Dear Dr. Haider Abdulkareem Al-Mashhdani,

in a previous question relating to this topic here in ResearchGate, Dr. @Angel Cuesta, gave the following, very clear, explanation:

The Tafel slope simply tells you how much you have to increase the overpotential to increase the reaction rate by a factor ten. This will be determined by the magnitude of the change in the activation energy for a given increase in overpotential (when you change your potential 1 V, the Gibbs energy of the process will change by 1 eV per electron transferred, but the activation energy will only change by a fraction of this, and this fraction determines the Tafel slope). In a reaction involving only one step with one electron transfer, the Tafel slope will be determined by the symmetry factor, which is usually 0.5 (corresponding to a Tafel slope of 120 mV). In a more complex reaction involving several steps and several electron transfers, the Tafel slope will be determined by the rate-determining step and by the number and nature (i.e., involving an electron transfer or not) of the preceding steps. So, essentially, from the Tafel slope you can deduce whether your rate-determining step involves an electron transfer or not, as well as the number of electrochemical (involving an electron transfer) and chemical (not involving an electron transfer) steps that precede it. If you propose a reaction mechanism, you can calculate what the corresponding Tafel slope should be and, if it does not coincide with the experimental one, the mechanism cannot be correct.

For more details, you can have also a look at the following notes:

-Tafel Plot and Evans Diagram

By Palm Sens

Available at: https://www.palmsenscorrosion.com/knowledgebase/tafel-plot-and-evans-diagram/

- Quantitative Corrosion Theory

By Gamry

Available at: https://www.gamry.com/Framework%20Help/HTML5%20-%20Tripane%20-%20Audience%20A/Content/DC/Introduction_to_DC_Corrosion/Quantitative%20Corrosion%20Theory.htm

Best regards, Pierluigi Traverso

Hello,

1m V/decade means a variation of 1 mV for a ratio of 10 between two currents.

In the plot log(i) vs E, it means 1 mV is measured between log(i1)=n and log(i2)=n+1.

Or in scale log for the plot i vs E, it means 1 mV measured between i1 = n and

i2 = 10 x n.

I hope it will help you!

Best Regards

Thomas

Dear Dr. Haider Abdulkareem Al-Mashhdani,

in a previous question relating to this topic here in ResearchGate, Dr. @Angel Cuesta, gave the following, very clear, explanation:

The Tafel slope simply tells you how much you have to increase the overpotential to increase the reaction rate by a factor ten. This will be determined by the magnitude of the change in the activation energy for a given increase in overpotential (when you change your potential 1 V, the Gibbs energy of the process will change by 1 eV per electron transferred, but the activation energy will only change by a fraction of this, and this fraction determines the Tafel slope). In a reaction involving only one step with one electron transfer, the Tafel slope will be determined by the symmetry factor, which is usually 0.5 (corresponding to a Tafel slope of 120 mV). In a more complex reaction involving several steps and several electron transfers, the Tafel slope will be determined by the rate-determining step and by the number and nature (i.e., involving an electron transfer or not) of the preceding steps. So, essentially, from the Tafel slope you can deduce whether your rate-determining step involves an electron transfer or not, as well as the number of electrochemical (involving an electron transfer) and chemical (not involving an electron transfer) steps that precede it. If you propose a reaction mechanism, you can calculate what the corresponding Tafel slope should be and, if it does not coincide with the experimental one, the mechanism cannot be correct.

For more details, you can have also a look at the following notes:

-Tafel Plot and Evans Diagram

By Palm Sens

Available at: https://www.palmsenscorrosion.com/knowledgebase/tafel-plot-and-evans-diagram/

- Quantitative Corrosion Theory

By Gamry

Available at: https://www.gamry.com/Framework%20Help/HTML5%20-%20Tripane%20-%20Audience%20A/Content/DC/Introduction_to_DC_Corrosion/Quantitative%20Corrosion%20Theory.htm

Best regards, Pierluigi Traverso

Decade means for example 1 mA to 10 mA, or 10 mA to 100 mA, in log scale -3 (0.001 A) to -2 (0.01 A), or -2 (0.01 A) to -1 (0.1 A).

So the Tafel slope of +/-120 mV/dec means that between 1 and 10 mA potential increase (+) or decrease (-) for 120 mV.

Dear Haider,

The unit of the Tafel constant is either mV/decade or V/decade. A decade of current is one order of magnitude. Tafel constants are used to calculate corrosion rates from polarization resistance data.

Hope it is helpful to you.

Ashish

Nice Question Haider Abdulkareem Al-Mashhadani , Following

Thank you Dr Abdelkader Mohamed Elsayed

Thank you for answering me Ashish Thakur

Thank you so much for this answer Pierluigi Traverso

Thank u Dr Branimir N. Grgur