Corrosion potentials 198 Double electrical layer

Transfer of charge carriers from the metal to the electrolyte solution as well as movement of ions in the electrol3de are hampered by the corrosive system. The electrolyte solution induces the formation of a surface layer in the vicinity of the metal electrode surface consisting of spatially separated positive and negative charge carriers, which is called a double electric layer. Spatial separation of charges is always accompanied by a potential difference, therefore the double layer exerts a perceptible influence on the rate of electrode processes. The double layer consists of two parts a compact layer and a diffusive layer (Fig. f.2). 

The potential difference across the electric double layer A. This cannot be determined in absolute terms but must be defined with reference to another charged interface, i.e. a reference electrode. In the case of a corroding metal the potential is the corrosion potential which arises from the mutual polarisation of the anodic and cathodic reactions constituting the overall corrosion reaction see Section 1.4). 

From this physical model, an electrical model of the interface can be given. Free corrosion is the association of an anodic process (iron dissolution) and a cathodic process (electrolyte reduction). Ther ore, as discussed in Section 9.2.1, the total impedance of the system near the corrosion potential is equivalent to an anodic impedance Za in parallel with a cathodic impedance Zc with a solution resistance Re added in series as shoxvn in Figure 13.13(a). The anodic impedance Za is simply depicted by a double-layer capacitance in parallel with a charge-transfer resistance (Figure 13.13(b)). The cathodic branch is described, following the method of de Levie, by a distributed impedance in space as a transmission line in the conducting macropore (Figure 13.12). The interfacial impedance of the microporous... 

The capacitance. The electrical double layer may be regarded as a resistance and capacitance in parallel see Section 20.1), and measurements of the electrical impedance by the imposition of an alternating potential of known frequency can provide information on the nature of a surface. Electrochemical impedance spectroscopy is now well established as a powerful technique for investigating electrochemical and corrosion systems. 

The previous section discussed the structure at the junction of two phases, the one a solid electron conductor, the other an ionic solution. Why is this important Knowledge of the structure of the interface, the distribution of particles in this region, and the variation of the electric potential in the double layer, permits one to control reactions occurring in this region. Control of these reactions is important because they are the foundation stones of important mechanisms linked to the understanding of industrial processes and problems, such as deposition and dissolution of metals, corrosion, electrocatalysis, film formation, and electro-organic synthesis. 

Pyrrole electropolymerization on noninert metallic substrates has been specially optimized for aluminum [33,34] and stainless steel [35,36]. In the case of aluminum electrodes, highly conductive polypyrrole films are obtained from solutions of t-butylammonium p-toluenesulfonate in acetonitrile [37]. Their conductivities range between 10 and 350 S cm", as a function of the electrical and chemical variables of synthesis. On stainless steel, highly conductive polypyrrole films can be obtained by means of square waves of potential [36]. In this case, charging of the electrical double layers, oxidation of pyrrole molecules, and formation of a porous oxide layer occur during the application of the anodic step and promote the polymerization process. The application of the cathodic potential seems to avoid corrosion... 

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