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Selecting a DC Voltage for EIS Measurements.
We have seen several examples where a customer has recorded an open circuit
voltage of 3, 4, or even 5 volts when running EIS on a coated sample. Are these
voltages real, and should you run EIS on these samples at 0V vs. Eoc?
The Eoc values that were recorded in these experiments are probably not real
Eoc readings. They are probably due to an instrumental artifact. The nature of
this artifact is covered in our FAQ “The open circuit potential for my coating is
4.9 V. Is there something wrong?”
For very, very good coatings, such as the ones that these researchers were
studying, we recommended that the potentiostatic EIS measurement be done at
a potential vs. Eref, not vs. Eoc. The Eoc values are often not related to the
coating or to the substrate: They are a function of the length of time the
potentiostat has been connected to the cell, as is explained in that FAQ.
The question then arises "What potential should I choose?" If the coating
resistance is very high, then only very small DC currents can flow across the
coating. Because the currents are small, very little damage can be done to the
coating or to the underlying substrate. Consequently, 0V vs. Eref can be
selected as the DC potential for the EIS scan. This value is less stressful to the
coating than +5V, which may have been measured as the Eoc!
However, as the coating degrades in a long-term test, or if a hole or 'holiday'
forms in the coating, some unwanted damage may happen to the underlying
substrate at 0V vs. Eref. For this reason, it is a good idea to run these very good
coatings at the open circuit voltage measured for the bare, uncoated substrate!
There is nearly no effect of the value of the applied voltage while the coating is
still intact and a very good insulator. However, if the coating fails, there will be
little accidental and catastrophic damage if the substrate’s Eoc is used.