I assume that the epoxy layer is a dense coating. The coated steel might immerse in the electrolyte solution and measure the cell capacitance. It will characterize the coating quality. 

I assume that the epoxy layer is a dense coating. The coated steel might immerse in the electrolyte solution and measure the cell capacitance. It will characterize the coating quality. 

If you are interested in coating failure or coating-substrate interface corrosion performance then you have to make a scratch in the coating and use a paint test cell (PTC). The cell confines the active area to the electrolyte and then 3 electrode system can be used to study the electrochemical behavior of the coating. Some details about the test cell is given below.

The Gamry Instruments' PTC1 Paint Test Cell kit is a simple, inexpensive cell used for electrochemical corrosion testing. This cell was originally developed by Dr. Richard Granata of Lehigh University.

The PTC1 was designed for electrochemical impedance testing of flat, coated metal specimens. The PTC1 can also be used to test base metal samples using PortHole Electrochemical sample masks.

https://www.gamry.com/assets/Uploads/PTC1-Manual.pdf

OCP will not give you much. You can run EIS and Tafel on the coatings to measure the impedance/ current density of the coating. You can use the simple 3 electrode set-up for both. Lots of published paper can be found on how to run the test. 

Dear ajay

First off all you have to run an OCP for getting an idea of over circuit potential. From that OCP value you will run EIS.

For tafel you have to choose potential range. Range is towards 500mV of negative potential and towards 500mV of positive potential from the OCP.