Dear Dr. Krishna Kant Pandey,

the open circuit potential (OCP) is the potential of the working electrode relative to the reference electrode when no potential or current is being applied to the corrosion system.

Generally, the OCP trend of a working electrode shows the tendency of the metallic substrate to be subject in the electrochemical corrosion reactions in a specific environment.

If the trend of the OCP vs. time increases and reaches a stable value, the metallic substrate has a noble behaviour and it can resist to the electrolite aggressive action, forming a protective film during the exposure time. If the trend is contrary, the metallic substrate has a more propensity to corrosion phenomena.

If the trend is particularly unstable during the exposition in aggressive solution, the metal substrate often could be affected by localized corrosion attacks.

Morphological and/or chemical surface analysis on the exposed metallic substrates can be useful to validate these conclusions.

Best regards, Pierluigi Traverso

Dear Krishna

I do agree with Mr.Traverso. He gave an excellent views/comments about the behavior of the OCP of a metal in an aqueous solution.

In addition to the above, the OCP is the potential which forms across the interface of the metal/ the aqueous solution as soon as the metal submereges in the aqueous solution.

In general, the higher- stable value of the OCP of a metal in an aqueous solution, is the less corrosive the metal as compared to the lower-stable value of the OCP of the metal in the aqueous solution. Also, the variation of the value of OCP is due to the instability of the electrochemical activity of the formed film,i.e., an oxide film or due to localised corrosions, on the metal surface in the aqueous solution.

Thank you for your comment Mr. Traverso and Mr. Habib for your comments.

Mr. Traverso, can you please tell me what type of morphological and/or chemical surface analysis on the exposed metallic or coating substrates can be done?

Dear Dr. Krishna Kant Pandey,

on exposed metallic or coating substrates you can carried out a lot of surface analysis in order to investigate the chemical and/or morphological situation: Optical Microscopy, Scanning Electron Microscopy (SEM, with EDAX probe for chemical analysis), 3D No Contact Optical Profilometry, X-ray Photolecton Spectroscopy (XPS, for the study of the composition of the firsts atomic layer of the surface), FT-IR Spectrophotometry (to evaluate coating degratation), etc.

Best regards,

Pierluigi TRAVERSO

Dear Dr. Krishna Kant Pandey;

If the electrolyte conditions are under control (temperature, agitation, oxygen content), the stability of the open circuit potential is an indicator that the material resists environmental attack, possibly by passivation. The more unstable the potential, is an indicator that the material is subject to an active corrosion process. However, the temperature, agitation and the higher or lower oxygen content, change the conditions of the electrolyte, and move the potential. The temperature and the agitation can vary the potential by the action in the convection in the electrolyte and replacement of reactive species, but also by the increase or reduction of oxygen; or by the detachment of the films from the surface.

Best regards

Dear Dr. Osby Garcia, I am in total agreement with you. In the OCP measurement, but in general in all chemical reactions related to the surface of a material, the morphological conditions and the chemical state of the substrate (initial and during the test) together with the chemical-physical parameters related to the solution (....and to the whole measurement environment), play a pivot role on the obtained result.

Best regards,

Pierluigi Traverso