I have some EIS data of corroded cast iron coupons in NaCl solutions. I want to model my data by taking into account two components. A charge transfer layer and a porous layer. For both of these components there would be a capacitive and a resistive effect in the equivalent circuit (EC) which are represented by a constant phase element (Q) and a resistant (R). I can understand why Qct and Rct are in parallel but I can't figure out why charge transfer components are in series with just the Rp? Please find the EC picture attached.
Hi Farhad,
Equivalent circuit models are meant to have a significance to what is physically/chemically occurring in the cell. In general for coated (oxide or paint) material with corrosion under the coating, the charge transfer resistance and associated Cdl all comes from current that must pass through Rpore to get to the surface. The coating capacitance (Q in your model) is parallel to that entire circuit. (Note the models also need to have value for the actual spectra collected, and if you only see one time constant it is not possible have any confidence in values from a fit to a 2 time constant model.)
Hi Farhad, Jacob is on the money. I always have to make a picture of these sort of systems in my head. In your system you have the porous film Rp and Q and the interaction with the electrolyte (Rct and Qct). If the film is perfect (has no pores) then charge cannot get to the solution therefore there will be no contribution from Rct and Qct as Rp will be too big for any charge to get into that part of the circuit. All you will see will be Q which will depend in the thickness, dielectric constant and area of the film. When there are pores, Rp becomes much smaller and charge can get to Rct and Qct so you will measure them. That is why the electrolyte interactions are in series with Rp.
Hi David,
Do you have any reference where they have used an equivalent system in which more than one CPE element in parallel connection has been mentioned ...?? I
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