Dear Ms.Ibrahim. As shown in first pic I attached, Nyquist plot could be obtained from EIS test. Also Icorr calculated from the equation in second pic that Ba and Bc are anodic and cathodic tofel constant. Other parameters were shown in first pic. At the end you could use the equation in third pic to obtain corrosion rate. In this equation, a, n, f are atomic mass, valence change and Faradays constant respectively.

Dear Dr. Mihad Ibrahim,

If you have a suitable software that you can use to fit the data to a physically realistic equivalent circuit model, then you should be able to calculate the polarisation resistance, Rp. If you then run a short a linear DC polarisation experiment (and construct a Tafel plot), you can obtain the anodic and cathodic Tafel coefficients and using these values you can calculated for Rp from your EIS fit, and the Tafel coefficients to calculate the corrosion current density, icorr..

In order to have a corrosion rate value, you can use the following equation:

Corrosion rate = (icorr.M) / (ρ.F.n.)

Where icorr is corrosion current density in A.cm-2, M is the molar weight of the species corroding in g.mol-1, ρ is the volume density of the species corroding in g.cm^-3, F is Faraday's constant (96490C.mol^-1), and n is the valence of the corrosion reaction. This will give you a corrosion rate in cm.s-1.

You might find these notes useful for processing your data:

-Basics of Electrochemical Impedance Spectroscopy by Gamry

http://www.gamry.com/assets/Application-Notes/Basics-of-EIS.pdf

and

-Comparison of Corrosion Rate Calculated by EFM, LPR, and EIS by Gamry

Available at: https://www.gamry.com/application-notes/corrosion-coatings/corrosion-rate-efm-of-iron/

-Corrosion Part 3 – Measurement of Polarization Resistance by Metrohm

Available at:https://www.ecochemie.nl/download/Applicationnotes/Autolab_Application_Note_COR03.pdf

-Corrosion Testing via Electrochemical Impedance Spectroscopy (EIS) by VLC

Available at: https://vlci.biz/corrosion-testing-via-electrochemical-impedance-eis/

- Analysis and Interpretation of EISData for Metals and Alloys by Solartron Analytical

Available at: https://www.ameteksi.com/-/media/ameteksi/download_links/documentations/library/solartonanalytical/electrochemistry/technical%20report%2026%20mansfeld%20eis%20metals%20and%20alloys.pdf?la=en

Good job and best regards, Pierluigi Traverso.

Dear madam, the following links may help you.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.sciencedirect.com/science/article/pii/S0927024818305336&ved=2ahUKEwjj777m5dLoAhW34nMBHSyhCGMQFjAEegQIBhAB&usg=AOvVaw2YwGyMsC8dF4AOrgT-JIk3&cshid=1586141664923,

https://www.google.com/url?sa=t&source=web&rct=j&url=https://vlci.biz/corrosion-testing-via-electrochemical-impedance-eis/&ved=2ahUKEwjj777m5dLoAhW34nMBHSyhCGMQFjADegQIEBAL&usg=AOvVaw0mWe5oIWqlG7jbwoPNNamq&cshid=1586141664923,

https://www.sciencedirect.com/science/article/pii/0010938X95000044

Dear Mihad,

Corrosion kinetic parameters play an important role in researchers’ ability to understand and predict corrosion behavior. The corrosion kinetic parameters of metallic specimens can be immersed in 3.5wt% NaCl solution for 1–2 h were determined using linear polarization resistance (LPR), Tafel-curve multiparameter fitting, electrochemical impedance spectroscopy (EIS), and electrochemical frequency modulation (EFM) methods. Slight perturbation of the corrosion system, EIS and EFM/TC4 in collaborative application are the recommended techniques for determining the kinetics and the corresponding parameters for the homogeneous corrosion of the naked metal.

A saturated calomel electrode was used as the reference electrode, and a 30 mm × 30 mm platinum plate was used as the counter electrode. The morphologies of the coupons before and after immersion in the test solution were observed using a scanning electron microscope.

EIS Methods

EIS experiments were carried out using ±5 mV excitation from 105 to 10−2 Hz with 5 points per decade. Subsequently, a fitting process based on the simple Randles circuit can be used to estimate the value of Rp. In addition, single-frequency (0.01 Hz, ±5 mV) EIS experiments were also performed because the solution resistance was far smaller than the system polarization resistance Rp (i.e., the solution resistance can be ignored if not it is always smaller than Rp). The Rp value increased with increasing scan rate from 0.1 to 0.3 mV/s is similar to the impedance behavior in the low-frequency region of the Bode plot can be constructed from the EIS data. There will be the dramatic decrease/increase of Rp (depend on the homogeneous samples in solution percentage….) measured data at potential scan rates can be taken around 1.0 mV/s or little less.

EIS analysis

The Nyquist and Bode plots based on the results of the EIS experiments in the frequency range from 105 to 10−2 Hz( Use Mr. Maryam has shown the plot for reference) calculate Rp values. During the fitting process, a constant phase element was used; its impedance (Z) can be calculated by appropriate equation.

Z(omega)= 1/ Y0(j. omega)n , j= root under square-1,

Rp= delta E/delta I= babc/ 2.3(ba+bc). 1/icorr , babc can be found from tafel plot, and icorr can be calculated from Butler–Volmer equation by using , babc and Ecorr values. (this is enough)

Experimental and fitted curve you can find by using the EIS data Z vs. frequency which can demonstrate the oxygen diffusion etc.

It’s not very tough to calculate. you have to calculate Ecorr and Icorr (from tafel plot use ba bc and ecorr)using appropriate formula. (follow this reference https://link.springer.com/content/pdf/10.1007%2Fs12613-017-1502-6.pdf)

Hope it is helpful to you.

Ashish

Thank you Maryam Soleimani , Pierluigi Traverso , Nelaturi V Raghavaiah , and Ashish Thakur for your useful suggestions.

I have an additional enquiry. I will run the experiment using a coated cylindrical carbon steel coupon, how can I determine the constants used in the equations (molar weight and valence change)?

Regards,

Mihad

Hi, I attach some solved examples that determine the constant. Hope it would be helpful.

Thank you Maryam Soleimani for your help.

I use EIS analyzer software, i think you can predict the corrosion rate from the EIS parameters.

In my opinion quite complex and nice question.

In theory, the corrosion rate of a coated metal can be estimated from the Stern-Geary equation, but you need at least two time constants in the Nyquist spectra and to know the delaminated metallic area