How much deviation of measured OCP from Ecorr (potentiodynamic curve) is acceptable? and how could we explain this deviation?
I assume OCP measurement was not satble enough before running potentiodynamic measuerement. I appreciate your comments.
Dear Dr. Rachel Partovi-Nia ,
the open circuit potential (OCP) is a kinetic value that measured as a function of immersion time and respect to a reference electrode or another electrode when no current flows to or from it, representing the progress time for the transient and study state stage of the corrosion process on the metal surface. Now one of these values of OCP, during the immersion time, is the corrosion potential (Ecorr, obtained by Tafel equation by cathodic and anodic polarization of the sample). These values could be equal or very similar only if the metal corrosion is in fully steady state. This condition is a function of various parameters (e.g. type of material, corrosive solution, experimental set up of the measurement, etc.). However, in general, they almost never show very different values.
Best regards, Pierluigi Traverso
The potentiodynamic measurement may be itself a source of error. A linear polarisation record starts at a potential about -300 mV lower than OCP, thus this start is far from a steady state. Allow some time for equilibration (current stabilisation) at this initial imposed potential before starting the record. Use a low scan rate (< 1 mV/s).My opinion is that Ecorr is commonly deviated from OCP, not vice versa.
Dear Rachel partovi-nia
you should apply lower scan rates. please read carefully this paper
X.L. Zhang, Zh.H. Jiang, Zh.P. Yao, Y. Song, Zh.D. Wu, Effects of scan rate on the potentiodynamic polarization curve obtained to
determine the Tafel slopes and corrosion current density, Corrosion Science 51 (2009) 581–587.
The open circuit potential (OCP) is a kinetic value that measures the potential of the metal electrode as function of immersion time to reach a steady state when no current flows to or from it. However, this value differs slightly from Ecorr (that is obtained from potentiodynamic polarization).Such difference is attributed to the fact that firstly the electrode is polished using a mechanical polisher and the surface is clean however when DC starts the cathodic reaction and the bubbles of hydrogen gas are responsible to clean up the surface better than cleaned before and thus becomes as a mirror again causing a slight difference between Ecorr and OCP. In general both values are very similar to each other
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