Dear RG Family,
I am working on certain aluminide-based coating systems for anti-corrosion applications. I performed different electrochemical tests including EIS and Tafel after immersion in 3.5 % NaCl for different intervals (from 1 hour to 100 days). For 1 hour, I am getting the normal charge transfer resistance (RCT) value (up to few thousand Ohm) after circuit fitting. As the immersion time increases, the impedance value was expected to increase due to the formation of a sacrificial layer. However, in my case, I have observed an exponential increase in RCT after the immersion for 100 days. The new Rct value is in millions, about ten thousand times as compared to 1 hr of immersion time (The Nyquist plot is attached). Even after the repetition of tests with a different specimen, I got a similar trend. A few Tafel Plots are also complementing the results from EIS, as the corrosion rate decreased significantly after immersion. As far as I understood, I am going right and obtaining a significant reduction in corrosion rate. My question is am I missing something? What significance do these results really have?
Sincerely,
Dear Dr. Nisar Ahmed ,
according to what you are reporting, it seems to me that you are carrying out the electrochemical part relating to the protective power of the surface layer in a rigorous way, using "standard" electrochemical cells.
Repeating the measurement with other equal samples gave you a similar trend, even comparing the different exposure times. What intrigues you, however, is the cause of this trend. What I suggest you do is a chemical morphological analysis of the surface of the unexposed samples, after a short exposure (e.g. 7-20 days) and after 100 days of exposure (if you want on the samples subjected to EIS measurement which is not destructive or more simply on samples exposed to free corrosion) using an SEM with EDX microprobe.
If this gives you interesting results, you could also take measurements on the section of the samples and see, in even more detail, how the surface layer changes over time. You should notice quite evident changes that could help you in the interpretation of the chemical-physical phenomena that have generated such a highly protective surface layer.
P.S. Of course, I assume you have done an initial test of your electrochemical system with a dummy cell.
My best regards, Pierluigi Traverso.
Dear Nisar Ahmed,
frankly this is really high value after 100 days. My experience is that Rct decreases over increasing immersion time from long-term horizont as the degradation of the coating occurs. Could you provide us with some closer informatios about the coatings character ?
Sincerely,
Milan Štrbák
I would first check the layout used for the research. With resistances of tens or hundreds of megaohms, it is hard to imagine making a Tafel curve.
stefan krakowiak
Could you provide the same results but in Bode format (modulus and phase angle)?
Corrosion behavior of self-organized TiO2 nanotubular arrays grown on Ti6Al4V for biomedical applications
Corrosion resistance, Tafel curves clearly explained.
https://www.sciencedirect.com/science/article/pii/S0254058423007198?ref=pdf_download&fr=RR-2&rr=7e2d4d415ca89633
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