Duplex stainless steel tested in 3.5%NaCl solution using potentiodynamic polarization method for pitting behavior.
Dear Jastej Singh
The higher the pitting potential, the better localized corrosion resistance and much better when the difference between pitting potential and corrosion potential is higher.
So, the relatively low Ecorr and high Epit is the case of more pitting resistant.
Best regards
Let see if I can explain my view on this in clear terms. First, since the free energy of a reaction is directly related to the measured potential (deltaG=-nFE), a higher Epitt means that greater energy is required to drive the pitting reaction. This is what I would call a direct measure of the "pitting resistance." On the other hand, there is the question of the "probability of pitting." That is, what is the probability that pitting will actually occur during service. This is a completely different question. If we assume that pitting occurs when the free corrosion potential exceeds a potential for the critical amount of time required for the nucleation of stable pitting at that potential, then the probability of pitting in service is indicated by the separation of Ecorr and Epit. That is, the probability that the environment will contain enough oxidizer (ie. DO2) to raise Ecorr to a potential and keep it there long enough for stable pits to nucleate and grow. Keep in mind that Epit is actually a range of values with different pit nucleation times for each value, and the experimentally determined value is a function of time at potential as the potential is increased (ie. potential sweep rate, dE/dt). Ecorr, on the other hand, is a stochastic variable that depends on the oxidizer content of the environment and the thickness, composition, structure, and defect size distribution (and other variables) of the surface film as well as on the activity of the cathodic sites on the surface of the sample.
In the case of cyclic polarization tests, steel characterized by a lower value of Epitt is more susceptible to pitting corrosion. The high value of corrosive potential is information about the condition of the passive layer or about the possible earlier chemical treatment of the surface (passivation).
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