Yes dear

 you find the answer through the attached file

Thanks

Ikor for metal without coating and Ikor for coated metal , next you can use formula similar to "inhibitor efficiency". But I think you don,t use Tafel test to investigation of coatings which have resistance close to Gohm :). 

Thank you all for your helpful answers,

Dear Dr. Stefan would you tell me the reason to not conduct the Tafel test for coating systems with resistance in the range of Gohm or more

Dear Prof. Dr. Juan Genesca  thank you very much for your worthy advice but, unfortunately, I don't have access to the suggested articles.Any help in that will be much appreciated 

One more thing I would like to highlight also, that I have some knowledge about how to figure out the best coating system among several prepared systems via using Tafel tests but my main question is if I can use the parameters obtained from Tafel test as a coating performance indicators rather than perform comparative studies

If you have coating 1GOhm resistance the current don't flow and if flow your measure error is very big (iR drop).

I would recommend if you dealing with a coated material , you can classify the coating system from the EIS test.,from the polarization resistance which you obtain from the EIS, at low frequency ,f=0.16Hz, you can tell whether the coating is a resistive enough or not.    you are dealing with a material very corrosive resistant, so a subtle examination by EIS would be the best to characterize the coating behavior.

Proposition of Khaled is optimal. For high resistivity coatings AC methods are the best.

Coating resistance and coating capacitance well characteristic of a studied object.

Please distinguish during tefal test between the resistance of 100% defect free coating resistance and defected one.

The main possible defect in coating is connected porosity which is the easy path for electrolyte to reach the metal surface.

This mean that there are two resistances working in parallel (I repeat in parallel) across the coating layer the first is the coating material resistance which assumed by Dr Ammar Shafaamri  to be in Kohm and the second is the electrolyte resistance which fill the pores and should be very conductive.

So from engineering and practical point of view the working resistance of the coating layer during the scan test is not too much far from the electrolyte resistance and it directly related with the porosity amount.

Now the problem, if the electrolyte in your cell is not very conductive in this case IR compensation is needed.

Based on this explanations porosity percent can be calculated. So we can classify the relative coating system integrity in term of the parameters obtained from Tafel test.

I am asking all what wrong in my above answer to be down voted.  

Thanks

Dear Dr. Stefan, you give very interesting comment which for me one of the problems that make me confused in conducting and studying Tafel test. However, the literature still contains many studies that reported Tafel results for coating system having resistance in the range of Gohm

For example "Heidarian, M., Shishesaz, M. R., Kassiriha, S. M., & Nematollahi, M. (2010). Characterization of structure and corrosion resistivity of polyurethane/organoclay nanocomposite coatings prepared through an ultrasonication assisted process. Progress in Organic Coatings, 68(3), 180-188.'

Further discussion and explanation would be very much appreciated.

I totally agree with both of you, Dr. Khaled J. Habib and Dr. Stefan Krakowiak about the efficiency of EIS in ranking the coating system with the wide variety of parameters could be obtained. But still Tafel test could be considered as a powerful tool for determining the corrosion rate and the corrosion current and predict the performance of the coating system instantly. Furthermore, even I give detailed studies about EIS, some reviewer and examiner remain giving comments about the necessary of reporting Tafel test.

Dear Ammar

Please note that Tafel test of the coating would be  based on a DC method.   In contrast, the EIS test of the coating would be based on AC method.   There are many advantages of using the EIS test in the case of coating testing.   Among those advantages,you can control the frequency range of the applied current on the coating.   Most of the coatings are very corrosive resistance materials.   Therefore, a subtle method such as EIS would be far better than the DC method of Tafel test.   The application of EIS of coating testing prevents extraneous errors due to heat induction of the DC method.   Also, the EIS can provide readily interface parameters such as the double layer capacitance, the AC impedance, the polarization resistance(at low frequency,and so on of the coating via modelling the equivalent electrical circuit of the solution/coating/solid substrate system . This is the reason why the EIS became a standard method of testing coatings.    

Thanks a lot Dr. Khaled for the helpful information