Introduction
All My Impedance Spectra Look the Same!
"I’m an experienced polymer chemist. I’m trying to use Electrochemical Impedance
Spectroscopy (EIS) to predict the corrosion resistant properties of paints. I’ve recorded many
EIS spectra on painted metal samples immersed in electrolyte. All (or most) of these spectra
look the same, regardless of the changes that I makes in my paint formulation. I obviously
cannot use the results to evaluate paint performance. What’s going on?"
There are two common causes for this complaint:

1. You have a very high quality paint that gives very reproducible EIS spectra.
2. You are attempting to make measurements that are beyond the capabilities of the
potentiostat in your EIS system.
The second cause is more common. The spectra look the same because you are measuring
the characteristics of your potentiostat, not those of the paint.
The rest of this application note describes the effects that a potentiostat can have on EIS
measurements of coated metals. Gamry Instrument potentiostats will be used as examples,
but the discussion will apply to EIS systems built around any potentiostat.
This note concludes with some specific recommendations for getting meaningful EIS spectra
on coating systems that are difficult to measure.

Background
If you are not very knowledgeable concerning electrochemical instrumentation, read through
our Primer on Potentiostats. This Primer will introduce the terminology used in talking about
potentiostats and some potentiostat concepts that are used in the remainder of this
application note.
You will also need a basic understanding of EIS to follow the discussion in this note.
Experienced EIS users should have no problems with the level of the discussion. If you are
less experienced or you want to brush up on your basics, read through our Basics of
Electrochemical Impedance Spectroscopy for a primer on EIS theory and practice.

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