Uniform corrosion mixed electrodes

In review, consider a mixed electrode at which one net reaction is the transfer of metal to the solution as metal ions, and the other net reaction is the reduction of chemical species in the solution such as H+, 02, Fe3+, or N02 on the metal surface. For purposes of the present discussion, no attempt is made to define the individual sites for the anodic (net oxidation) and cathodic (net reduction) reactions. They may be homogeneously distributed, resulting in uniform corrosion, or segregated, resulting in localized corrosion. In the latter case, the cathode-to-anode area ratio is of practical importance in determining the rate of penetration at anodic areas. 

Standards define uniform surface corrosion as corrosion practically equal to mass loss over the entire surface (homogeneous mixed electrode), and nonuniform corrosion (shallow pit formation) as corrosion with locally different mass losses. 

The theory of mixed electrodes was developed by Wagner and Traud [1] and first applied to corrosion by Stern and Geary [2]. In the following we develop the Butler-Volmer equation for a mixed electrode by taking a specific example the uniform corrosion of iron in hydrochloric acid in the absence of concentration gradients (Cj s = Ci b). Two electrode reactions take place simultaneously ... 

Formally, a corrosion cell can thus be treated as a mixed electrode just as in uniform corrosion, but there is one important difference the anodic and cathodic surfaces are not necessarily equal. As a consequence, if according to (7.5) the intensity of anodic and cathodic partial currents are always equivalent, the respective current densities may differ depending on the surface area ratio between cathode and anode. 

Equation 10.2, which involves consumption of the metal and release of electrons, is termed an anodic reaction. Equation 10.3, which represents consumption of electrons and dissolved species in the environment, is termed a cathodic reaction. Whenever spontaneous corrosion reactions occur, all the electrons released in the anodic reaction are consumed in the cathodic reaction no excess or deficiency is found. Moreover, the metal normally takes up a more or less uniform electrode potential, often called the corrosion or mixed potential (Ecotr)-... 

A reference electrode scanned along the metal surface will measure the series of (E"x)n and (E"M)n interface potentials. From these values, solution potentials (t))s) at the metal/solution interface may be calculated (< )s = -E") and presented as in Fig. 4.6. When the anodic and cathodic sites are microscopic relative to the size and position of the reference electrode, identity of the anodic and cathodic sites on a macroscale is lost, and a single mixed or corrosion potential, Ecorr, is measured as discussed previously. There is essentially a uniform flux of metal ions from the surface, and cathodic reactants to the surface, which constitute anodic and cathodic currents. Since the relative areas to which these currents apply usually are not known, the total area is taken as the effective area for each reaction. It is these currents, however, that mutually polarize the anodic reaction potential from E M up to Ecorr and the cathodic reaction potential from E x down to Ecorr. 

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