Dear Hardhal

For the coated sample of the 316L stainless steel ,it would be an approepriate method to use electrochemical impedance spectroscopy (EIS) to study the corrosion behavior of the sample. The EIS method is suitable for high resistive materials, i.e.,coated samples, inhibited solution and so on. The EIS is a subtle method to the point that one can control the current frequency for those sensitive conditions. Also one should realize that coated samples can have localized regions,i.e.,holes,that are exposed to the aquous solution. Therefore,I would assume that your coated sample has a lower corrosion resistance than its counter part, the uncoated sample is because the coated sample has localized exposed areas to the solution.

The corrosion behavior of the coated stainless steel might be different if the sample tested by EIS as compared if the sample tested by LPR. In LPR ,one uses DC current to ultimately extrapolate,i.e.,extract the PR from LPR plot to measure the corrosion rate.

In contrast,in the EIS ,one uses AC current with a specific frequency range and a specific potential amplitude to plot Bode and simultaneous Nyquist in oder to determine the impedance of the sample at a low frequency ,ie., 0.16Hz, which is equivalent to PR,then determine the corrosion rate.

Dear Dr. Harshal Mehta

considering to make the measurement on two samples coated with the same coating (also applied in the same way) and without unprotected areas but having a different shape, both the geometry of the sample and the compatibility with an unsuitable counter, can lead to differences in the measurement . Using the EIS technique, which is also particularly sensitive to the arrangement and geometry of the cell, I use a cpunter electrode in Pt, in the form of a network with a different shape in order to optimize the current lines; square type. and always at a constant distance, if my sample is in the form of a coated foil. If, on the other hand, I am dealing with a coated tube or bar, I use a cylindrical counter in Pt that "wraps" the sample, as per the attached figures. Of course, in addition to the electrochemical tests, I also always carried out other resistance measurements of the coatings in parallel and, fortunately, I never had trends other than those I expected ...

My best regards, Pierluigi Traverso.

You have to look at the low frequency limit of the impedance spectra, to get polarization resistance (Rp). Often one has to use circuit fitting and find the low frequency limit by extrapolation. Within experimental error, EIS based Rp should match with the Rp obtained from LPR and Tafel extrapolation.

Beware - In some cases, if the coating is exceptional, it may appear as high solution resistance. One should check to make sure that the solution resistance calculated from the EIS of coated and uncoated samples in the same solution is same.

If you see more than one semicircle (depressed or otherwise), then what you are seeing in the first semicircle is charge transfer resistance. You can't compare that with LPR results

You have to carefully measure EIS spectra again. Generally, the Depressed semi-circle has lower resistance as well. On the other hand, coated specimens must have high resistance. What is your definition of depressed Nyquist circle here?

Khurram Shahzad, as the coated sample is showing high corrosion resistance according to Tafel extrapolation method so I suppose the Nyquist plot of it must exhibit higher resistance than the bare. but the coated is showing depressed loop than bare sample. that's why my question is in which case is it possible that the charge transfer resistance is low but the coating is resistive to corrosion. what can be the possible mechanism behind this phenomena?

Well, it's hard to answer your question without looking at your Nyquist plot. You mentioned here, you are getting better corrosion resistance for coated specimens it also means you are getting better absolute impedance in the bode plot as well for a coated specimens as Z=R+C+L, neglecting L implies Z=R+C, at the same time, you are getting depressed semicircle. A semi-circle depression usually means the low value of alpha or n. or the low value of capacitance as well. If so, then your coated specimen must have a low value of capacitance than bare metal (which is unusual). Only then you can get Z = R+C (very low C) lower than bare metal. In that case, you can get low Zimg (Depressed semi-circle) or high Zr (in the x-direction of Nyquist plot). For me, it just seems, you have improper measurements.