Hi, when you do EIS measurements, you will probably measure on the open circuit / corrosion potential. The AC excitation of 10 mV is not very invasive, i.e. the material corrodes under free conditions. In the polarization experiment, however, you polarized the steel strongly anodically (up to 0.7 V!) and thus forced the anodic dissolution of the iron. Therefore you see strong corrosion. You will probably have to wait longer under free conditions for visible corrosion to occur because the steel simply corrodes more slowly.

Best regards,

Andreas

Try to (re)measure, please, an EIS, time series, set, one EIS (Vdc=OCP[1]) per day, for about 5 working days, or more.

1. Vdc = OCP (of each day's OCV), for a wider spectrum: 100kHz down to 10 mHz, or lower.

Andreas Heyn Sir, Thank you for your quick response. I will take your points into consideration. I will try to increase the AC excitation signal and look for the required answers. Any inputs are welcomed.

Dear Ioannis Samaras Thank you for your suggestion. I will conduct the experiments as you have mentioned.

Andreas Heyn prof, how to know what voltage range and potential scanning rate should be applied during the potentiodynamic polarization test. (Taken into consideration I have performed the Open Circuit Potential test for 1 hour and the curve got stabilised at -0.5V.)

thank you. Willing to learn from you

1. If you really need to see corrosion during EIS, choose for DC imposed potential a higher value (not OCP but, e.g., 0.3 or 0.5 V) (look at a polarisation curve to see the corrosion domain).

2. For a polarization test:

- scanning rate: very low 1 mV/s

- scanning range: from OCP - 200 to OCP + 200 mV (it is enough for a further processing as Tafel plot)