I have collected few electrochemical impedance spectra of Cr2O3-based oxide layers formed during high temperature oxidation of alloys in a non-aggressive electrolyte. The qualitative Bode phase angle plot exhibits around 2-3 time constants (CPE peaks at high-, mid- and/or low- frequencies) indicating various dielectric features within the oxide layer. Generally, I attribute the time constant in the high-frequency region (above 103 Hz) with that of the bulk oxide because of its lower capacitance (from high thickness). Meanwhile, the other time constants located in the mid- and/or low-frequency time constants look complicated, though I suppose it must be the response from nano-porous fractured paths observed in the oxide that may have electrolyte accommodation till the metal-oxide interface. It is thus speculated because much thicker oxide scales did not exhibit these mid- and/or low-frequency time constants. In this regard, how can I associate these time constants with their respective dielectric features?
Further I need to model it for quantitatively comparing the electrolyte penetration within the oxides. The general oxide models (porous and sandwich structured) seems irrelevant when much electrical components needed to be added for fitting spectra with much dielectric components.
http://link.springer.com/article/10.1023/A:1018856808917
If we assume that the scale is single layer, dens, adherent to the substrate, free of connecting pores and the electrolyte is non aggressive, then the scale is stable and no variation in the dielectric time constants is observed. In your question ,you made clear that the scale composition is only chromium oxide single layer and the electrolyte is non aggressive, in this case only the physical interaction between the scale and electrolyte via connecting pores is effective in changing the dielectric property of the scale. If the connecting pores are regular in their geometry and the test period is much faster than the transport of electrolyte from the scale outer surface towards the surface of the underlying substrate through the connecting pores, then no significant variation in time constants is observed. Now if the connecting pores are not regular in their geometry and the test period is slower than the transport of electrolyte from the scale outer surface towards the surface of the underlying substrate through the connecting pores then significant variation in the dielectric relaxation time constants is observed.
Best regards
Dear Viswanathan Ramamoorthy,
say, please, more about your cell. Also, could you, please, show some other spectra, saying more on the measurements conditions for EIS (and the VDC polarization) ?
Dear Mohammed Hussein j. H. Al'Atia
Thanks for your answer. I have few queries from you answers and I have listed below:
1. 'Significant variation in the time constant' - Does it mean the variation along the time of exposure in the solution or along the frequency?
2. 'Irregular pores and slower electrolyte transport have variation in time constant' - Does it mean there will be variation in the lower-frequency time constants with respect to the time exposed in solution (or quantity of electrolyte penetration)?
3. I have also observed shifting of time constants with respect to the frequencies on different substrates and I am trying to qualitatively or quantitatively interpret this phenomena with respect to the oxide structure.
I hope your assistance in this regard would be great.
Thank you.
Dear Ioannis Samaras
I used a 3-electrode setup with SCE reference and graphite counter electrodes. The experiment was done in borate buffer solution (Na2B4O7 + H3BO3) and it was non-deaerated, since I assumed there wont be any interactions between the dissolved oxygen and oxide layer. EIS (10 mV AC amplitude) of the oxidized alloy was done at OCP after stabilization, without any polarization.
Maybe 10 mV Ac amplitude is too small. It is amplitude value used to typical electrochemical or corrosion investigastions. The high temperature oxide layer has (?) high resistance and realy amplitude is much lower.
Dear Stefan Krakowiak
Most of the researchers have done the analysis at 10 mV AC amplitude, as concerns may raise about the linear response on increasing the amplitude. Meanwhile, the oxide was some hundred nm thick and not much resistance change was observed from the non-oxidized specimens.
Dear Viswanathan Ramamoorthy,
could you show (add), please, an EIS measurement[1] for an non-oxidized specimen ?
1. Is it possible, please, to add a Nyquist plot(?), also ?
Viswanathan Ramamoorthy
Thank you for your interest in my previous answer.
1&2- Yes it means the variation along the time of exposure in the solution. For short immersion time, a single time constant may observed at high frequency due to very high associated resistance (little or no electrolyte transport). For longer immersion times, a second or more time constant may start to appear at low frequency due to decrease in the resistance as a result of electrolyte transport through the pores and other type of defects.
3- Yes, the oxide structure play essential rule in terms of associated defects within its matrix.
Best regards
Dear Karthik Bhat
Yes, it pleases me
Thank you
http://www.sciencedirect.com/science/article/pii/0010938X8390063X?via%3Dihub
http://www.sciencedirect.com/science/article/pii/S1572665714002756
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