Ideally, EIS is based on two hypothesis : linearity of the I vs. E curve and stationnarity (system don't change during the measurement, current vairation during EIS measurement is quite null). You have to verify the validity of these two hypothesis:

- plot a curve (CV) of the evolution of the current measured vs. the applied voltage for a range of +/- 50 mV vs. OCV. It has to be recorded at different speed, in agreement with the frequency range of your EIS measurement. You could then define the range of linearity of your system: may be 10 mV, may be 50 mV or only 1 mV...

- For stationarity, it's more easy, plot the values of the current recorded during EIS measurement vs. time. If the current changed (significantly), don't lose time to study your spectrum... Your system changed during the measurement and no equivalent circuit can be used to fit with something changing.

These two points MUST be verified for each measurement before analysis, then, you could understand why you only have sometimes nice results.

Be also very carreful with plugings the electrodes (avoid alligators plugs) and twisted wires.

Good luck, Nicolas

Do you have an example and type of system you are working with? Likely your cell and electrodes are not stable or your parameters (amplitude, number of scans, selection of frequencies) are not optimized. Try increasing the amplitude or averaging over a larger number of scans per frequency.

The working electrode is 316SS in 0.5 M HCl solution, Platinum as counter electrode and Ag/AgCl as Reference electrode. frequency range 10^5 to 10^-2 and voltage excitation 10 mV/s. These are the condition. I will try larger number of scan for next time.

yes i am using strong acidic media. The frequency range i am using what amplitude should be better 50mV or 10mV ? what you suggest to get better results. Mostly I measured the OCP is around -0.3V.

10mV amplitude seems rather low to me. You can try the larger amplitude but you should make sure applying a potential away from OCV will not affect the corrosion reaction. A Faraday cage is also a good idea for low frequencies.

Dimensions of your studied surface ?

What does 'the best Bode and Niquist diagram' mean ?

Your registered signal is noisy or you get different Z values on same surface during different runs?

What does' the OCP I always measure is not good enough' mean ?

When do you consider your system at echilibrium (mV)?

How do you prepare your surface ?

Thanks,

Cheers

Ideally, EIS is based on two hypothesis : linearity of the I vs. E curve and stationnarity (system don't change during the measurement, current vairation during EIS measurement is quite null). You have to verify the validity of these two hypothesis:

- plot a curve (CV) of the evolution of the current measured vs. the applied voltage for a range of +/- 50 mV vs. OCV. It has to be recorded at different speed, in agreement with the frequency range of your EIS measurement. You could then define the range of linearity of your system: may be 10 mV, may be 50 mV or only 1 mV...

- For stationarity, it's more easy, plot the values of the current recorded during EIS measurement vs. time. If the current changed (significantly), don't lose time to study your spectrum... Your system changed during the measurement and no equivalent circuit can be used to fit with something changing.

These two points MUST be verified for each measurement before analysis, then, you could understand why you only have sometimes nice results.

Be also very carreful with plugings the electrodes (avoid alligators plugs) and twisted wires.

Good luck, Nicolas

I do not know whether this is relevant, but I did some exps with passivating dental NiCrMo alloys about 25 years ago at pH7, with oxygen (from air) being the cathodic reaction. I noticed that the OCP potential slowly increased after immersion, linear vs log t. This was explained by a 1 / t behavior of the anodic current, in combination with regularTafel behavior for the cathodic current. It is all in my thesis, Corrosion of dental NiCrMo alloys, and I have published several papers, for instance in the Journal of the Electrochemical Society (containing the wrongly phrased term 'potentiostatic deaeration method') and Corrosion Science. I mention in my thesis that one could estimate how long it would take for the OCP to reach the pitting potential, what would permit to say something about long term behavior of such alloys. If you are interested, I can send you my thesis. I won the Borghardt-VOM prize for this work, but no funding, so I moved on.

Success,

Anthonie Muller

You should try to make constant the experimental conditions as much as possible you can otherwise the results will not be the same. Any infinitesimal change in the conditions will change the results. EIS is so sensitive to the reaction conditions. Check again and again your electrodes fittings, the concentration of electrolyte, software and CV of your system. If the CVs are reproducible during different runs, it will give you the system stability facor and before EIS exps you should check the electrochemical window of the system.