Impedance at the Corrosion Potential

Remember 9.1 In an equivalent electrical circuit, boxes should be used to designate impedances that cannot be generally described in terms of passive elements. 

the total current consists of the sum of charging and Faradaic currents. At the corrosion potential, the sum of the anodic and cathodic Faradaic currents is equal to zero, i.e., ig + k = 0. The Faradaic impedance must, therefore, be a parallel combination of Zg and Zg. The contribution of the double-layer capacitance is added in parallel. Expressions for the impedances Zg and Zc must be developed separately according to proposed reaction mechanisms (see Chapter 10). 

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Figure 9.2 Equivalent electrical circuit of the interfacial impedance at the corrosion potential, where ic represents the charging current and I c represents the cathodic current.

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