Cathode polarization

Then stray current enters the pipeline and the pipe/soil potential becomes more negative. The recording in Fig. 15-8b shows the behavior with a direct stray current drainage to the rails. With > 0, a current flows off the pipeline via the stray current return conductor back to the rails so that there is no anodic polarization of the pipeline. With Uf g < 0, a current flows over the connection in the pipeline and anodically polarizes it. Direct stray current drainage is therefore not possible in this case. Figure 15-8c shows the result of a rectified stray current drainage to the rails. Now the pipeline is always cathodically polarized. Cathodic protection is, however, also not fully attained. 

As in aqueous electrochemistry it appears that application of a potential between the two terminal (Au) electrodes leads to charge separation on the Pt film so that half of it is charged positively and half negatively8 thus establishing two individual galvanic cells. The Pt film becomes a bipolar electrode and half of it is polarized anodically while the other half is polarized cathodically. The fact that p is smaller (roughly half) than that obtained upon anodic polarization in a classical electrochemical promotion experiment can be then easily explained. 

When the electrolyte solutions are not too reactive, as in the case of ethereal solutions, there is no massive formation of protective surface films at potentials above Li intercalation potential, and most of the solvent reduction processes may occur at potentials lower than 0.3 V vs. Li/Li+. Hence, the passivation of the electrodes is not sufficient to prevent cointercalation of solvent molecules. This leads to an exfoliation of the graphite particles into amorphous dust (expholiated graphene planes). This scenario is demonstrated in Figure 2a as the reduction of the 002 diffraction peak21 of the graphite electrode, polarized cathodically in an ethereal solution. 

The surface reactions of graphite electrodes in many nonaqueous solutions have been investigated intensively,29 30 and the major reaction paths in a variety of alkyl carbonate solutions seem to be quite clear. Both EC and PC decompose on graphite electrodes, polarized cathodically, to form solid surface films with R0C02Li species as major components,31 and ethylene or propylene gases, respectively, as co-products. 

However, an improved electrochemical redox methodology using a flow cell fitted with two consecutive porous electrodes of opposite polarities (cathode then anode), allows a rapid and total oxidation at the anode of the hydroxylamine intermediate produced at the cathode. Various nitroso compounds may be obtained in high yields without... 

A further problem is the tendency for the ITO itself to be reduced. If there is little or no analyte in solution, and the ITO is polarized cathodically in the presence of moisture, then the indium and tin oxides are themselves reduced to metal, according to the following ... 

Electrolyte 0.3% KC1 aq. solution prepared with twice-distilled water, containing 0.02 M benzotriazole. Polarization Cathodic 5 V for 10 min and 10 min without potential (with air bubbling). Joints were tested after 24 h in boiling water. Adhesive 100 parts diglycidyl ether of bisphenol A (Epon 828 from Shell Chemical Co.) cured with four parts dicyandiamide at 175DC for 2 h. 

By contrast, O2 passing the membrane of a PO2 electrode enters an irreversible reaction at a polarized cathode so that current generated by the reaction is proportional to the amount of O2 reduced. The amount of O2 available for consumption depends on the rate of diffusion of O2 through the membrane. A steady state is achieved when the rate of diffusion equals the rate of reduction. Thus the electrode responds to a greater degree to O2 diffusing from a gaseous phase than from a liquid phase. 

Tomita et al. pointed out that Ag ion easily leaks from an Ag/AgCl reference electrode employed in the electrolysis cell with AN electrolyte. Ag contaminates the Pt electrode, and CO is consequently the major product from the contaminated Pt electrode. A Pt electrode without Ag contamination gives (COOH)2 as the major product. When polarized cathodically in CO2 saturated electrolyte, Pt is readily covered with strongly adsorbed CO as evidenced by IR spectroscopy, and is chemically inert like Hg and Pb,... 

It is further clear from Fig. 5.19 that the n-electrode has to be polarized cathodically with respect to the equilibrium potential, and the p-electrode anodically, in order to reach the corresponding flatband situation (see lower part of Fig. 5.19), provided that the positions of the energy bands at the surface are the same for the two types of electrodes. Keeping in mind that the electrode potential refers to the Fermi level of the electrode, then the difference of flatband potentials corresponds exactly to the difference of the two Fermi levels. Since the Fermi level in the bulk of a semiconductor with the usual doping (>1() cm ) is rather close to the corresponding band, the difference in the flatband potentials approximates the bandgap of the semiconductor as found with GaP. 

Instead of an asbestos membrane and an anion exchange membrane, Li et al. [46] suggested a Model 3 type cell in which a Nafion membrane was employed as the electrolyte to keep borohydride from crossing to the cathode. It was verified that cation (Na+) was the charge carrier in the Nafion membrane. Compared with anode polarization, cathode polarization was the main reason for the cell voltage drop. The power density of a Model 3 type cell can reach 190mWcm, as shown in Fig. 8.22. 

Equation 62 predicts Tafel behavior only for anodic (positive) polarization. Cathodic polarization is predicted to be potential independent at large negative polarizations. However, for most corrosion systems, this region of potential independence is small due to the presence of other cathodic partial processes, e.g., solvent decomposition to form hydrogen gas. 

Protection effect. MacroceU currents can have beneficial effects on rebars that are polarized cathodically. This is indirectly evident for patch repair of chloride-contaminated structures when only the concrete in the corroding areas is replaced with alkaline and chloride-free mortar, but surrounding concrete containing chlorides is not removed. Before the repair, the corroding rebars behave as an anode with respect to those in the surrounding areas, which are polarized cathodically and thus are protected by the macrocell. After the repair, formerly anodic zones no longer provide protection, and corrosion can initiate in the areas surrounding repaired zones (these have been called incipient anodes) [3]. Consequences for repair are discussed in Chapter 18. 

In Eq. (18), the first term represents the anodic partial current density I a (taken as positive) and the second term is the cathodic partial current density ic (taken as negative). The net current is the sum of the two terms. It is positive when the electrode is polarized anodically and negative when the electrode is polarized cathodically. 

T. Zakroczymski, Z. Szklarska-Smialowks, M. Smialowski, Effect of arsenic on permeation of hydrogen through steel membranes polarized cathodically in aqueous solution, Werkst. Korros. 26 (1975)... 

The main principle of CP is to impress an external current on the material, which forces the electrode potential down to the immune region, or, for protection against localized corrosion, below a protection potential. In other words, the material is made the cathode in an electrochemical cell it is polarized cathodically as illustrated by potential-log current curves in Figure 13.2. 

Gordon ef a/. studied adsorption of bacteria on polarized Cu and Pt metal surfaces in seawater, and attributed their adsorption results, in general, to pH changes at the interface. A copper surface was polarized cathodically from —0.12 to —0.24 V (NHE), and anodically from —0.01 to 0.07 V (NHE). Platinum was polarized cathodically from 0.40 to 0.22 V (NHE), and anodically from 0.60 to 1.30 V (NHE). While anodic polarization was seen to decrease bacterial content on both metals, the cathodic polarization produced mixed results for different kinds of bacteria. 

ATR on Mineral-Bed Electrodes. ATR at a mineral-bed electrode was employed to study the anodic oxidation of ethyl xanthate (EX) on chalcocite, chalcopyrite, pyrite, and galena [513, 514]. The optical scheme of the SEC cell is shown in Fig. 4.51. Prior to the addition of xanthate to the buffer, the electrode was polarized cathodically in order to remove any oxidation products that are formed during sample preparation. After a polarization period of 15 min at the selected potential, the electrode was pressed against a Ge IRE and the spectrum was measured while applying a potential less than or equal to -l-0.1 V. Otherwise, the spectra were recorded at open-circuit potential (OCP) just after the polarization to avoid corrosion of the Ge IRE. 

Since (Cu )j is less than (Cu ), the potential of the polarized cathode is less noble, or more active, than in the absence of external current. The difference of potential, ( )2 - ( )i, is the concentration polarization, equal to... 

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