The ASTM standard recommends a scan rate of 10mV/min but at faster reverse scan rates, more noble repassivation potentials are obtained.
With AISI 316L in artificial seawater (3.5% NaCl), at 25 C, 10 mV/min scan rate gives a repassivation potential close to OCP (ca. 0 mV/SCE), while with a reverse scan rate of 10 mV/sec, repassivation potential increases to more noble values, ca. 300 mV/SCE.
Both scans are limited/reversed at current density, 0.1mA/cm2. It is also clear to see a stable pit growth (almost 2 dec current density increase) with the lower reverse scan rate, compared to the scan with a faster reverse scan rate.
Has anyone tried something similar, and possibly has recommendations on which scan rates are more appropriate to simulate real life repassivation events?
Hello Sunday, I think the reason for the more negative repassivation potentials at lower scan rates is the fact that you are in the stable pitting area for a longer time. This leads to deeper pits that are harder to repassivate.
For the selection of materials based on electrochemical polarization tests, lower scan rates or static holding tests should be preferred, as the values are more conservative and therefore safer.
Best regards, Andreas
Thanks for the feedback Andreas Heyn and Pierluigi Traverso .
Pierluigi Traverso , from your response, it is unclear to me what scan speed works for you when scanning from -100mV vs Efc to Ep, and why you have selected to work with such scan rates.
Dear Sunday,
in addition to the above answers I would like to point out that it depends on what you want to achieve. If you want to obtain a quick comparison of the repassivation potential of different metals in a given environment than any scan rate is fine, as long as you use the same scan rate for all tests - so it makes sense to use the ASTM standard scan rate as these is bound to be used also by other researchers. As to the influence of scan rate on the measured (apparent) value of the repassivation potential, Antoine Pourbaix has done extensive research on this subject and presented a fine paper during the first International Workshop on the Long Term Prediction of Corrosion in Nuclear Systems, held in 2002 at Cadarache. I will be glad to provide you his contact information.
Best regards
Frank Druyts.
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