I investigated the corrosion resistance PEO coated AZ91 magnesium alloy. When I compared the results from electrochemical impedance spectroscopy (EIS), namely Rp values were much higher compared to the Rn (noise resistance) obtained from Electrochemical noise (EN) measurements. Could you help me to explain this phenomenom because literature says these characteristics should be in correlation.
Thanks for any comments.
Could you provide more details ? (Values that you obtained from using the two techniques, the method you used to get Rp from EIS, one of the references saying that the two methods should give the same value).
Dear Abdelmoheiman Zakaria Benbouzid
I attach the Noise resistant (Rn) evolution during 168h of exposure in 0.1M NaCl for plasma electrolytic oxidation coated AZ91 magnesium alloy. On the contrary, the values for polarization resistance ranged between (600 000 - 150 000 Ω.cm2) and the data were extracted by using quivalent circuit from Nyquist plots.
Here is the reference with the information:
Chen, J. F., & Bogaerts, W. F. (1995). The physical meaning of noise resistance. Corrosion Science, 37(11), 1839-1842.
Many thanks,
Milan
First of all the reference you are using is too old, please check some recent papers.
Second, the analysis they used in the paper is limited to the case of high corrosion current in comparison to the coupling current.
Third, keep in mind that Rp by definition is the limit at zero frequency of the impedance spectra, which is not what we get experimentally in most cases (We can't reach zero frequency and we are not 100% sure that what we get experimentally is the real value of Rp; many other time constants can exist at very low frequencies).
Forth, Rp=Rn only when we are under activation polarisation control (charge-transfer control of the elementary corrosion processes). In other word, anodic and cathodic kinetics following Tafel's equation.
Thus, at very low frequencies and when we are dealing with inductive loops related to the relaxation of the coverage fraction of adsorbed intermediates ,"the value of Rp will reflect side effects and is no longer related to the derivative of the Tafel kinetics with the potential and provides a biased estimation of the corrosion rate". In this particular case, we should use the charge transfer resistance instead of the polarization resistance (This fact may explain the huge difference between Rp and Rn that you obtained in addition to what I'm going to explain below).
Fifth, keep in mind that Rn is very sensitive to many parameters such as:
- The method used for the DC component removal (There is no universal methods and the result will depend of course on the method you use for that);
- The frequency range;
- The sampling frequency;
- Size of your working electrodes and especially the asymmetry between your working electrodes (Even if it is initially complied with, it my be changed over time);
-..., Etc.
Conclusion: in many cases, electrochemical noise is not an acurate method as it meant to be for corrosion rate assessment.
At last, "The techniques essentially provide what they measure in the temporal domain where they observe the surface."
Hope it helps.
Best Regards,
Zak.
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