Equivalent Circuits The Electrical Approach

The equivalent electrical circuit approach has already been introduced in connection with analysis of mass sensors (Chapter 4). Its application is older and somewhat 

The equivalent circuit representing the electrochemical cell in Fig. 5.1 (insert) is shown in Fig. 5.6a. Its overall impedance Zcen is 

The symbol ft) is angular frequency. Note that Jim denotes an imaginary number, and not a symbol for current density. Next, we perform the separation of the real and the imaginary parts by multiplying the numerator and the denominator of the second term of (5.26) with (j o)RciC -1). Thus, we obtain for the cell impedance  

the second term is the real part of the electrode impedance ZRe and the third term is its imaginary part Z.  

A plot of Zim against Zre, called the Nyquist plot, is a semicircle centered on the real axis (Fig. 5.6b). In this plot, the angular frequency ft) is the parameter that increases in the anticlockwise direction. We can now perform a simple phenomenological analysis of the plot. From (5.27), we see that at low frequencies (i.e., at almost DC behavior) 

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Of these different modeling methods, the equivalent electrical circuit approach is the most widely used to serve the need for precision of the model without too high a degree of complexity of its implementation. 

The equivalent electrical circuit approach adopted maps the processes occurring in the corrosion systems and enables the determination of parameters relevant to these processes. The parameter values of individual elements of equivalent electrical circuit representing investigated corrosion systems are summarized in Table 2. 

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