In Nyquist plot, high frequency region represents ESR (equivalent series resistance), but low frequency region show Rch charge transfer resistance and curve shape which indicates capacitive or inductive behavior. So, if you have capacitor you will obtain the same curve in low region.

Thank you Sir. Could you please explain this behaviour in detail. I have attached the Nyquist plot.

Dear sripriya

The maximum allowable potential amplitude is 10 mv in all EIS studies. The amplitude of 50 mv is destructive. It is the reason or different quantities of Rs. In other word the amplitude of 50 mv should not be used in EIS experiments.

Dear Sripriya Rengaraju,

1) apply the lowest possible amplitude (10mV or even less) value in all your EIS studies to avoid non linearity effects, as the (nearly bad) Z-spectrum (50mV) at high frequencies. So, the low amplitude, the better (Z-spectrum).

2) what is the high frequency limit, because you note 0.1MHz (to 0.01Hz) in your question, and you note 1MHz in the attached Nyquist plot ?

3) what is the CE (counter/auxiliary electrode, Graphite?) and it's surface is quite high ?

4) the mortar in your study is a workable paste or it is, nearly, dry ?

Also,

5) what is, the main goal in your studies, testing steel embedded in mortar ?

You must break your goal(s) in two parts, if you want useful results. One potential goal might be, the Z(-spectra) study of the:

a) WE (the steel as a Working Electrode, embedded in mortar), and it's (steel's) evolution in time. In this case we need a 3-electrode configuration[1,2]. A second potential goal might be, the Z(-spectra) study of the:

b) media (mortar) as an electrolyte (workable paste, not dry), and it's (mortar's) evolution in time. In this case we might need a more complex[3-5] (4-) electrode configuration[4]. You can use four probe electrical conductivity method to measure directly the electrical conductivity of your media (mortar). 

1. http://www.gamry.com/application-notes/electrodes-cells/two-three-and-four-electrode-experiments/

2. a) Simultaneous impedance and ... Data Simultaneous impedance and volumetric studies and additional...

b) https://standards.globalspec.com/stds/topic/251136370

c) https://www.researchgate.net/post/What_is_the_difference_between_two_electrode_and_three_electrode_photoelectrochemical_characterization

3. Three electrode configuration measurements of electrolyte-diffusion barrier-cathode interface Article Three electrode configuration measurements of electrolyte-di...

4. https://en.wikipedia.org/wiki/Van_der_Pauw_method

5. http://pvcdrom.pveducation.org/CHARACT/4pp.HTM

Dear Ioannis Samaras,

Thanks for pointing out the frequency. I have corrected that in the question.

I am trying to find the polarisation resistance of steel embedded in mortar. Before that, I wanted to determine the effect of test parameters. I got the curves as mentioned earlier and want to arrive at the inference.