Hi,
I am doing work on corrosion protective coating on mild steel sample, I have extracted EIS data from electrochemical workstation. I am using Zsimpwin software to fit Nyquist and Bode plot to get equivalent circuit. My EIS data is not fitting to simple circuit like RQR, RCR etc but it is well fitted for R(Q(R(QR))) circuit. After fitting curve now I am in confusion in finding out which one is coating resistance and which one is charge transfer resistance in the final data. Please suggest me the way to deduce Rc and Rct.
Thank you.
Dear Mahesh,
I can give you general idea about the interpretation of the impedance behavior of coated samples. Please go through the following discussion
1. If you have very protective and uniform coating (without any defects and holidays), your data may either fit to the single or two time constants depending on the dielectric characteristics of the coating. If you have two time constant in this case, these should be in series combination and the first time constant (after electrolyte resistance) would be for coating.
2. If you have defective coating (as i can see from your results) on the steel surface, you can also use two time constants (TCs) (as you used here) but in this case you have used parallel combination of these TCs which is correct. In this case the first TC would represent the Rct and Cdl and the next would be related with the coating. Simply, as you know current always follows the least resistive path and double layer is formed very rapidly (so high frequency response can be used to predict this). Whenever you have defects in the coating (examine through other physical analytical techniques), the ingress of electrolyte at the interface would establish the ionic transport to the electroconductive surface and may initiate electrochemical processes.
Note: In Case 1: The high frequency regime of the impedance spectra will be affiliated with the coating reponse. In case 2: this will represent the double layer and charge transfer processes
thanks
Dear Kashif Mairaj Deen ,
If R(Q(R(QR))) from Zsimpwin software best fitted your plots, then, the likely physical meanings are: R (the first R) represent solution resistance; Q(the first Q) represents coating capacitance (Qcoat); R (the second R) represents coating pore resistance (Rpore); while the (QR) can be ascribed to double layer capacitance(Qdl) and charge Transfer resistance (Rct). You can get more details on the attached paper and the associated references
Kind Regards
No need to guess. You should prove each circuit element meaning, using different thickness protective films or different surface area electrodes.
Dear Demian,
This depends on the dielectric characteristics of coating. If the applied coating has large dielectric constant than the surface charge (within the defect), then definitely the high frequency response would be related with the coating. But in this case as i perceived from the question I consider the defective coating which has small dielectric constant and has not absorbed enough water yet. In this case the first response would be related with the double layer charging. I too support the comment by Dr. Alexey as in the true sense we need to develop a kinetic model to validate each parameter (element) included in the equivalent circuit. In this case we can validate the physical model with kinetic behavior.
Thanks for sharing a very good paper.
Thanks much for your kind support and suggestions Mr. Kashif Mairaj Deen, Mr. Alexey L. Klyuev and Mr.Demian Ifeanyi Njoku.
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