External currents, mixed-electrode cathodic

The separate anodic and cathodic processes will occur simultaneously but statistically independent of one another. The rate of each reaction will be governed by the electrical potential difference which exists across the metal-solution interface and the appropriate values of i0, a and 0 for each system. In the absence of an external disturbance, for instance an external current, a steady state will usually be reached where the sum of the rates of the cathodic reactions will equal the sum of the rates of the anodic reactions, viz., Sia =21. . The electrode potential will assume some value, Ej p, which is designated the mixed potential, and the electrode is considered as a poly-eieccrode (11, 16,17,18). 

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. 

Fig. 4.14 Representation of a mixed electrode with anodic reactant, M, and cathodic reactant, X. (a) Freely corroding condition, (b) Net external oxidation current, (c) Net external reduction current...

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

Upon polarization of either electrode, the cell potential moves along the oxidation and reduction curves as shown in Fig. 1.1. When the current through the cell is f, the potential of the copper and zinc electrodes is Cj cu and e zn > and each of the electrodes have been polarized by (Ceq.cu i.Cu) and (Ceq.zn i,z )- Upon further polarization, the anodic and cathodic curves intersect at a point where the external current is maximized. The measured output potential in a corroding system, often termed the mixed potential or the corrosion potential (Tcorr)> h the potential at the intersection of the anodic and the cathodic polarization curves. The value of the current at the corrosion potential is termed the corrosion current (Icon) and can be used to calculate corrosion rate. The corrosion current and the corrosion potential can be estimated from the kinetics of the individual redox reactions such as standard electrode potentials and exchange current densities for a specific system. Electrochemical kinetics of corrosion and solved case studies are discussed in Chapter 3. 

The procedure for determining a current density-potential curve is to immerse the metal electrode in the corrosive medium. After an incubation period, the mixed potential corresponding to the outwardly currentless state becomes established on the metal. If a circuit is now placed on the electrode and external current is applied, the result is a shift in potential polarization. Current density-potential curves are therefore also known as polarization curves. Depending on the direction of the external current imposed, this is termed anodic or cathodic polarization, and anodic potential becomes positive while cathodic potential becomes negative. 

The copper thus corrodes without any external current. The open circuit potential of a mixed electrode undergoing corrosion, is called the corrosion potential (in the literature it is sometimes also called the free corrosion potential). The corrosion potential has a value that lies in between the equilibrium potentials of the partial electrode reactions. In contrast to the equilibrium potential, which is a thermodynamic quantity, the corrosion potential is determined by kinetics its value depends on the rates of both the anodic and the cathodic partial reactions present. 

Bubbles of hydrogen are observed from the surface of zinc electrode, and formation of bubbles of hydrogen is a cathodic reaction. Hydrogen is reduced and not oxidized. Similarly, zinc is oxidized and not reduced. Hence, only the two reactions (a) and (b) proceed. Under the condition of rest (no outside current), the potential of the electrode cannot be computed by the Nernst equation as it is not reversible. Also, the above electrode would not corrode in the absence of an external current. The potential assumed by the electrode under the above condition is the mixed potential and its value lies between the value of equilibrium potential of hydrogen and zinc. The value of the potential would depend on the metal and the environment. It is to be observed that the corrosion potential (Ecorr) is not the equilibrium potential of either of the reactions, but some intermediate potential determined by the two partial anodic and cathodic reactions. Both the reactions... 

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