Tafel Plot Question We use VersaStudio software to plot Tafel slopes, for our recent of carbon steel in hyper saline brine at 45DegC surprisingly, we got passive curve but our metal sample is almost dissolved. And when we use software to determine the Tafel slope it gives a very less corrosion rate which doesn’t match as the coupon got almost dissolved indicating higher corrosion. Please find below the anodic curve of the run and slope determined by Tafel plot from software [url=https://ibb.co/d8z7ow][img]https://thumb.ibb.co/d8z7ow/Software_plot.png[/img][/url] Based on theory, if drawing a slope for a most linear region of the anodic curve is right then is below manual extrapolation is right? Manual extrapolation of Tafel slope [url=https://ibb.co/hbwRgG][img]https://thumb.ibb.co/hbwRgG/Manual_plot.png[/img][/url] Which one should I consider as right the one software, products or our manual extrapolation? - KIndly suggest
The corrosion current density is intersecting point of anodic Tafel, cathodic Tafel, and corrosion potential lines. This was well done by software program. The total metal loss during test period is sum of metal loss at anodic polarization scan time and not represents the metal loss at corrosion potential. Now to be sure, try to immerse the specimen freely in the same solution and temperature for the same scan time you will find the specimen corrode almost at the rate calculated by the software program.
Best regards
The corrosion current density is intersecting point of anodic Tafel, cathodic Tafel, and corrosion potential lines. This was well done by software program. The total metal loss during test period is sum of metal loss at anodic polarization scan time and not represents the metal loss at corrosion potential. Now to be sure, try to immerse the specimen freely in the same solution and temperature for the same scan time you will find the specimen corrode almost at the rate calculated by the software program.
Best regards
The Tafel slope calculation for estimating instantaneous corrosion rates is only relevant for the region 10 to 20 mV on either side of the open circuit potential. Your manual extrapolation is not valid. To correlate the total mass loss you measured in the anodic region of your test, you need to calculate the total coulombs consumed. One Coulomb = 1 Ampere*second. For carbon steel, each Coulomb corresponds to a dissolution of approximately 0.289 grams of steel in the anodic region, assuming 100% conversion efficiency. Your software probably has a menu or dropdown choice for calculating the total number of Coulombs used in your test. To do this manually, you need to plot current versus time and calculate the area under the curve to yield coulombs. Remember to convert current density to current.
Tafel experiment is destructive by nature because it induces corrosion through the artificial polarization of the electrode over a relatively wide range of voltage ( -250mV to 250 mV). The weight loss during the experiment doesn't reflect the weight loss of a free metal hanged in the same solution for the same time. To compare the weight loss with the corrosion rate found by the software, a coupon of a well-known surface area , made from the same metal , prepared in the same way as your electrode should be hanged freely in the same solution, at the same temperature, with the same stirring velocity (in case you use a magnetic bar to homogenize your solution during Tafel experiment ) and for the same time . I am pretty sure you will find the CR of Tafel higher than the CR calculated from the WL
@Mohammed Hussein j. H. Al'Atia,
Thanks for the reply, do you mean to conduct a weight-loss test with similar condition? And on doing it, I got the corrosion rate of weight-loss is much similar to software value. Which comes close to 4mpy.
But am really confused to see such a low corrosion rate for test solution of 40% KCl and NaCl saturated solution at 85DegC - Do you have any understanding over this?
Dear Vinith Gandhi Rajendran
For steel, you need to obtain more or less 6.6 mg weight loss per 1 cm2 of exposed specimen area during one month exposure. Yes it is reasonable experimentation to get 4mpy corrosion rate as recorded by software .
You can follow the ASTM G-1 ( Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens) as standard reference.
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