Hi, corrosion current for copper in water seem to be ok in a range of some microAmps/cm2. But for steel it seems not to be correct n that range, its typical at least ten times higher. Is there any corroison inhibitor involved in your soap/water? Or did you maybe made a mistake with electrode area? Corrosion potential for the steel is also quite positive with regard to copper, maybe the steel is indeed kind of anodically inhibited.

Best regards, Andreas

p.s. If steel and copper are coupled, the steel will be anodically polarized and then it will corrode much stronger

Hello,

Thank you very much Andreas Heyn for your reply. I don't know the composition of my soap, it is a commercial product and no indication of the composition. I think that the potential of the steels are correct but as you told me the corrosion current seems too low. But if copper and steel are coupled, there is no risk of important corrosion as i(corr) for the steel is smaller than i(corr) for the copper, even if E(corr) for copper is higher than E(corr) for steel. But I will do again my experiments with steels to ensure the results I have. Do you know if others experiments could be useful to determine if a galvanic coupling is possible between my 2 materials?

Thanks a lot,

Best Regards

Tom

Hi Tom,

Your statement:

But if copper and steel are coupled, there is no risk of important corrosion as i(corr) for the steel is smaller than i(corr) for the copper, even if E(corr) for copper is higher than E(corr) for steel.

Thats not correct. If you polarize the steel in anodic direction (electric coupling with the more positive copper electrode), the anodic dissolution of the steel will increase. If the steel is passive or anodically inhibitited, then there is the danger of localized corrosion, because every little pore or failure in the protective layer will lead to a very small anode and you get huge currents, driven by the bigger copper cathode.

And, by the way, surface area ratio of anode/cathode is also very important.

You can perform current measurements (ZRA) between copper and steel (varying the surface area ratio) and check how current(coupling) and potential are influenced.

Best regards,

Andreas

Hello,

Thank you Andreas Heyn for your comment. Ok, I think I understand what you mean. So with my experiements I can't say if my galvanic coupling will give a rapid corrosion of the steel or a very small corrosion. What I can say is that my steel can be corroded if it is in contact with copper only by the fact that E(corr) for copper is about 100 mV higher than E(corr) for steel. I can't say anything about my TAFEL plots concerning the galvanic coupling. So it was not ncessary to do these experiments in my case :(

Am I right? Only a ZRA could give me more information... I'm going to try that :)

Best Regards

Tom

Hi Thomas,

you can still read some useful information from your curves, provided the curve for steel is correct (I think it's too low). If you assume that steel and copper are short-circuited (and you neglect the resistance of the electrolyte), the intersection of the anodic curve of the steel and the cathodic curve of the copper is the current density and corrosion potential of the system when it is short-circuited.

In real use, however, the area ratios of the different electrodes must always be taken into account. ZRA measurements can substantiate the results and there the short-circuit currents are measured (but not the self-corrosion at the electrodes).

Best regards, Andreas

Hello,

Thank you Andreas Heyn . I repeated my measurements and I confirm : the curves for steel are correct. they seem too low, but maybe as you told me steel could be passivated in my electrolyte. I am goind to try to combine my i-E curves (for steel and copper) to obtain E(corr) and i(corr) for my system. I will do also ZRA measurements to complete my measurements. So one more time, thank you very much. With your help, I will be able to finish my study :).

Best Regards

Thomas