Greetings,
I'm trying to find a proper way to analyze the Tafel slope of a potentiodynamic polarization curve. I've read all the answers on a similar question here (https://www.researchgate.net/post/What_is_the_right_way_to_calculate_the_Tafel_slope_in_Echem_Analyst), but still didn't get a clear point of view. I found a few helpful 'rules' though:
- Choosed region for calculating Tafel slop shouldn't be too far from Ecorr (+/- 50mV)
- When cathodic reaction is under diffusion control, it's better start with the anodic slope and then adjust the cathodic one in accordance to it.
Therefore, here are the questions:
1) Pics 1A and 1B represents the same sample but with different regions to calculate Tafel slop. I deliberately didn't change the cathodic point and altered only anodic part. But still got so different results in the corrosion rates. Although my eye can't see that much of a difference in the anodic slopes. Pics 2A and 2B compare only the anodic part of that curve. And still - different results, even without cathodic part.
My question is what to choose? What else do I need to pay attention to?
2) Next file. Pic 3A and 3B. Different sample, different (the more 'classical' form) of the polarization curve. It seems simpler to choose the region for the Tafel slopes. Both, cathodic and anodic, angles look the same. So I assumed that a corrosion rate should be the same no matter what part of the curve I processed. However, no, the new result in corrosion rate only by anodic slope is twice less.
So, I guess I always have to take into account both slopes and the point of their intersection? Can't use only anodic part, right?
Thank you for reading it. I will be grateful on any useful articles or books that can help me to understand the right way to process potentiodynamic polarization curve (complicated, not classical) to get Tafel slopes and a corrosion rate. Thank you all one more for answering.
Dear Dr. Ekaterina Mansurova,
I understand your doubts, the results of the experimental measurements are not "simple2 as the theoretical ones. Also keep in mind that there are many parameters that can give rise to trends that cannot be perfectly reproduced .... sometimes you have to accept some compromises and mediate complex situations .
I suggest you to have a look at the following, interesting notes:
-Getting Started with Electrochemical Corrosion Measurement by Gamry Instruments
Available at: ttps://www.gamry.com/application-notes/corrosion-coatings/basics-of-electrochemical-corrosion-measurements/
-Tafel Plot and Evans Diagram – By PalmSens
Available at: https://www.palmsenscorrosion.com/knowledgebase/tafel-plot-and-evans-diagram/
-Electrochemical Corrosion Studies of Various Metals by Metrohm
Available at: https://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwj-qePgorTsAhXSsaQKHSK5CMsQFjABegQIAhAC&url=https%3A%2F%2Fpartners.metrohm.com%2FGetDocumentPublic%3Faction%3Dget_dms_document%26docid%3D3347651&usg=AOvVaw3DOgpH4l6qq3FFmrQ0TAE2
-Tafel Curve in Electrochemistry by Wolfram Demontrations Project
Available at: https://demonstrations.wolfram.com/TafelCurveInElectrochemistry/
Good work and best regards, Pierluigi Traverso
Musa, Ahmed Y., Abdul Amir H. Kadhum, Abu Bakar Mohamad, Abdalhamid Ahmad B. Rahoma, and Hussein Mesmari. "Electrochemical and quantum chemical calculations on 4, 4-dimethyloxazolidine-2-thione as inhibitor for mild steel corrosion in hydrochloric acid." Journal of Molecular Structure 969, no. 1-3 (2010): 233-237.
Pierluigi Traverso Thank you, it helped a lot.
As I understand, the best way is to be consistent in processing the curves, do it the same way every time. And the other thing is when we have diffusion control (bc =1V/decade to ∞) in cathodic branch and trying to calculate corrosion rate by Tafel, the true value is somewhere between the rate calculated with bc=1V/decade to ∞ and the rate calculated with bc=0.12 V/decade for activation control. So if I want to compare the worst case scenario, its 'better' to adjust cathodic slope to a point of intersection of Ecorr and anodic slope taking into account the fact that bc should be more then 1V/decade.
Don't know if what I said make sense :)
Dear Ekaterina Mansurova, Looking the polarization curves Enchem1-2 and Enchem3, where the current density is in logarithmic scale, I don´t see Tafel regions in anodic or cathodic curves, because they could appears as straight lines, well defined. The cathodic straight line usually appear in a region of polarization between 60 and 120 mV (vs the initial open circuit potential) and the anodic line appears in a shorter region of 50 to 90 mV polarization. In these cases, the use of the software (potentiostat) is not recommended. I suggest to obtain the Rp applying polarization of ± 25 mV vs the initial, open circuit potential. After that, you can calculate the corrosion current (because in this region of polarization, Ohm´s lays is valid).
Thank you very much Professor L. Veleva for your helpful answer. I looked more thoroughly on the regions you noted above. To do that I reduced the scan rate (pic. 1) and also did a polarization experiment from 0 to 200 mV from OCP (pic. 2) to see an anodic slope without taking into account a cathodic one. I think now I can actually see a good straight line or is it just my imagination?
I'm also trying to find a way to compare LPR and EIS results with mass loss tests as recommended in ASTM G102 in case when Tafel slopes cannot be obtained. And it looks like corrosion rate for mass loss is 5-10 times less than LPR.
Thank you very much Professor L. Veleva for your helpful answer. I looked more thoroughly on the regions you noted above. To do that I reduced the scan rate (pic. 1) and also did a polarization experiment from 0 to 200 mV from OCP (pic. 2) to see an anodic slope without taking into account a cathodic one. I think now I can actually see a good straight line or is it just my imagination?
I'm also trying to find a way to compare LPR and EIS results with mass loss tests as recommended in ASTM G102 in case when Tafel slopes cannot be obtained. And it looks like corrosion rate for mass loss is 5-10 times less than LPR.
Dear Ekaterina, Look well, there is not straight lines !!!! Do not use the software. It is very difficult to beleve that the anodic process is controled by charge transfer....What is the metal and its surface, as well as the electrolyte in the formed system? Kind regards, LV
Dear Prof. L. Veleva thank you again for your help. I used the software just to get an anodic slope in a region I thought is straight (pic3). I did not trust the results, only tried to figure it out if it even possible to find a straight line in anodic curve at all.
The working electrode is carbon steel, polished with 1 micron final. Electrolyte solution is 2.0%wt NaCl, deaerated from O2 with N2, and then CO2 -saturated, pH=4.0. Experiments from pic1-3 were performed at room temperatures.
Dear Ekaterina, The carbon steel electrode, even polished, used as electrode for anodic curve is difficult to accept to present the Tafel region. I was asking in what kind of electrolyte was registered the anodic curve? In this curve I can not appreciate the region of mV (polarization) where You consider that there is Tafel region. (For me it is not a straight line...).
Dear L. Veleva ,
Electrolyte solution is 2.0%wt NaCl, deaerated from O2 with N2, and then CO2 -saturated, pH=4.0.
The main reason I'm trying so hard to find any straight region on an anodic curve is that to calculate Corrosion rate by using LPR technic I still need to know Ba or/and Bc to calculate B-coefficient first. I can see that ASTM G102 specifies (in cases like mine, when it's not possible to obtain Tafel slopes) to determine the Stern-Geary constant by mass loss experiment. But I can't do that.. Because I'm doing this electrochemical experiments to find a different comparison method for mass loss experiments I've already done in that electrolyte.
Thank you again for the help. Your advises actually cleared tones of my questions!
Dear Ekaterina, Now I am sure that there is not Tafel region in the anodic and cathodic curves, because of the absence of O2. So, the cathodic process is controled by the diffusion: at pH=4 in the absence of O2, the content of H+ ion is very low. What about the oxidizer agent in this solution ?!?!
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