Reference electrodes experimental polarization measurements

The experimental arrangement is shown in Figure 9.17. It is preferable to use a separate reference electrode for potential measurements in addition to the current-supplying counter electrode. The reference electrode shonld be attached to the electrolyte near the sample in order to avoid large contributions from ohmic polarization losses. (Under certain experimental conditions, e.g., in the case of constant cnnents, this is nnimportant, however). 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

In the diode the cathode is usually a W filament which can be flashed and maintained as a reference electrode at a temperature above which adsorption occurs. The anode may be of similar construction or take the form of a metal film evaporated from an adjacent filament. Experimentally, current polarization curves are obtained, first for the clean anode surface A and then for the covered anode surface A. Alternatively, resistance-voltage characteristics are measured (SO). The potential difference comprises the applied polarization and the C.P.D. between the emitter and collector. For a given anode current j,... 

Polarization experiments on a corrosion system are carried out by using a potentiostat. The experimental arrangement of the cell consists of a working electrode, reference electrode and a counter-electrode. The counter-electrode is used to apply a potential on the working electrode both in the anodic and the cathodic direction, and measure the resulting currents. The electrochemical cell is depicted in Figure 1.26. 

The earlier sections of this chapter discuss the mixed electrode as the interaction of anodic and cathodic reactions at respective anodic and cathodic sites on a metal surface. The mixed electrode is described in terms of the effects of the sizes and distributions of the anodic and cathodic sites on the potential measured as a function of the position of a reference electrode in the adjacent electrolyte and on the distribution of corrosion rates over the surface. For a metal with fine dispersions of anodic and cathodic reactions occurring under Tafel polarization behavior, it is shown (Fig. 4.8) that a single mixed electrode potential, Ecorr, would be measured by a reference electrode at any position in the electrolyte. The counterpart of this mixed electrode potential is the equilibrium potential, E M (or E x), associated with a single half-cell reaction such as Cu in contact with Cu2+ ions under deaerated conditions. The forms of the anodic and cathodic branches of the experimental polarization curves for a single half-cell reaction under charge-transfer control are shown in Fig. 3.11. It is emphasized that the observed experimental curves are curved near i0 and become asymptotic to E M at very low values of the external current. In this section, the experimental polarization of mixed electrodes is interpreted in terms of the polarization parameters of the individual anodic and cathodic reactions establishing the mixed electrode. The interpretation then leads to determination of the corrosion potential, Ecorr, and to determination of the corrosion current density, icorr, from which the corrosion rate can be calculated. 

At open circuit the interface with the silver electrode is In thermodynamic equilibrium. The interface with the zinc electrode has a mixed potential, which is defined by adding the zinc oxidation and proton reduction currents, with the latter reaction characterised as being very slow at the zinc electrode. The polarities of the electrodes in open-circuit conditions are defined by the experimentally measured potentials of each electrode vs a saturated calomel reference electrode. 

Fig. 9.4 Potential difference between the working and reference electrodes (a) and relative error in the determination of WE polarization resistance (b) as functions of misalignment between the WE and CE normalized by the solid-electrolyte thickness (s/d), as calculated by the finite-element analysis assuming linear electrode kinetics [8, 25]. At high s/d ratios, the experimentally measured value stabilizes at a small Nemst potential due to gas-phase polarization of the working electrode,...

Consider a three-electrode electrochemical cell comprising an electrolyte solution of metal ions with a bulk concentration c, an inert, ideally polarizable working electrode, a counter electrode and a reference electrode with a fixed potential Eref. The whole system is kept at a constant temperature T. For the sake of convenience, we select a reference electrode made of the bulk metal M whose ions are present in the solution. The only requirement that we have to meet for the purpose is the bulk metal M to have a stable and reversible equilibrium potential JE x(c). That being the case. Ere/ = Ej(c) and any external potential E which we apply to the working electrode polarizes it directly to the electrochemical overpotential t] = EJ/f) - E, which is a measurable quantity with a clear physical significance. Certainly we can use any other reference electrode with a fixed potential Ere/ and this is what we should do if the bulk metal M has no reversible potential in the particular experimental system or if this potential is not known. However, then the external potential E will polarize the working electrode to the overpotential rjre/ = Ere/ - E, which relates to rj according to //re/ = // + AEref where AEre/= Ere/- Eao(c). Namely the difference Ere/-EJ/S) is which we eliminate with the special choice of our reference electrode. 

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