Am confused in deciding the DC Voltage to select for EIS measurement of my ss316L (Coated and Surface modified). Just wanted to crosscheck before proceeding forward. I found this article bit helpful but still it would be great to discuss with experts here. Could you please suggest me something ?
Dear Gopinath,
I have few questions before i proceed for the answer
1. Are you working with organic, semiconducting (metal oxide etc. )or metallic coatings?
For metallic and semiconducting coatings you can go to run EIS vs. OCP and then you may apply DC potential vs. OCP. The magnitude of DC potential is selected based on the requirement. If you are interested in the corrosion studies then you may select at-least 50 mV away from the OCP value. But you must need a stable potential during EIS scan over which you apply the small AC excursion. For this you must run potentiostatic test (at the potential same as you will apply during EIS test (DC potential vs. OCP)) for few minutes to get steady current value prior to EIS measurement. Otherwise you will initially get impedance (mostly capacitive response) values under transient conditions following by unreliable resistive response (within low frequency regime).
In case of organic coatings, you should preliminary use potential vs ref. electrode not vs. OCP. Since the OCP, you will get from potentiostat will be an artifact and this does not depict the state of the coating. Additionally, if you test thick and resistive organic coatings by EIS, possibly you will not get useful information from the results due to scattering of data. This may be because the frequency range you selected (specially the highest frequency) is too less to get the true capacitive response of this type of coatings. Or for this coating system the frequency for the capacitive response exceeds the instrument limits.
In this way you may do the following:
a) Either decrease the thickness of the applied coating
b) or you may increase the exposed surface area of the coated sample
If you have more questions please let me know. Thanks
Dear Gopinath Perumal,
could you illuminate, please, your cell and focus[1] on WE ?
1. focus on : "POLARISATION BEHAVIOUR OF CORRODING METALS" (in :THE ELECTROCHEMISTRY OF CORROSION) http://www.npl.co.uk/upload/pdf/the_electrochemistry_of_corrosion_with_figures.pdf
Dear Gopinath,
I have few questions before i proceed for the answer
1. Are you working with organic, semiconducting (metal oxide etc. )or metallic coatings?
For metallic and semiconducting coatings you can go to run EIS vs. OCP and then you may apply DC potential vs. OCP. The magnitude of DC potential is selected based on the requirement. If you are interested in the corrosion studies then you may select at-least 50 mV away from the OCP value. But you must need a stable potential during EIS scan over which you apply the small AC excursion. For this you must run potentiostatic test (at the potential same as you will apply during EIS test (DC potential vs. OCP)) for few minutes to get steady current value prior to EIS measurement. Otherwise you will initially get impedance (mostly capacitive response) values under transient conditions following by unreliable resistive response (within low frequency regime).
In case of organic coatings, you should preliminary use potential vs ref. electrode not vs. OCP. Since the OCP, you will get from potentiostat will be an artifact and this does not depict the state of the coating. Additionally, if you test thick and resistive organic coatings by EIS, possibly you will not get useful information from the results due to scattering of data. This may be because the frequency range you selected (specially the highest frequency) is too less to get the true capacitive response of this type of coatings. Or for this coating system the frequency for the capacitive response exceeds the instrument limits.
In this way you may do the following:
a) Either decrease the thickness of the applied coating
b) or you may increase the exposed surface area of the coated sample
If you have more questions please let me know. Thanks
Dear Prof.Kashif,
thank you for your elaborate answer. Yes, I do metallic coatings.
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