In general, when performing a Tafel analysis for corrosion resistance measurements, a line voltage scan is performed from say -250 to +250 mV vs. OCV at low scan speed around 0.125 mV/s. However, there is a certain ambiguity in various publications concerning the speed and scan direction, when measurements are done for hydrogen evolution reaction. In one paper the LSV scan is performed from -0.2 to +0.2 V vs. RHE with speed 5 mV/s. The second one tells to scan it in negative direction at 2 mV/s. The third one doesn't even specify such details. The problem is that the direction of LSV scan greatly affects the polarization curve in terms of corrosion potential (hydrogen redox potential), linear regions, Tafel slope and exchange current density. So which one of these curves should be used to determine such parameters as Tafel slope and exchange current density?
Tafel analysis should always be performed under steady-state conditions. If the scanning direction influences the Tafel slope or the exchange current density, it indicates the influence of dynamic effects and cannot be taken at face value. For best results, the current should be measured at different static potentials across the potential range of interest.
Correct way to obtain STEADY-STATE polarization curve is to measure it potentiostatically or galvanistatically point-by-point. For lazy people using scanning modes: scanning must be done in both directions, and anodic scan must coinside with cathodic scan. Do not forgen about IR correction in any case.
Dear Dr. Vitalii Yu. Izai,
I am sending you, my “standard” procedure that I generally follow for electrochemical tests, in order to study the corrosion behaviour of different metallic materials immersed in several corrosive environments. These measurements were carried out with 250µV/s scanning rate on specimens previously free corroded (for example for 2, 72, and 168 hours).
A conventional three-electrode cell was used for electrochemical tests.
The counter-electrode was a platinum coil and all potentials related to the saturated calomel electrode (SCE) were recorded.
The working electrode was an abraded sample (down to 800 grit by SiC emery paper, suitably prepared) incorporated into an epoxy resin with stable electric contact, in order to have a flat and regular surface.
The exact working electrode area for every sample, ranging from 0.57 cm2 to 0.78 cm2, was inserted in the software for electrochemical data normalization.
The corrosive solution was an aqueous 3.5 wt.% NaCl solution, pH = 5.8 and D.O. (Dissolved Oxygen) = 6.5 ppm.
All the experiments were conducted at room temperature (25 °C), P = 1 atm, in unstirred conditions and all the additional experimental parameters were fixed in order to allow measurements inter-comparison.
In order to drawn polarization curves measurement were started from -80 / -100 mV (SCE) with regard to free corrosion potential (open circuit potential), Efc, then the working electrode potential was increased toward anodic direction. This start potential was chosen in order to avoid any possible surface changing from the steady state. Passive film breakdown potential (critical pitting potential, Ep) and pitting protection potential (Epp) were also measured. For this purpose, the working electrode potential was increased up to Ep value, indicated by a sharp anodic current increase. When Ep was established, the working electrode potential was decreased until the backward curve crossed the forward curve. The difference between Efc and Ep was named passivity domain (PD), while the difference between Efc and Epp was defined as perfect passivity domain (PPD).
In addition, a domain of about +/- 25 mV around Efc, in which log I vs. E shows a linear relationship, was selected to calculate polarization resistance Rp using a suitable software.
Hoping to be helpful to you, warm regards, Pierluigi Traverso (CNR-IAS-Genoa).
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