Impedance at open circuit

EIS is a method that is not very well suited for the study of pitting corrosion because, as described earlier, it should be applied to electrodes at steady state, and pitting is a non-steady state condition. Nonetheless, EIS can be used to assess the low-frequency impedance at open circuit, which provides a measure of corrosion rate without the need to polarize away from open circuit. It is therefore possible to use EIS to determine if pitting is occurring at open circuit. 

Thus, measuring a single small-current spectrum, the Tafel slope can immediately be obtained from Equation 5.123 and the double layer capacitance from Equation 5.124. Note that the ratio a/e does not contain the exchange current density and, hence, this parameter cannot be determined from the spectrum. To determine (, a spectrum close to OCP has to be measured (the section Impedance at Open-Circuit Potential ). 

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