Please have a look at this figure, does it ever reflect that an EIS measurement has been made at t=5000 s and at t=10,000 s?
The figure which you are showing is only showing OCP vs. time. The OCP is not changing a lot (2 mV/33 min. for the first 4000 s) and 4 mV per 150 min for the second point of 9000 s.
This change seems it happened during taking the measurement which can be normal due to affecting the interface. If you change the scale of the Y-axis, the changes will not appear.
Have you the Nyquist plot or other EIS plots, I can have a look. Cheers.
Thank you Adel for the notes, the Nyquist plots are not ready yet actually. You can see the abrupt change in the OCP vs. time after holding the OCPs for nearly 300 s during an EIS test from 10 kHZ to 0.01 Hz at 4000 and 9000 s. That what made me think of the reliability of the readings. Thank you again.
Here is the Nyquist plots. The EIS measurements were made after 5000, 10000 s and at the end of the OCP monitoring, at 15000 s
May I know about the system you study, it seems a bit strage to have one semicircle, but I need to know about the 3-electrode system you are using and the WE, is it for a coat or something else. Why you choose to work in 1% NaCl, if it is for corrosion study, it is preferred to have 3.5% of NaCl to simulate almost the same salinity of sea water. Waiting for your reply. Kind regards.
The WE tested is a high-strength steel immersed in a low-pH CO2-saturated solution of 10 g/L chloride. This Nyquist plot is typical for similar CO2-saturated systems. The plot seems composed of two depressed semi-circles overlapped at low frequencies. They reflect the continuous dissolution of the substrate and the simultaneous build-up of the corrosion products that raise the charge-transfer resistance over time. You can see the continuous increase in the size of the Nyquist profiles. If you have any comments please share.
Thank you for sharing the information.
1) why you are using 1% NaCl instead of 3.5% NaCL?
2) You can leave the high-strength steel for longer time before you take the EIS measurements. I could not notice the 2 semicircle from the Nyquist plots.
3) Can you provide the Bode plots?
4) Do you have any gravimetric experiments (wieght loss)?
Hi Adel,
I am using actually a range of chloride concentrations to study their effects on the corrosion behavior. The chloride concentrations in the pipeline flows are on a large scale of variation. Also, the chloride concentration in a pipeline flow is subject to change in association to many environmental factors if the flow is of a high water content. Yes, I have bode phase plots, but I depend on them for longer periods of immersion and in mildly-alkaline (bicarbonate) solutions - for which I conducted weight loss measurements.
Thank you Binod for your input.
Hi Faysal....
Whether yo can study lower or longer immersion periods, Bode plots gives realistic picture of your system. If your Nyquist plots exhibit two loops in semicircle, you will also get two phase angle maxima in your Bode plots. In your environment you can immerse the sample for 30 min or 1 h before the experiment. During this time OCP will be stabilized, later you can proceed with your experiment. In our case we have been tested our samples for a maximum immersion period of 90 days. In your case the Rct value is increasing with immersion time, because, there is formation of metal oxide on the metal surface. This metal oxide will protects it from corrosion up to some extent. So my sincere advice is increase your immersion time, until the Rct values are decreased.
During the EIS the WE potential is set by the instrument while during the OCP the WE reflects the free evolution of your electrochemical system. You should look at the DC part of the current during the EIS. If you expect the system to not be modified by the EIS then the mean current should be zero. Any drift in the current will indicate an undergone polarization of the WE. You may want to try a galvano EIS and set the mean current to zero.
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