During the electrochemical analysis (EIS) of my coating for corrosion performance project, I come across a few negative readings from both coating resistance, Cp and charge transfer resistance, Rct.
My question is as stated, what is the meaning of these negative values? Do I need to repeat the analysis?
An example is given below.
Thank you very much for your kind answers.
Dear Dr. Wan Mohamad Ikhmal,
Measuring an EIS spectrum takes time (often many hours).
The system being measured must be at a steady state throughout the time required to measure the EIS spectrum.
A common cause of problems in EIS measurements and their analysis is drift in the system being measured.
In practice a steady state can be difficult to achieve. The cell can change through adsorption of solution impurities, growth of an oxide layer, build up of reaction products in solution, coating degradation, temperature changes, to list just a few factors. Standard EIS analysis tools may give you inaccurate results on a system that is not at a steady state.
For more detailed info, you can see at the source:
https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/
Best regards, Pierluigi Traverso
Dear Dr. Pierluigi Traverso
So, how can I create this steady state? Do I need to change any parameter before starting the measurement?
Dear Dr. Wan Mohamad Ikhmal,
it is not always possible to change some parameters before the Eis measurement in order to obtain a steady state in (a long) time. Something, however, can be attempted. In the case of a coating that tends to degrade strongly over time and leading to conditions of great instability, the situation could be improved, for example, by reducing the concentration of the electrolyte (or even sometimes changing it with a less aggressive one) so as to limit the drastic decrease in power protective of the coating. Another suggestion could be to monitor, for a definied time before the EIS measurement, the stability of the potential correlable directly to the stability of the system. EIS measures are, for better or for worse, a mix of theory and experience, to which we must approach with different methods of measurement set-up.
Best regards, Pierluigi Traverso
How to relate polarization curve with equivalent circuit? can anyone help me by explaining this?
Dear Dr. Muhammad junaid Anjum,
I think that the follow link:
https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/
already mentioned before, can provide useful answers to your question.
Best regards, Pierluigi Traverso
Dear Dr. Muhammad junaid Anjum
Thats a very good question. Now that u mentioned about it. I also don't really touch about that particular subject. But, based on my previous experimental procedure, I doubt that eq. circuit will have any effects on polarization curve or Tafel Plot. I mean, when I set the eq. circuit, it always directly affect the outcome of nyquist plot.
But, maybe someone else can give a better answer.
Dear Dr. Muhammad Junaid Anjum,
Polarization curve and equivalent circuit are two different set of data. Equivalent circuit is employed to model the impedance data, to understand the physical-electrochemical processes occurring in the desired system basically at the electrode/electrolyte interface. So, from the circuit you may obtain the polarization resistance of the system in study and compare with the polarization resistance acquired from the polarization curve.
Polarization resistance from polarization curve could be attained using tafel extrapolation.
Hope it helps. You may also like to check the gamry technical notes available on the gamry website for brief understanding.
Best Regards,
Prince
I think this is a measurement error or the optimization program has problems with matching experimental points to the model. 60 days of exposure is a time sufficient for the system to be stable. In addition, very low capacitance values indicate the electrical connection capacity rather than the actual shell capacity.
stefan
Prince Kumar Baranwal thank you for your kind answer. it is really helpful
Dear Dr. Wan Mohamad Ikhmal
Concerning your results, what was the equivalent circuit used?
Maybe you could also show one of your results. Thhat way we could have a clear view of your system.
Regards
Dear Wan Mohamad Ikhmal ,
the meaning of the most negative values (Zreal) is that there are some (incoherent) OCV changes, negative OCV-drifts[1], during these messy P-EIS (conventional Potentiostatic, Z-spectra) measurements.
So, yes, you need to repeat, at least, those old EIS measurements having negative[1,2] Zreal (bizarre, and unrealistic) values.
Next :
1) you could try, either:
a) to preset higher Vac amplitude values (up to 20mV), and you could, additionally, measure
b) a much faster P-EIS spectra (FLow>>10mGHz), under a certain VDC(?=OCV) value, in order to avoid a risk for some, corrosion based, unyielding OCV (drifts[1]) changes, mainly, during a single (messy[4] , P-)EIS spectrum measurement.
or you could try to switch, if possible, your EIS mode, e.g.
2) switch from the everyday P-EIS mode[3] (of measurements), to a special G-EIS (Galvanostatic mode), recommended for these case(s), like noted in your question, having negative Zreal values (in P-EIS modes).
1. https://www.researchgate.net/post/What_could_be_the_reason_behind_the_drift_in_EIS_data
2. https://www.researchgate.net/post/How_to_get_good_curves_in_impedance_measurement_technique
3. Potentiostatic mode (PEIS) for the impedance measurements : https://www.gamry.com/application-notes/EIS/eis-potentiostatic-galvanostatic-mode/
4. https://www.researchgate.net/post/Why_is_the_EIS_analysis_on_a_bare_metal_in_an_artificial_seawater_produce_this_result
Negative resistance values can occur when fitting an inductive loop with an R/C circuit.
See page 3 of the following application note: https://www.bio-logic.net/wp-content/uploads/20120817-Application-note-42.pdf
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