Dear Ms. Shahriari

Check the connection between the sample and the wire. Connect one end of the Ohmmeter to the sample and the other end to the wire. If the connection is correct, the resistance between the two must be zero.

Also, make sure that the water does not penetrate the contact point of the sample and the wire (Galvanic corrosion between sample and wire).

In addition, be sure to calibrate your potentiostat and reference electrode.

Best regards,

M. Sabouri

The resistance of your epoxy coating is higher than 1 Mohm and your potentiostat is not able to measure it.

I suggest you the following tests:

- try to measure OCP on a 1 Mohm resistor (AUX and REF connected together); if you will obtain a similar record, I am right and your instrument can't do more (what you record is in fact the instrument instability when all electrodes are ...in air/disconnected).

- try to measure OCP on your cell with a pH-meter in mV mode (the input impedance of a pH-meter is very high, more than 1 Gohm) ; here I am curious to know the result.

- my guess is that sample resistance is less than 1.5 Mohm ; if it is possible, try to prepare a thinner coating and to use a higher class potentiostat.

Dear Ms. Shahriari,

Yes, this is normal behavior of highly resistive coatings. I endorse Dr. Cornel answer, it is true if the coating resistance is high and beyond the detection limits of potentiostat, you may have this kind of response but most of the potentiostats can measure upto 100 Gohms (at low frequency). So in your case this may not be applicable. In addition to coating resistance, the most important factor is the high frequency impedance behavior and very large coating capacitance for which the critical frequency needed to present capacitive response is beyond the potentiostat maximum frequency limit. In other words, the highly capacitive response of the coating can not be detected due to the constraint in applying such a high frequency with the existing potentiostats (See the accuracy contour plot of potentiostat you are using) in your case (thick coating and probably the small exposed area) is causing this problem.

Now coming to the solution of this problem, you may get reliable data by either decreasing the thickness or by increase the exposed surface area of the coated sample depending on which suits you well. Also there are other ways to get rid of noise and to achieve reliable results for such systems. But i think you should adopt the above stated way first and if you find this problem again let me know, i can suggest few more options. Thanks

In completing the above mentioned, I suggest that the subject matter presented in the following address be studied:

https://www.gamry.com/application-notes/EIS/accuracy-contour-plots-measurement-and-discussion/

Best regards,

Mohsen

Dear Shahriari

can you explain coating surface condition (crack, morphology and etc) and substrate?

Dear Faride Shahriari,

what is the Vdc polarization parameter in your EIS[1] ?

Also:

1. Try, please, in some new EIS the following Vdc (minimum) list: -1.5V, -0.5V, +1.5V.

2. Take if possible, some SEM images along the time (unit, in weeks).

1. I hope that your electrical contact on your WE is, still, good (alive). Next time, I suggest to make two(!, ohmic) electrical contacts on your (same) WE, for many reasons, apart from the obvious diagnostics (ohmic check-up, along the time).

There are several solutions to overcome this problem:

1. Use thinner coatings (50 microns)

2. Use higher amplitude sinusoidal voltage in your measurements such as 20 or 50 mV (However, it could be destructive)

3. Perform your EIS measurements at longer immersion times such as 20, 40, and 60 days

4. You could report only Bode plots which have less noises in comparison to Nyquist plots

Dear Dr. Radu and Mr. Deen

Thanks a lot for collaboration in finding solutions. Your helpful advises are highly appreciated. I will work on suggested solutions as much as possible (due to my resources and available instruments) and I would share results.

Best regards

Dear Dr.Izadi

Thank you for advises you have shared kindly.

Best Regards.

Thank you Dr. Sabouri,

First af all thanks for helpful advices and comments.

But up to our knowledge and considering results from bare samples the setup and instruments are working well because the results of the bare samples. However, suggested solutions are absolutely useful.

Best regards

Dear Mr. Farzane,

The coating surface is almost dense and defect free, and the substrate surface was sand blasted to Sa 2 1/2 before painting.

Best regards

Dear Ms. Shahriari

I am agree with solutions suggested by Dr.Izadi and Dr.Radu as well as Mr.Deen. I think If you prepare a thinner coating (note that you may become away from industial coating process condition which result in thicker coatings) and longer exposure results would be more regular since diffusion may take place through coating. Also I think you should test (one test at least) with another potentiostate like EG&G 273 or other available devices to check ability of your Ivium device. According to Mr.Zahedi each device usually work well with a specific test and sample condition.

However, I dont know the scientific reason of such behaviour in OCP.

Bests

Dear Mr. Jafari

Your consideration is greatly appreciated, thank you. I will do my bests.

Best regards

Dear Dr. Samaras,

Thanks for your neat answer. I think I should study more since I did not get the point completely about some statements including Vdc range that you said. I hope you can provide me further information or suggest related references.

and also do you mean SEM image from surface or cross section?

thanks again.

Best regards

EIS spectra (please, measure) as well as Tafel plots depend[1,2] on the Vdc polarization parameter.

Yes, try both methods for SEM images.

1. An electrochemical method based on OCP fluctuations for anti-corrosion alloy composition optimization Article An electrochemical method based on OCP fluctuations for anti...

2. Application of Electrochemical Impedance Spectroscopy to evaluate cathodically protected coated steel in seawater Article Application of Electrochemical Impedance Spectroscopy to eva...

Dear Dr. @Samaras

Thanks for immediate answer and suggestions.

Cheers.