Dear researchers,
I would appreciate if you could guide me on the following question:
I put a mild steel specimen in 0.1 M HCl solutions without and with an amino acid corrosion inhibitor.
I measured the corrosion behavior of the coupon, as a function of time. I mean I did not wait for OCP to reach an steady-state condition and started the measurements immediately after immersion (0 min), 30 min, 1 hour, 2 hours, 4 hours, 8 hours and 24 hours.
In the presence of the inhibitor: It was observed that the diameter of the Nyquist plots increased during the first 2 hours but it did not change during the rest of the experiment. But the OCP decreased and became more negative over time after 1 hour or even less.
In the blank solution: the diameter of the Nyquist plots decreased with time but the OCP increased.
TO your opinion: can the increase in diameter of the Nyquist plots in the presence of the inhibitor (during the first two hours) be related to the surface adsorption of the inhibitor?
Why did the OCP become more negative? can it again be related to the surface adsorption of the inhibitor?
Thank you so much in advance
I think that your results (Rp increase, more reductive OCP) confirm that the inhibitor is effective.
The mechanism of the inhibition is a different issue. But in this case the amine adsoption (and oxygen removal) is commonly accepted as primary mechanism.
I hope that the applied DC potential was the actual OCP, measured before every new EIS experiment.
Before any experience, you need an OCP for mild steel you need 30 min - you can verify that in several article- (H.Lgaz)
When you have an equilibrium (dE/dt =0 ) you can proceed the other step of experience.
I agree with Mr Cornel Radu about your inhibitor effective
Dear Taher, For "potentiostatic EIS" studies the prior OCP stabilization is necessary due to following reasons:
The baseline (OCP) of the applied AC potential perturbation is shifting (in your case) during the course of EIS measurement and the point one after another in the impedance spectrum (as a function of frequency) does not link with the true situation at the electrode/electrolyte interface. In other words the dynamic conditions (time dependant) conditions during potentiostatic EIS experiment can only give you the estimate of your inhibitor effectiveness but your results may not be used to evaluate the exact situation on the electrode surface (e.g. adsorption etc.) For time resolved studies of inhibitor effectiveness i would recommend you to used Rp & Ecorr Trend and Electrochemical Frequency Modulation Trends which are specifically designed for this purpose. Please see the following references:
https://www.gamry.com/support/software/efm140-electrochemical-frequency-modulation/
https://www.gamry.com/support/software/dc105-dc-corrosion-software/
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