Dear Dr. Asst. Prof. Sameer K. Fayyadh ,

for the potentiodynamic polarization curves on metal samples in corrosive environments I have always used a scannig rate of 250 µV/s which combines the achievement of a good "equilibrium" of the system with a reasonable time for carrying out this electrochemical test.

I have always obtained excellent experimental results and no negative comments in this regard from any international journal referee.

Scan rate too high makes the Tafel slopes and corrosion current density deviate from their actual values. Moreover, the production of a distortion of the potentiodynamic polarization curve can lead to misunderstanding of the real electrochemical processes.

My best regards, Pierluigi Traverso.

Thank you for your interest and answer

Dear Dr. Asst. Prof. Sameer K. Fayyadh ,

The scan rate used in a potentiodynamic polarization (or linear polarization, or cyclic voltammetry, etc.) test to assess corrosion behaviour can have an impact on the resulting measured curve. A slow scan rate typically allows for more time for steady state to be achieved, and thus, a more accurate determination of Tafel slopes, etc. In addition, a slow scan rate minimizes the capacitive current contribution [Elgrishi, 2018, see ref below]. A faster scan rate is typically preferred to save time in running experiments, in particular when you can demonstrate that increasing the scan rate does not significantly change the polarization curve. As Dr. Pierluigi Traverso mentioned, the main idea is to select a scan rate that allows for proper interpretation of the electrochemical behaviour.

A faster scan rate may also be required due to limitations of your sample. For example, when performing potentiodynamic polarization of a thin sample (e.g., a coating of limited thickness, thin amorphous foils), a faster scan rate may be needed to prevent the complete degradation (i.e., dissolution) of the sample prior to the completion of the polarization scan (faster scan rates take less total time to complete). In these cases, you should assess literature in your related field to understand common rates others have used. Another example of when a faster scan rate may be required is if you expect/hypothesize that your sample will change substantially during the time of the test (e.g., increased roughness due to high dissolution rates, accumulation of adsorbed layers, dealloying) and you want to characterize the behaviour prior to that change fully occurring.

In my experience, the most common scan rates I have observed in literature are between 0.1 – 1 mV/sec; this would be a good starting point, in particular since ASTM G5 (“Standard Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements”) recommends a scan rate of 0.6 V/hr (~0.1667 mV/sec) when running a polarization test on their standard specimen. I have also seen some studies that used up to 5-10 mV/sec; however, I would not recommend using such high scan rates unless you have a good reason why such high scan rates are required, as they are not common and likely introduce significant differences in polarization behaviour compared to slower rates. In either case, I recommend performing a preliminary study on your sample at multiple scan rates to assess the effect on your measured data (e.g. at what scan rate you begin to observe differences?). I would also recommend examining literature studies in your specific area (sample and electrolyte) to obtain a preliminary idea of scan rates that have been used.

I recommend these publications:

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. https://doi.org/10.1016/j.corsci.2008.12.005

D.A. Fischer, I.T. Vargas, G.E. Pizarro, F. Armijo, M. Walczak “The effect of scan rate on the precision of determining corrosion current by Tafel extrapolation: A numerical study on the example of pure Cu in chloride containing medium” Electrochimica Acta. 313 (2019) 457-467. https://doi.org/10.1016/j.electacta.2019.04.064

A. Guyader, F. Huet, R.P. Nogueira. “Polarization Resistance Measurements: Potentiostatically or Galvanostatically?” Corrosion. 65 (2009) 136-144. https://doi.org/10.5006/1.3319118

N. Elgrishi, K.J. Rountree, B.D. McCarthy, E.S. Rountree, T.T. Eisenhart, J.L. Dempsey. “A Practical Beginner’s Guide to Cyclic Voltammetry”. Journal of Chemical Education. 95 (2018) 197-206. https://doi.org/10.1021/acs.jchemed.7b00361

All the best!

Leo Monaco

Dear All,

thanks to the kind suggestion of Dr. Leo Monaco I proceeded to change the scanning speed which previously was not correct.

I apologize but in the previous indication the Greek character had not been inserted and an incorrect data appeared.

My best regards, Pierluigi Traverso (CNR-IAS-Genoa).