Charge cathodic partial reaction

Mechanism, The overall anodic partial reaction, Eq. (8.5), usually proceeds in at least two elementary steps (like the cathodic partial reaction) formation of an electroactive species, and charge transfer. The formation of electroactive species (R) usually proceeds in two steps through an intermediate (Redinterm)-... 

Eq. (44), usually proceeds in at least two elementary steps (like the cathodic partial reaction) the formation of the electroactive species and charge transfer. 

An electrode may be defined as a solid electron conductor which is in contact with a liquid (solid, gaseous) ion conductor (electrolyte). At the interface charge transfer reactions take place. During corrosion processes this charge transfer involves anodic and cathodic partial reactions of various kinds which are dependent on many parameters and which have to be taken into account when designing an electrode. 

Equations (4.91) to (4.93) can be applied to any cathodic partial reaction for which the charge-transfer step obeys the Butler-Volmer equation. In corrosion, oxygen reduction is often under mixed control. Figure 4.25 shows the cathodic polarization curve for oxygen reduction, measured on a platinum electrode [6]. The shape of the curve suggests a relatively low value for the ratio I o/ltil-... 

This equation was first postulated empirically by Wagner and Traud (1938). In Eq. (7-26), b+ and b are the slopes of the Tafel lines of the anodic and cathodic partial reactions. The fundamentals of polarization resistance measurements have been described in more detail by Mansfeld (1976). This concept has also been adopted for the interpretation of EIS (Mansfeld, 1981 Mansfeld et al., 1982). For the simplest case of a purely reaction controlled corrosion process, the Faraday impedance Zp in Fig. 7-3 may be replaced by a potential dependent charge transfer resistance / (( ), which is composed of the charge transfer resistances of the anodic and cathodic partial reactions. At the corrosion potential, the polarization resistance corresponds to Rp = R (Eco ) Th overall impedance of the equivalent circuit in Fig. 7-3 can then be described by... 

The main correction term, A 2 is related to the potential dependence of 2 for the corroding metal in the given environment through Eqs. (19)-(21). An example of this potential dependence at seven solution concentrations is shown in Fig. 2. The second correction term, (zo)chc > is related to the cathodic partial reaction. The concentration of the oxidant for this reaction (for example, the hydrogen ions or dissolved oxygen) may be affected by the potential distribution in the diffuse double layer. The effect is cotitrolled by the ionic charge of the oxidant and the stoichiometric number of the cathodic reaction mechanism. 

It is an experimental fact that whenever mass transfer limitations are excluded, the rate of charge transfer for a given electrochemical reaction varies exponentially with the so-called overpotential rj, which is the potential difference between the equilibrium potential F0 and the actual electrode potential E (t) = E — Ed). Since for the electrode reaction Eq. (1) there exists a forward and back reaction, both of which are changed by the applied overpotential in exponential fashion but in an opposite sense, one obtains as the effective total current density the difference between anodic and cathodic partial current densities according to the generalized Butler-Volmer equation ... 

Corrosion is defined as the spontaneous degradation of a reactive material by an aggressive environment and, at least in the case of metals in condensed media, it occurs by the simultaneous occurrence of at least one anodic (metal oxidation) and one cathodic (e.g. reduction of dissolved oxygen) reaction. Because these partial reactions are charge-transfer processes, corrosion phenomena are essentially electrochemical in nature. Accordingly, it is not surprising that electrochanical techniques have been used extensively in the study of corrosion phenomena, both to determine the corrosion rate and to define degradation mechanisms. 

The theory can be generalized by also taking into account the anodic term of the Butler-Volmer equation. If the anodic partial reaction is controlled by charge transfer, and if the cathodic reaction is under mixed control, the polarization curve is described by equation (4.94). 

At the beginning of the wetting period, the iron oxidation rate (expressed as partial current) significantly exceeds that of oxygen reduction. To satisfy the charge balance, a second cathodic reaction is therefore required, namely, the reduction of 7-FeOOH. The anodic partial current is then equal to the sum of the two cathodic partial currents ... 

The Heyrovsky reaction in Equations 3.7 and 3.8 is a pure charge-transfer reaction. The reaction rate in the cathodic reaction direction is proportional to the degree of surface coverage of atomic hydrogen 0), and the concentration of (acid solution) or H2O (alkaline solution). On the other hand, the anodic partial reaction is proportional to the concentration of molecular hydrogen h and the free surface (l -1 ). Based on the current-overpotential equation for the charge-transfer reaction, the Heyrovsky current density () expression can be written as Equations 3.37 and 3.38 for acid and alkaline solutions, respectively ... 

Charge of the reactant in the cathodic reaction Transfer coefficient of the anodic partial reaction of an electrochemical process... 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

When an electrode is at equilibrium the rate per unit area of the cathodic reaction equals that of the anodic reaction (the partial currents) and there is no net transfer of charge the potential of the electrode is the equilibrium potential and it is said to be unpolarised ... 

From a kinetic point of view a describes the influence of a change of the electrode potential on the energy of activation for the charge transfer reaction which in turn influences the partial current density. The transfer coefficients % for the anodic charge transfer reaction and for the cathodic reaction add up according to... 

The potential-decay method can be included in this group. Either a current is passed through the electrode for a certain period of time or the electrode is simply immersed in the solution and the dependence of the electrode potential on time is recorded in the currentless state. At a given electrolyte composition, various cathodic and anodic processes (e.g. anodic dissolution of the electrode) can proceed at the electrode simultaneously. The sum of their partial currents plus the charging current is equal to zero. As concentration changes thus occur in the electrolyte, the rates of the partial electrode reactions change along with the value of the electrode potential. The electrode potential has the character of a mixed potential (see Section 5.8.4). 

As demonstrated in Section 5.2, the electrode potential is determined by the rates of two opposing electrode reactions. The reactant in one of these reactions is always identical with the product of the other. However, the electrode potential can be determined by two electrode reactions that have nothing in common. For example, the dissolution of zinc in a mineral acid involves the evolution of hydrogen on the zinc surface with simultaneous ionization of zinc, where the divalent zinc ions diffuse away from the electrode. The sum of the partial currents corresponding to these two processes must equal zero (if the charging current for a change in the electrode potential is neglected). The potential attained by the metal under these conditions is termed the mixed potential Emix. If the polarization curves for both processes are known, then conditions can be determined such that the absolute values of the cathodic and anodic currents are identical (see Fig. 5.54A). The rate of dissolution of zinc is proportional to the partial anodic current. 

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