The OCP tells you if your material is in equilibrium with the electrolyte. Once the OCP remains stable, it means the equilibriuim has been reached.

Suppose we have two different alloys and alloy first has lower E (mV) value with respect to time as compared to second alloy. so what is it means.......??

Please refer to the electromotive series of the elements. A more positive potential means the materials has a noble behavior. This means is less affected by the electrolyte. A more negative is more prone to be affected (corrosion, oxidation, ..). A material that is more positive than the other is normally called the cathode and the other would be the anode. 

Two terms are mainly used when we interpret the result of corrosion i.e. Ecorr and Icorr. what is it means..??

Ecorr means corrosion potential and icorr means corrosion current..

The variation of the corrosion potential vs. time measurement helps to predict the corrosion behavior of a material in an electrolyte. If the OCP trend to increasing and stable values, the material can resist the action of the electrolyte, generally forming a protective film during the first hours of immersion.

If OCP curve is in decreasing order then what it means.........?

Generally, OCP in decreasing order indicates a disolution process of the metal in the electrolyte. Anyway, you can check with other measurements as polarization curves, linear polarization, etc.

actually i just want to know the corrosion behavior and corrosion rate of Co-Cr-Mo alloy so If you have any idea about it then share with me.......

One more thing how we measure Ecorr and Icorr value through Gamry software...??

Ecorr is equal the OCP once the system is in equilibrium

Icorr run a LPR test get the value . I am pretty sure the Gamry software has this test already and the software fitting to get the Icorr.

you can easily find a primer with the basic knowledge you need, for example here: http://www.gamry.com/application-notes/basics-of-electrochemical-corrosion-measurements/

With Gamry Potentiostat you need the DC105 DC Corrosion Techniques Software and you have several choice http://www.gamry.com/products/software/dc105-dc-corrosion-software/

The open circuit (OCP) potential is the potential of the working electrode (metal specimen) relative to the reference electrode when no potential or current is being applied to the corrosion system. In general, OCP of an electrode is a thermodynamic parameter which tells about the thermodynamic tendency of that metallic materials to participate in the electrochemical corrosion reactions with the surrounding medium.

It is the potential of the electrode at which the cathodic and anodic reaction rates are in equilibrium in the absence of applied potential in a given corrosive environment. The metallic materials with nobler OCP are more thermodynamically stable than materials with less noble OCP. Thus, the potential below OCP is more thermodynamically stable (shows less tendency to participate in the corrosion reactions) whereas the potential above OCP is considered thermodynamically unstable and susceptible to corrosion.

OCP is a potential at which there is no current. It is the potential of working electrode (metal) with respect to reference electrode when no potential applied. More negative OCP shows the tendency of corrosion of metal in solution while less negative OCP shows the low tendency of corrosion. At OCP the cathodic and anodic reaction rates are in equilibrium.

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Best wishes.

Dear Amit

In my opinion, the difference between chmistry,i.e., materials in vaccum or in air, & electrochemistry,i.e., materials submereged in aqueous solutions, is that materials in aqueous solutions normally develop a potential cross the interface between the materials surface & the electrolytes. This potential is called the open circuit potential or the corrosion potential. Therefore, role of the open circuit electrode potential (OCP) graph versus ,in corrosion test ,is important for indicating how stable the materials in the electrolytes.

OCP shows the relationship between the electrode potential and the immersion time, indicating the stability of the sample in the electrolyte

See the Open Circuit Potential Test section in this article:

Article Preparation and performance of nanometric Ti/TiN multi-layer...

Dear Dr. Amit Aherwar ,

as i said in a similar question, long-term change of corrosion potential (open-circuit potential) reflects a change in a corrosion system because the change in corrosion potential depends on the change in one or both of the anodic and cathodic reactions. For example, an increase in corrosion potential can be attributed to a decrease in the anodic reaction with the growth of a passive film or the increase in the cathodic reaction with an increase in dissolved oxygen. A decrease in corrosion potential can be attributed to an increase in the anodic reaction or a decrease in the cathodic reaction. The monitoring of corrosion potential is therefore often carried out (ISO 16429, 2004; JIS T 6002). For the test solution, saline, phosphate buffer saline, Ringer solution, culture medium, serum and artificial saliva are typically used. The corrosion potential of the specimen can be monitored against a reference electrode using an electrometer with high input impedance (1011 Ω ~ 1014 Ω) or a potentiostat.

For more details, please see the source: Monitoring of corrosion potential by S. Hiromoto, in Metals for Biomedical devices, 2010.

The most widely used electrochemical method of determining the corrosion rate is the Stern-Geary method which allows to evaluate the corrosion current (i corr), an essential parameter from which to derive the corrosion rate of the material in that particular environment.

My best regards, Pierluigi Traverso.