This EIS data are obtained for an Aluminum alloy coated with a relatively thick and porous crystalline oxide layer. Also, a thin barrier phosphate layer may be formed on the Al surface just below the porous oxide layer. The oxide layer also contains phosphorous compounds.
Which type of element may be used in case of EIS data analysis?
dear Mohsen Sabouri,
give us, also, an additional figure. Note, please, the contacts (contacts' geometry, a scheme) on your sample.
Dear Dr. Samaras ,
The sample is a Aluminum plate (2024 Al alloy) (1.5cm *1.5 cm) that is coated by plasma electrolytic oxidation method at 500 V in Na4P2O7 (10 g/l) +NaAlO2 (3 g/l) +KOH (3g/l) solution for about 10 min (Coating thickness is about 20-30 micrometer).
EIS experiments is conducted on the coated sample in 3.5% NaCl solution with time at room temperature under static conditions. Graphite rod and SCE were employed as counter and reference electrode, respectively.
Based on XRD analysis, the obtained ceramic coating is mainly composed of α-Al2O3. γ-Al2O3 and AlPO4 were also identified in the coating composition.
EIS experiments were conducted using an EG&G flat cell.
The EIS bode and Nyquist plots reveal that your experimental data can be analyzed based on an equivalent circuit consists from three time constants in series connection with the solution resistance (Rs). The first time constant (RctCdl) describes the behavior at the HF range, the second one (RcotCcot) is due to the coating response in the MF range. While the third time constant appearing at the LF range is for the inductive behavior of the system at very low frequency region. You are welcome to visit me in my Lab at Faculty of Science Cairo University to further discuss your results if you want ( Mobile: 01002449048).
From the graph it looks like your coating is absorbing electrolyte during the measurement process and hence change/decrease in the low frequency impedance.
dear Mohsen Sabouri,
1) as Vinod Upadhyay and/or
2) a following corrosion.
To verify these proposal mechanisms, remake some EIS, as time series (better in i-current mode with iDC=0, If possible).
Dear Dr. Upadhyay and Dr. Samaras,
Thank you for your responces. I do the EIS experiments in potentiostatic modes with time (about 1000 hours). According to the results, this happens repeatedly with time.
Dear Dr. Samaras, Could you please explain to me why you prefer the EIS experiments in current mode? Also, in current mode (GEIS), a very low value of current peturbation (for example 1 microamp of perturbation) may needs a high voltage of response (about 15 V). In fact, I am not sure about the optimum value of current peturbation to keep the GEIS experiments nondestructive.
Also, as this is a repeatedly process with time, which circuit element may be suitable in case of data fitting.
In other words, should we consider these low frequency data in the fitting process or disregard them?
dear Mohsen Sabouri,
1) remake time series EIS set only for a next (new) fresh sample. Mohsen: I do the EIS experiments in potentiostatic modes with time (about 1000 hours). According to the results, this happens repeatedly with time.
2) during an incomplete reaction (corrosion? etc.) the electrochemical potential V(t) is free to follow the reaction (use iAC~nanooamps). From the EIS time series set and V(t) you may have infos for the reaction rate Mohsen: Could you please explain to me why you prefer the EIS experiments in current mode?
3) remake one higher F-resolution EIS (only for the old sample) and then we will discuss-consider again. Mohsen: ...should we consider these low frequency data in the fitting process or disregard them?
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