when compared to base metal the OCP will be noble

that is more positive

see my paper on ORIGIN OF CORROSION POTENTIAL - Bull of Electrochem

contact  www.cecri.res.in    library

Thanks Prof. Muralidhara,

if its opposite than your observation; what is the error in the system or measurements as I described?

dear Student

now the question is electrical contact

assuming everything is perfect   IR in the coatting  will not play any contribution to E corr

Many thanks Prof. Muralidharan

I suggest you use electrochemical impedance to compare the coating properties. Please refer my paper about hybrids coatings or my dissertation. The coating resistivity changes once you immerse the sample in the electrolyte. Despite the same coating the OCP response will change with different substrates.

Dear Enrique,

      We have done EIS of the sample after  OCP even sometimes more than 80 hours. Obviously, OCP varies drastically with time and get stabilize.  EIS was performed before performing the polarization experiments. Could you please send me your paper to [email protected]

Dear Nam,

As the coating is done by electrochemical deposition directly on aluminum, I am will not be able to perform the experiment as you have suggested, but we are doing OCP sometimes, as mentioned to reply to Enrique, more than 72 hours to obtain OCP with drift less than a mV before doing EIS and polarization.

Thanks

Dilip

5 microns coating is probably leaky and you measure OCP of copper substrate. In cause of tight coating the OCP measurements can be difficult.

Unless your coating impedance is at least 10-100 times higher then impedance of substrate corrosion cell. Your OCP will be determined roughly with substrate and electrolyte composition.

Two reasons:A) thickness of coating; B) different oxide film on the surface substrate:Copper and Aluminum. 

Dear Yong,

   Thanks for your comments. Could you please send me a few references on the effect thickness of the coatings and interfacial oxide led lowering of corrosion potential?

Regards

Dilip

The method of deposition of the coating can substantially alter the nature of the resulting interface between coating and substrate.  For example, electrodeposition will provide a very different interface than simple application of coating from either solution or dispersion.  Please describe at least the essence of the two deposition techniques, so we can explore the role of deposition technique on your results. 

Regards,

Lou Floyd. 

There are the effects of types and thicknesses on OCP.