EIS and Tafel measure the general or you can say overall corrosion behaviour of the material. However, CPP is generally conducted to understand the localized effect of the

Typically, the larger the perturbation from steady state corrosion (i.e. applied potential) the greater the probability of measurement error due to changes in sample condition (surface films, roughness, etc). So, we tend to use small perturbation techniques for measuring corrosion rates (i.e. EIS, LPR) and the larger perturbation techniques for more general characterizations such as pitting resistance and microstructure dependent behavior. In the case of duplex st steels, we found that in one case it took over 15 hrs to reach a steady state OCP (Fig. 7: Metl. Matls. Trans. A, 35A (8) 2427 (2004)). We do not use the hysteresis in CPP as an "official indicator" for anything because we have found it to be too dependent on surface conditions that are easily influenced by difficult to control experimental variables (i.e. pit size, shape, and depth, how far you went past the pitting potentia; size, shape, and mounting of the sample; crevice attack between the mount and sample during the experiment, etc.). In the case of duplex st steel, I would expect the size and shape distribution of the pits to be strongly influenced by the morphology of the more susceptible phase (which relates to the changing OCP behavior), but getting a reliable indication from hysteresis on relative performance in service still seems unlikely. A better indicator would be metallography of samples before and after OCP, or potentiostatic, exposures bracketing the expected range of exposure conditions. Keep in mind that the measured potential is a weighted average value for the surface and not the same everywhere.

I agree with Dr. Richard E. Ricker .

The large charge transfer resistance observed with EIS at OCP may not be directly correlated to repassivation behavior observed during CPP test, because the interfacial conditions of the specimen are totally different. If you want to compare, perform EIS at OCP and in the vicinity of pitting protection potential and compare the two EIS spectra.

Thank you very much Sir for the answer. I understand your point. The issue that I am facing here is that I have performed tests under identical conditions for 4 samples that differ in terms of the aging treatments they have received and accordingly they are to be ranked based upon their corrosion performance. For this task, I did EIS and Tafel scan (solution is 3.5% NaCl solution) for their general corrosion behavior and CPP for their localized corrosion susceptibility (solution is 3.5% NaCl solution). Now once I go for analyses, I find that the order I am getting with CPP differs from what EIS and Tafel results divulge. That becomes a bit problematic for me as I do not understand now what to make out of these results.

Does these results suggest that aging treatments affect general corrosion and pitting corrosion of these samples differently??

Moreover, going by the fundamentals, Is it a fact that a material exhibiting good uniform corrosion behavior (checked by EIS, LPR, Tafel etc.) should also show good pitting corrosion resistance too (checked by PAP, CPP etc.)??

Thank you.

Stainless steels, in general, show very good resistance to general corrosion in oxidizing environments. It is well established. Therefore, if I were you, I would not focus on general corrosion resistance in simulated sea water conditions. The real issue is localized corrosion that includes pitting, crevice, and stress corrosion cracking. Duplex stainless steels are used because of their higher strength and better resistance to localized corrosion than their austenitic/martensitic counterparts. Welds of DSS have complex microstructures and heat treatment further complicates the metallurgy. Therefore, good general corrosion resistance need not translate into good localized corrosion resistance because the tests are not done at identical conditions. You determine the icorr near OCP, whereas pitting resistance is determined by a significant polarization. When you send your manuscript for review, most of the reviewers will agree to this point. Focus on the correlation between microstructure and localized corrosion. SEM images before and after pitting tests will give you sufficient information that will lead to a better understanding.

Thank a lot respected sir Krishnan Raja for sharing your opinion. Although as it is well known that SSs have good general corrosion behavior, the reason why I am going for general and localized corrosion behavior is more inclined towards academic contribution. I am working on investigating the effects of welding conditions as well as thermal aging treatments (that can result into deleterious intermetallic precipitation) on the general and pitting corrosion performance of duplex stainless steel welded joints. Any suggestions from your side about this work would be respectfully welcomed. Thank you.

Sir, can you please share ur opinion about how to decide the potential range selection for Tafel extrapolation on Tafel scans to calculate icorr and hence corrosion rates. The testing has been done from -250mV to 250mV about Ecorr at a very slow scan rate of 0.125mV/sec. Should I go about selecting 100mV on either side of Ecorr or greater value?

Secondly, suppose we have 4 samples which have been tested using Tafel scan and they are to compared for their corrosion rates using Tafel extrapolation.

Now should the potential range selection needs to same for all these samples for a reliable output or potential range can be different but still genuine corrosion results can be obtained??

A clear linear region in the E Vs. log i plot is required for determination of Tafel slopes. I would not shrink the potential range. In my lab, we do not run a separate experiment for Tafel slope calculations. We run separate cathodic polarization from OCP to -600 mV Vs OCP, and a separate anodic polarization or cyclic polarization run, using a different specimen or the repolished specimen that was cathodically polarized. This gives us a large potential window to observe a clear linear portion.

I prefer to use similar potential ranges for all the specimens to carry out Tafel extrapolation. The polarization data need to be statistically analyzed because +/_15% scatter is not unusual in these measurements.

Wishing you the best!

Dear Sir,

The Ecorr and OCP should come same values, but always find the little difference between them. Because, the OCP values is measured in open circuit condition without disturbing the surface and before measure the Ecorr values, we disturbed the surface cathodically.

Generally, the Ecorr values are usually measuring by considering -300 to -200 mV with respect to OCP following to the positive potential. Basically, we are polarizing surface cathodically and coming towards the equilibrium potential and thereby the small change is usually observed in Ecorr and OCP values. Hence, in my knowledge, keep the lower negative potential like -300 to -200 mV and in this way we able to keep less disturbance on the surface cathodically before reaching to the equilibrium potential for better Tafel extrapolation.