Equivalent circuit model pitting

Equivalent circuit analysis is well suited for analysis of EIS measurements of conversion coatings and is the primary method for interpreting EIS spectra from conversion coated metal surfaces. A widely accepted generalized equivalent circuit model for the EIS response of pitted conversion coatings is shown in Fig. 22a (66,67). Several related models discussed below are also shown. In the gener-... 

Figure 22 Equivalent circuit models for conversion coatings, (a) Generalized model for pitting conversion coated surfaces, adapted from Ref. (b) Model for a barrier conversion coating, (c) Model for the early stages of CCC breakdown.

Using the generalized equivalent circuit model for conversion coated surfaces shown in Fig. 22, it is possible to track the time-dependent changes in the resistances and capacitances of the intact coating and evolving pits. Figure 25 shows representative Bode plots for CeCl3-passivated and bare A1 7075-T6 immersed in 0.5 M NaCl solution (81). Spectra like these were collected over 35... 

Breakdown of anodic films is yet another phenomenon for which EIS is well suited. Equivalent circuit analysis has been used to analyze EIS spectra from corroding anodized surfaces. While changes in anodic films due to sealing are detected at higher frequencies, pitting is detected at lower frequencies. Film breakdown leads to substrate dissolution, and equivalent circuit models must be amended to account for the faradaic processes associated with localized corrosion. 

Figure 32 (a) The equivalent circuit model and physical model for a passive pit in a... 

If the data collected do not fit the simplest equivalent-circuit model (Fig. 6.18), more complex models are analyzed. A number of equivalent circuits have been developed to model corrosion processes involving diffusion control, porous films or coatings, pseudoinductive mechanisms, simultaneous electrochemical and chemical reactions, and pitting corrosion (Ref 14-18). 

Figure 7.25 Equivalent circuit models proposed for the interpretation of EIS results measured in corroding systems (a) simplest representation of an electrochemical interface (6) one relaxation time constant with extended diffusion (c) two relaxation time constants and (d) the impedance of pitting processes of Al-based materials.

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