Localized corrosion mixed electrodes

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

Figure 4-3a indicates the ideal case of a mixed electrode in free corrosion. Such situations do not arise in soils or aqueous media. Usually the attack is locally nonuniform (see Fig. 4-3b) in which the current balance is not equalized at small regions along the surface. This is a case of free corrosion without extended corro-... 

Fig. 11-1. Mixed electrode model (local cell model) for corrosion of metals i = anodic current for transfer of iron ions i = cathodic current of electron transfer for reduction of hydrogen ions.

Analysis methods for electrochemical noise data can be separated into three categories, (1) deterministic, (2) statistical, and (3) spectral. Deterministic methods involve the use of mixed potential theory to explain the oscillations that occur. For example, if the ZRA current increases suddenly while the potential difference between the two current electrodes and the potential electrode increases, localized corrosion has likely initiated on one of the current electrodes. A common pitfall in such a measurement is that if a nominally identical reference electrode is used, it could pit as well, leading to no change in potential versus the coupled electrodes. Due to the need for careful interpretation, deterministic methods are not widely used. 

Solid materials, in general, are more or less subject to corrosion in the environments where they stand, and materials corrosion is one of the most troublesome problems we have been frequently confronted with in the current industrialized world. In the past decades, corrosion science has steadily contributed to the understanding of materials corrosion and its prevention. Modem corrosion science of materials is rooted in the local cell model of metallic corrosion proposed by Evans [1] and in the mixed electrode potential concept of metallic corrosion proved by Wagner and Traud [2]. These two magnificent achievements have combined into what we call the electrochemical theory of metallic corrosion. It describes metallic corrosion as a coupled reaction of anodic metal dissolution and cathodic oxidant reduction. The electrochemical theory of corrosion can be applied not only to metals but also to other solid materials. 

The conventional approach to corrosion is to start directly with the concept of a mixed electrode of indistinguishable distribution of sites for the anodic and cathodic reactions. The approach taken in this chapter is to first examine the behavior of distinguishable anodic and cathodic sites. This is the classical case of galvanic couples of joined dissimilar metals in contact with a common solution. In this case, local movement of a reference electrode through the solution can map the... 

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 cause of nonuniform corrosion is the presence of corrosion elements, that is, heterogeneous mixed electrodes. The common feature of both types of corrosion is the geometry of the damaged area. The surface spread of these areas is generally greater than the depth. Accordingly, compared with localized types of corrosion such as pitting, these types of corrosion are limited in extent. 

The dissolution of zinc in a mineral acid is much faster when the zinc contains an admixture of copper. This is because the surface of the metal contains copper crystallites at which hydrogen evolution occurs with a much lower overpotential than at zinc (see Fig. 5.54C). The mixed potential is shifted to a more positive value, E mix, and the corrosion current increases. In this case the cathodic and anodic processes occur on separate surfaces. This phenomenon is termed corrosion of a chemically heterogeneous surface. In the solution an electric current flows between the cathodic and anodic domains which represent short-circuited electrodes of a galvanic cell. A. de la Rive assumed this to be the only kind of corrosion, calling these systems local cells. 

The kinetic nature of mixed potentials is, in most cases, responsible for the lack of reproducibility. In a given solution under the same conditions, the same metal with different surface characteristics may adopt a different corrosion potential and, even at a given polycrystalline electrode, the corrosion potential is an average of different local regions of different properties crystal orientation, defects, and chemical heterogeneities. 

It follows that the corrosion potential on a heterogeneous metal corroding by local-cell action is virtually equal to the mixed potential at an electrode on which electronation and deelectronation reactions are occurring on spatially separated sinks and sources and is identical to a mixed potential when the metal is corroding homogeneously by a Wagner-Traud mechanism. The concept of the corrosion current /corr and the corrosion potential 40corr will now be treated quantitatively. 

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