I have 3 coatings and my interest point is to investigate the weldment corrosion behavior under these coatings. I'm gonna use Potentiostat EIS with scribed coated samples since I'm focusing on weldment corrosion.
does that work for my purpose? because even if I have scribed samples, coating still works and that will give me which coating type will be better.
ONCE the scribed samples work, the exposed surface area I should log in Gamry is just scribed area or total exposed area for electrolyte.
Thanks a lot
If your intention is to study the degradation of the coating on the weld, then EIS can be very informative since it is sensitive to very small changes over time. However, if your intention is to study the corrosion of the weld beneath the coating, then you would be better off using the ZRA function of your gamry unit after cutting the sample, as described by ND Nam.
Thanks a lot Stephen N Smith,
I'm gonna study weldment corrosion in coated samples. that means I should investigate the influence of presence the coating and what difference among all these coating on weldment corrosion. for that I'm gonna scribe all sample because the coating degradation isn't my main purpose, you know even if the sample scribed but coating still work in certain percentage to minimize the effect of corrosion in welds joints.
what do you think about that?
Performing an EIS on the scribed weldment is going to provide you with a result that cold be quite complicated. Depending upon the weld procedure used an d the chemistry of the base metal, you probably would end up with a result that shows indications of galvanic corrosion between different parts of the weld *cap, HAZ, ...) and the base metal. You will also have indications of initial coating failure.
If your real interest is in the weld zone galvanic corrosion, you should probably think about removing the coating and using your gamry unit to investigate the galvanic corrosion by coating everything on the piece of metal being used for the test. That way you get a better measure of the galvanic corrosion reaction without the complication of induction loops.
On the other hand, if your interest is to get an idea of the possibility of coating disbondment in real life. It would be best if you could perform EIS curves on both tests on both the coated and uncoated specimens. The coating selection will depend upon a number of factors. These EIS curves should be taken at a variety of times from about 1 to 24 hours or even a week or two. By comparing plots, you should be able to see some changes that you can get an idea of how the coating begin to fail (along witth the bounds of your test conditions.
Thanks a lot , I'll take your second recommendation, but which surface area should I use, scribed or all exposed area for electrolyte?
The surface area used for the EIS evaluation is the area of metal that is exposed during the test. Initially this is the area exposed by the scribing. However, the exposed area can become a variable during the test is the coating begins to disbond from the metal. Disbondment can expose metal beneath the coating, which will cause the surface area to increase.
Thanks and I appreciate your help
Alright! So if we log in the scribed area value initially and the area will be increased during the test. that will lead us for incorrect data. how could we control that?
I'm not sure I see some people enter 1 cm^2 , what about that?
The surface area is not going to change to any significant degree during your EIS run. That occurs when you expose the sample for a period of days. Even then, having the wrong surface area is only going to create an error in the current density values. The current is still going to remain the same. If you see coating disbondment after an extended exposure, you can always go back after the test when you know the degree of disbondment and adjust the surface area to correct your data.
As for entering a default value like 1cm^2, that will satisfy the computer but will cause some of the output parameters to be incorrect. If you do not care about any of the parameters that depend upon surface area, then it is not going to cause a problem.
Sure, you can test the corrosion behavior of coated specimens after mounting the samples to get surface area of welded and coated regions and then investigate the polarization with SEM.
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