Equivalent electrical circuit metal-electrolyte interface

Many treatments of this subject have used an electrical equivalent circuit model to simulate the corroding metal/ electrolyte interface [1,38,39]. The simplest form of such a model is shown in Fig. 4. The three parameters discussed above... 

FIG. 4—Electrical equivalent circuit model simulating a simple corroding metal/electrolyte Interface. See also Fig. 5. R. is the solution resistance. R, is the polarization resistance. C Is the double layer capacitance. 

Fig. 6 presents the system of A1 corrosion in 0.5 M NaCl solution, its frequency impedance characteristic in the form of Nyquist plot and the equivalent electrical circuit. Individual parts of the electric circuit reflect the electrochemical and electrical characteristics of the corrosion systems. In this arrangement, the spectral characteristic of the impedance in the Nyquist plot has the shape of a semicircle, whose intersection with the real axis in the high-frequency range determines the electrolyte solution resistance Rs. Conversely, the intersection of the real axis in the low-frequency range corresponds to the sum of Rs + Rci/ where Ret indicates the charge transfer resistance of the boundary metal/electrolyte, and characterizes the rate of corrosion. On the other hand, Cdi component of the circuit represents capacity of the double layer at the interface metal/electrolyte. 

Electrochemical methods are well adapted for characterizing the corrosion behavior of coated metals in solution. Because of the high resistance of organic coatings, ac methods are generally more suited than dc polarization methods. In electrochemical impedance spectroscopy (EIC) one measures the response of the coated electrode to a small amplitude ac perturbation as a function of frequency (Chapter 5). The interpretation of the measured frequency response, in principle, requires a physical model. However, for coated metals useful information is more easily obtained by representing the metal-coating-electrolyte interface by an electrical circuit (equivalent circuit). 

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