum/ platinum family metals/ and silver are classified in the second group. The oxygen electroreduction on these metals occurs both directly to water and via intermediate formation of hydrogen peroxide. The hydrogen peroxide formed in the parallel reaction is decomposed by chemical or electrochemical mechanisms. The relation between the rates of these processes depends in a complicated way on the nature of the metal, the potential, the coverage of the electrode by the chemisorbed species, and their adsorption character. The polarization curves are characterized by a single wave with a limiting current close to the diffusion current for the four-electron process. 

As stated above, Ejj and Eprot often dejiend strongly on the method by which they are determined and, therefore, do not uniquely define intrinsic material properties. The Eprof values determined from the scanning method can be complicated by scan rate, pit size or depth, vertex potential/current, polarization curve shape, and specimen geometry [86,87]. Investigators have found more consistent Eprof values after a critical charge has passed, while others report a single critical potential [85]. Often this potential is difficult to choose from E-I data and has been taken at various points on the reverse scan of a cyclic potentiodynamic polarization curve [89]. 

The steady state of such a system can be described by mean values and time averages of accessible parameters, e.g., the steady state current density as a function of the electrode potential, i ( ) polarization curve, and its dependence on other system parameters. However, for elucidating complex reaction mechanisms, steady state measurements are not appropriate and in general not suitable for separation of the kinetic parameters and transport constants of interfacial reactions with different time constants. For the study of complicated corrosion systems. 

If adsorption is strong the equation of the polarization curve becomes more complicated. It can, however, be shown [15] that there is a maximum on the curve at the potential ... 

Whereas the use of conventional fast atom bombardment (FAB) in the analysis of polymer/additive extracts has been reported (see Section 6.2.4), the need for a glycerol (or other polar) matrix might render FAB-MS analysis of a dissolved polymer/additive system rather unattractive (high chemical background, high level of matrix-, solvent- and polymer-related ions, complicated spectra). Yet, in selected cases the method has proved quite successful. Lay and Miller [53] have developed an alternative method to the use of sample extraction, cleanup, followed by GC in the quantitative analysis of PVC/DEHP with plasticiser levels as typically found in consumer products (ca. 30 %). The method relied on addition of the internal standard didecylphthalate (DDP) to a THF solution of the PVC sample with FAB-MS quantitation based on the relative signal levels of the [MH]+ ions of DEHP and DDP obtained from full-scan spectra, and on the use of a calibration curve (intensity ratio m/z 391/447 vs. mg DEHP/mg DDP). No FAB-matrix was added. No ions associated with the bulk of the PVC polymer were observed. It was... 

Fig. 6.3-5 shows the spectra taken with a CdSe calibration plate on a Bruker IF.S-66 FT-IR spectrometer. Theoretically four curves should result. For calibration purposes, however, it is only necessary to record two curves, as the two other curves are mirror images of the latter. The curves shown in the figure were taken with only a single orientation of the CdSe calibration plate, but one with parallel and another with crossed polarizers. They are the results of 32 coadded interferograms transformed without apodization. As result we chose a power spectrum, since this is less noisy. A further advantage is that we do not need to use the (in the case of calibration spectra) complicated phase correction. The curves cross at certain points. Interpolating between those crossings, we get a curve with which we can multiply our spectra for the correction of Bessel function dependance. We also clearly can identify the node of the Bessel function at about 2450 cm . The modulator was tuned to quarter wave retardation at 1111 cm. ... 

---