I have been studying the corrosion inhibition characteristics of a molecule for mild steel (600grit finish) in 1M HCl. Except for the blank solution, I observe a significant inductive loop in the Nyquist plot obtained from electrochemical impedance spectroscopy of system with various concentrations of inhibitor molecule. Is there an explanation and protocol that helps understand the physics of inhibition and fit the circuit including the inductive loop? I have been using ZSimpWin for this purpose and there are several circuits which may fit the data, however, the physical significance could not be found for many of them. I have gone through the book "Electrochemical Impedance Spectroscopy" by MarkEOrazem and BernardTribollet and several other publications.
Through literature, I found that reaction intermediates(what are they?why not for blank?), relaxation processes on adsorption of Cl- or H+(then blank must also have inductive loop), presence of strong oxide film(this is mild steel, freshly prepared) and dissolution of passivated surface(is 10mV perturbation of EIS sufficient to dissolve the passive layer?several minutes of OCP measurement would have already dissolved passive layer), are some reasons reported for inductive loop. I could not find a fitting answer or may be I could not connect it to my case. Can someone help me understand the inductive loop in the context of corrosion, adsorption and double layer? And physical significance of the circuit with inductive loop? Is there any textbook or standard to fit such data?
We have observed a similar feature when studying methanol oxidation at various conditions (0.5 M sulfuric acid electrolyte/Pt WE). Through mechanistic modeling, we found that the inductive loop indicates that an adsorbate is involved in the process where the generation and consumption of this adsorbate is at comparative reaction rates. That it is inductive also indicates that the increase in potential is removing the adsorbate (dTheta/dE) < 0.
Some relevant literature:
- Interpreting inductive loops as a single-adsorbate mechanism:
https://www.sciencedirect.com/science/article/pii/0013468687800051
- Using this approach on a dataset:
https://www.sciencedirect.com/science/article/pii/S0378775399003316
- Modeling of a reaction mechanism giving an inductive loop:
http://ecst.ecsdl.org/content/85/12/167.short?casa_token=XYPsw_dgh0sAAAAA:8jhxUgmDB7kT8wxZbaBJt9n1QS-KZL1-u9uI84Xl8VTcD1V2aJTSSoxL0Qprc56XsGwSLQieR_I
- A review of inductive loops for fuel cells:
https://www.sciencedirect.com/science/article/pii/S037877531630831X
I do not work on corrosion directly. However, some examples of inductive loop in studies of corrosion may include:
- Mg corrosion showing mass transport features and an inductive loop:
http://jes.ecsdl.org/content/154/2/C108.short?casa_token=0BMPt7HS1moAAAAA:KWZU3mnJA7LsLdLGxcJX0Apz46Cw83uMmdj6cKU5dUXA_A6CAmhE07uwP0qvp5yAb_O8MndWVoo
I hope it helps!
What frequency is the loop at? What potential is your impedance experiment at? Consider the processes that are probed in the frequency domain in which it appears.
I'd suggest you go look at the work of Digby D. Macdonald, his work is extremely informative with regard to mapping corrosion/inhibition processes using impedance.
Hi Andrew,
I am working on certain derivatives or a class of inhibitors in polyamines, for mild steel. The total frequency range in EIS is 100kHz - 100mHz.
For each derivative, the start and end frequencies of inductive(L) loop are different. For example, for the simplest molecule, start and end frequencies of L-loop are 1Hz and 100mHz; for a derivative of this molecule, they are ~30Hz and 100mHz, for another derivative the Nyquist plot did not enter inductive regime at all. It was an interesting observation that with increasing concentration, certain derivatives had their inductive loops increased in size while some derivatives had decreased size of inductive loops. So, I am curious to understand the mechanism of inhibition, for each of them.
Impedance experiments were carried out at respective open circuit potentials for each system which are in the range of -450mV - -510mV.
I will go through the work by Digby D.Macdonald, thanks for the suggestion.
Thank you,
Venkat.
It corresponds to the charge transfer resistance which is used to determine the rate of corrosion.
We have observed a similar feature when studying methanol oxidation at various conditions (0.5 M sulfuric acid electrolyte/Pt WE). Through mechanistic modeling, we found that the inductive loop indicates that an adsorbate is involved in the process where the generation and consumption of this adsorbate is at comparative reaction rates. That it is inductive also indicates that the increase in potential is removing the adsorbate (dTheta/dE) < 0.
Some relevant literature:
- Interpreting inductive loops as a single-adsorbate mechanism:
https://www.sciencedirect.com/science/article/pii/0013468687800051
- Using this approach on a dataset:
https://www.sciencedirect.com/science/article/pii/S0378775399003316
- Modeling of a reaction mechanism giving an inductive loop:
http://ecst.ecsdl.org/content/85/12/167.short?casa_token=XYPsw_dgh0sAAAAA:8jhxUgmDB7kT8wxZbaBJt9n1QS-KZL1-u9uI84Xl8VTcD1V2aJTSSoxL0Qprc56XsGwSLQieR_I
- A review of inductive loops for fuel cells:
https://www.sciencedirect.com/science/article/pii/S037877531630831X
I do not work on corrosion directly. However, some examples of inductive loop in studies of corrosion may include:
- Mg corrosion showing mass transport features and an inductive loop:
http://jes.ecsdl.org/content/154/2/C108.short?casa_token=0BMPt7HS1moAAAAA:KWZU3mnJA7LsLdLGxcJX0Apz46Cw83uMmdj6cKU5dUXA_A6CAmhE07uwP0qvp5yAb_O8MndWVoo
I hope it helps!
I have seen some credible works attribute the inductive loop to diffusion phenomena in corrosion systems. In such systems, the equivalent circuit will differ from the normal simple randles circuit but a warbug element in series with the Rct.
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