Nikeset values ​​the value of the response and its angle to each value from zero to infinity, where the frequency is the variable in the Nyxt diagram, which represents the axis of the senate, while the response is the axis of the antagonists. The system is stable or unstable.

Your (Nyquist plot, WE case, of an Organic coating) cell could be analyzed in two experimental Equivalent Circuit (EC) steps.

1) Start with the simplest EC1, case1:

ZEQ = Z0 + Z1 + Z2 , where

Z0 = R0 (R0 ~0.53 Ohm)

Z1 = R1 // C1 , (R1 ~1 Ohm), a 1st simplified Randles' cell1[1,3]

Z2 = R2 // C2 , (R2 ~2 Ohm), a 2nd simplified Randles' cell2[1,3]

2) If the above simple EC1 (in case1) is not delightful, healthy enough for your experimental parametric conditions and other further needs, then proceed with a more complicated one[2,3] (case2), one good (optional) case from a candidate ECs list, more specific cases for coated metals.

1. Simplified Randles cell (Figure 14)[3] : The Simplified Randles cell is one of most common cell models. It includes a solution resistance, a double layer capacitor and a charge transfer (or polarization resistance). The double-layer capacitance is in parallel with the charge-transfer resistance. In addition to being a useful model in its own right, the Simplified Randles Cell is the starting point for other more complex models.

2. EIS of Coated Metals (Figure 22)[3] : The interpretation of impedance data from failed coatings can be very complicated.

3. https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/

There are some strategies and bases in Nyquest analysis, and basically it cannot be analyzed perfectly in the absence of Bode spectrum.

As a base, this double circle plot indicates the presence of two time factors ( or two reactions or reaction areas), for an example a coating with high porosity, or under-coating corrosion , thus you have to include two interconnected RC loops or may be more complex form of circuit model

Gamry Technical note reffered to by Ioannis Samaras gives a lot of information as well. There are other technical notes about EIS from Gamry and Princeton Applied Research (or Ameteksi)

Dear Dr. Wan Mohamad Ikhmal,

in agreement with the answers of the previous colleagues, I would add a case that seems very similar to what you indicate and which can be found at: https://www.palmsenscorrosion.com/knowledgebase/

Most coatings will behave like an ideal coating which means that they are a resistor and capacitor in series. More complex behavior will be developed over time due to water penetrating the coating. As long as no corrosion has started, the EIS will look like a simplified Randles circuit. The serial resistor would stay the Rsol, but the charge transfer resistor Rct would be replaced by the pore resistance Rpor and the double layer capacity Cdl by the coating capacitance CC.

When the water reaches the metal interface, corrosion can appear. This situation is reflected by the equivalent circuit. The corrosion can lead to gas formation under the coating, leading to blisters and disbonding. The solution inside a blister, as long as the blister is closed, can be quite different from the bulk solution. The corrosion reactions often lead to a low pH value.

Attached Figure:

Equivalent circuit and Nyquist plot of a corroding, coated surface.

Best regards, Pierluigi Traverso