attack ... It may be minimized by suitable surface and groove preparation between .... This selection process considers only organic coating systems and does not ...... of the bare substrate, βa is the anodic Tafel slope of the bare substrate, and ΔEcorr 

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https://www.payment.mdpi.com/2079-6412/7/2/25/htm

Dear Muhammad

For coated metals, it is more suitable to use EIS to determine the corrosion rate rather than Tafel plot.   This is because the coated materials on the metals  are far resistive than the uncoated ones.   Therefore, an AC technique such as EIS has been proven to be more subtle technique than the DC technique of the Tafel plot for determination of the corrosion rate for coated metals.  

One can determine the absolute AC impedance from the EIS spectra, Nyquist (complex plane) or Bode plots, at low frequency, to correspond to the polarization resistance of the coating at the open circuit potential of the coated sample in the solution.   Then one can use Stern-Gary equation to determine the corrosion rate of the coated sample by substituting the obtained polarization resistance from the EIS spectra. The value of anodic & cathodic Tafel constants can be assumed based on a cited reference in literature.  

Dear Muhammad

Tafel method for determining the corrosion is suitable for coatings that are of a metal base (CVD PVD, etc.).

For more DETAILS you can get information on:

http://www.caltechindia.com/Corrosion-Testing-Instruments-and-Accessories.htm?gclid=CjwKEAjwkq7GBRDun9iu2JjyhmsSJADHCD_HxRjoiczyM6q4k49Latshkc_vNM_5hI8i08iXYAhsaBoCZ-bw_wcB

attack ... It may be minimized by suitable surface and groove preparation between .... This selection process considers only organic coating systems and does not ...... of the bare substrate, βa is the anodic Tafel slope of the bare substrate, and ΔEcorr 

read the link :

https://www.payment.mdpi.com/2079-6412/7/2/25/htm

The short answer to your question is no, Tafel plots are not suitable to determine the corrosion rates of organic coatings.  In part, that is because the coatings themselves are not supposed to corrode - the measurement technique is actually checking the breakdown/moisture uptake of the coating and the onset of corrosion of the substrate.  However, Tafel plot extrapolation is over-rated as a method of detecting even corrosion rates on metallic materials - the results are always highly subjective and can be very difficult to interpret. 

Impedance measurements are more typically utilized because the technique gives more information during the period during which the coating properties - in particular its resistive and capacitive values - change in the exposure environment.  Care needs to be exercised when evaluating corrosion behaviour because the effective area being evaluated by the technique is really only that of the 'holidays' in the coating and not of the whole area of the substrate.

I would recommend that you do not overlook comparatively simple (and inexpensive) techniques such as potential/time logging when testing coating performance - you can get a significant amount of information about how a coating behaves over time in an exposure environment just by checking changes in the corrosion potential of the substrate.

If you are interested in more advanced investigation of coating properties, electrochemical noise analysis can be very useful.  It is far more sensitive than impedance measurement, because the applied perturbation of the alternating signal itself can affect the corrosion processes (though as stated by Khaled Habib you can use EIS to obtain quite a lot of mechanistic information), whereas in the case of electrochemical noise analysis all of the measurements you make are generated by the electrochemical activity at the coating/substrate interface.  Check out work by Skerry and Eden for more information.

Yes dear Tafel plot can used to determine the corrosion rate of coated and uncoated metals. The main possible defect in any type of coating is connected porosity which is the easy path for electrolyte to reach the metal surface where the corrosion process take place, also you can ranking the coating of different filler compositions (1%-5%) based on their performance in terms of porosity% change which can be calculated based on the obtained data from Tafel plot. Please go through the article below for more deep details.

Best regards

 https://www.researchgate.net/publication/222628469_Porosity_evaluation_of_protective_coatings_onto_steel_through_electrochemical_techniques

Article Porosity evaluation of protective coatings onto steel, throu...

how would you recommend the potentiodynamic polarization data be analyzed for polymer coatings on steel?