Hi

From my experience with EN, it is always quite difficult to get straight answers just from this technique. This means that you often need info from other techniques to help on your conclusions.

Noise measurements are more adequate for monitoring, provided that you have previously studied in detail your system, both with EN and other techniques, allowing for knowing the different responses of the system and their relation with the corrosion states. Once you have the fingerprint of your system you may use EN for corrosion monitoring.  

Dear Mohamed

I would suggest to use cyclic polarization (CP) test in order to evaluate the susceptibility of the steel plate  in NS4 to localized corrosion, i.e., crevice or pitting or localized galvanic  corrosion.

The three probabilities of  galvanic, crevice or pitting might be right.

Also useful to make use of the data from the basic article (the google link below),

"The Measurement of Localised Corrosion Using Electrochemical Noise" by K. Hladky etal., as they presented the following distinguishing factors.

"Measurements of self-generated electrochemical potential fluctuations on electrodes undergoing either pitting or crevice corrosion show that these forms of localized attack have quite distinct noise “signatures” and that these two types of attack can be detected within seconds of their initiation."

 https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&uact=8&ved=0ahUKEwi9so7r7OfUAhWLbhQKHev4DdoQFghLMAY&url=http%3A%2F%2Fwww.khdesign.co.uk%2Facrobat%2Fpaper1.pdf&usg=AFQjCNFEMX8Z--vw_7YUTqMzy75pY5QvfQ

The question with the "distinct noise signatures" is that they cannot be generalized. A potential fluctuation that is assigned to pitting for a given metal+environment may correspond to stable passivation for another metal+environment. That is the major problem of EN analysis.    

The data of EN on the susceptibility to localized corrosion of alloys are sometimes hard to interpret, as J.C.S.Fernandes has suggested   Therefore, I would suggest  to start with the standard cyclic polarization (CP) test in order to evaluate the susceptibility of the steel plate  in NS4 to localized corrosion.   So, when your CP data confirm the type of localized corrosion, then you can use the CP data to substantiate the EN data. 

Mohammed, your difficulty begins with the statement "I have an artificial inclusion....".  That means that you have automatically generated a crevice, and you may also have a galvanic couple.  Whether or not the base material will pit is a different question.

Typically, a crevice will give a regular charge/discharge/charge/discharge response.  However, it depends on the depth of the crevice how fast the charging and discharging process will take.  If you truly have just a small inclusion it may be that the crevice is very small, in which case the 'crevice' effect may be dominated by any pitting action and/or galvanic action that is taking place.

I would suggest that you proceed as follows:

1.  First use three identical pieces of your parent material.  Find out if you get pitting attack.  If so, characterize the noise response from your pitting condition, both by analysis of the simultaneous potential and current logging traces and also by evaluation of the 'localization' response from the noise signals.  If you don't get pitting you will get one type of response (typically, general/uniform corrosion with little or no indication of localized attack).  If you do get pitting you should be able to observe the pits using a microscope if the time traces exhibit the characteristic 'film rupture and repair' response of a material undergoing pitting attack.

2.  Once you know what pitting looks like, check what the difference in response is with your inclusion/crevice.  I would expect a distinctly different, and relatively rapid charge/discharge/charge/discharge response (the traces typically look U-shaped with a relatively short duration, rather than the much longer/slower film rupture and repair response of pit initiation).  Now you know what the crevice looks like.

3.  Any 'galvanic coupling' effect will simply serve to enhance the rate at which any other process occurs.  That means that the calculated corrosion rate from the noise responses should be higher if there is a differential corrosion potential/galvanic couple, but it will only show up as 'uniform corrosion' if uniform corrosion is the dominant corrosion morphology.  If the dominant morphology is pitting then the pitting rate will be enhanced, and if the dominant morphology is crevice attack, then the crevice attack rate will be enhanced, but you won't be able to diagnose galvanic coupling specifically from the noise responses - you'll just find that the overall corrosion rate of the predominant attack morphology is higher when galvanic coupling is occurring, compared to when it is not.  

The reason that your problem begins with the artificial inclusion is because the response from an artificial anything is never representative of the real condition.  So, you can carefully characterize your artificial sample but that won't necessarily mean that it has any relevance to the true situation that you are attempting to model.

First, I would like to thank all of you.

I really got a lot of experience from this valuable discussion.

This point seems to be important and takes a real concern from researchers.

I think every case, metal(s) and solution(s),  should have their own noise data library.

I mean, as W. Cox suggested, I should have the database of well-known cases of pitting, crevice, and galvanic corrosion, then match with the real data.

Also, we come to a conclusion, as Fernandes said, distinct noise signatures cannot be generalized. In other words, there is a real gap between theory and application.

Actually, I have got some data which seem to show (after removing the trend) charging/ discharging behavior (seems to be crevice).The interesting thing is that I have a corrosion (image sample after 4 days of immersion, the inner inclusion is Ti nitride and the matrix is pure iron) and it grows only on the surface in the opposite direction of gravity (I repeat this 4 times).

Finally, I think I have a lot of work.

1- I will start with cyclic polarization (as Habib suggested).

2- try to have a database for pitting, crevice and galvanic corrosions using noise. 

3- SEM to check where does the corrosion begin.

Please, I welcome your suggestion and comments.

Many thanks.

 , consequently, I have to try some of these devices.

These samples look like electrodes embedded in resin of some description.  You will always have a crevice between the meal and the resin in this case, and most of the noise response you will see will be from that crevice.

Why exactly do you need to embed the sample in resin? You would be much better off (I think) using a bare sample of material.  If you need to control the exposed area then you need to mask of the rest of it with something that will prevent crevicing (such as bees wax, for example).

It would also appear that you are not using three identical electrodes for your noise measurements.  If you want the technique to work properly that is an imperative.  You get noise if you monitor from a sample electrode and a reference electrode, but the noise cannot be used to give you a corrosion rate.

Also, if you wish to obtain a quantitative indication of the rate of corrosion rate you must measure the potential noise and the current noise simultaneously in order to be able to use Ohm's Law.

One last point:  It was discovered (by Hladky) that if you expose a very small surface area (like a pin head) and monitor the noise response from what should be uniform corrosion, the signal response is exactly the same as you would get from a pitting system.  What this tells you is that the electrochemical response in each case is precisely the same - what makes the difference in the noise response is the anode/cathode ratio for any particular corrosion metal/environment combination.  On that basis, you cannot build up a 'generalized library' of typical noise responses that work for all situations.  You need to characterize the behaviour of the system you are working with, especially if the material or environment is unusual.  However, once yo know what the signal response is in specific circumstances the noise response is extremely reliable and identifiable.

I am less certain that cyclic polarization will provide the information you need.  The problem is that as you sweep the polarity up and down the degree of response from the metal environment combination changes with change in the potential value.  So, you get some kind of qualitative response and the CP plot changes, but precisely what the change in the plot is telling you is anybody's guess.

It is only fair to say, however, that the degree of reproducibility of noise responses is universally lousy at the moment because there is huge variability in the quality of instrumentation that is available to apply the technique.  The 'round robin' tests undertaken originally by Hickling (EPRI) and more recently by Huet (France) have highlighted that the most confusing aspect of the technology is the sensitivity of the equipment being used, and that comes down to the quality of the ZRA in the measuring circuit and the degree to which the noise response is 'massaged' (or 'smoothed', whether deliberately or inadvertently.

Many thanks, William,

you said here I should have "three identical electrodes for your noise measurements" you mean even the reference should be the same as both W and C electrodes.

It is a goog point to replace epoxy by beeswax. this may be the next step.

Yes. 3 identical electrodes. Alternatively, if you have an artificial crevice or inclusion in one of them, use that as the common electrode and then measure potential nose between 1 and 2 and current noise between 2 and 3. That way you'll know that all/most of the pitting / crevice noise is coming from the common electrode. 

For quantitative measurements of corrosion rate, the noise responses should be measured in the same sense so you can't measure the potential noise against a reference electrode. You've gothe to use apples with apples. If you use apples and pears you end up with pineapples. 

Dear Mohamed

Refer to the following articles:

1-K.Habib,"Modified Electrochemical Emission Spectroscopy of Detection of Crevice Corrosion of Metallic Alloys”, Nondestructive testing and evaluation,(2002), Vol.18(3-4),pp.109-117.

2- K.Habib,”Modified Electrochemical Emission Spectroscopy (MEES) as NDT Technique for Detection of Pitting Corrosion of Mettalic Alloys in Aqueous solutions”, Nondestruutive Testing and Evaluation,, Vol.19, No.4,PP.143-151 (2003).

both articles deal with a combination of cyclic polarization , Electrochemical Noise (EN) technique , and holographic interferometry(Optical technique)for detecting localized corrosion, .i,e pitting & crevice corrosion.

Also, the second article is herewith attached.