Corrosion

[karlapfeiffer]

Hello,

I would appreciate very much if someone could answer my question. I'm not sure if I posted it in a right category but I hope it will get in "good hands".

So here it is:
By measuring the open circuit potential (OCP) of two different types of metals in the same solution, conditions etc., does the lower OCP indicate the lower resistance to corrosion? How can I interprate this "result"?

Thank you for your answer in advance.

Regards
Karla

[barretttomlinson]

Hi,

The OCP, or Open Circuit Potential, is a measure of the driving force of the reactions going on at the two electrodes being measured. The larger the magnitude or size of the voltage the more strongly rhe reactions want to occur. Translated to your question this means that an OCP of 0 means that no reaction or corrosion wants to occur., whereas an OCP of 0.5 volt or more suggests that corrosion will be quite rapid on at least one of the electrodes. Here is more information:

http://alpha.chem.umb.edu/chemistry/ch313/lecture10.pdf

http://bouman.chem.georgetown.edu/S02/lect25/lect25.htm

http://www.gamry.com/App_Notes/DC_Corro ... ements.htm

http://www.engineeringtoolbox.com/elect ... d_482.html

You may want to check out these Science Buddies project ideas on corrosion:

http://www.sciencebuddies.org/science-f ... p018.shtml

http://www.sciencebuddies.org/science-f ... p079.shtml

http://www.sciencebuddies.org/science-f ... p090.shtml

http://www.sciencebuddies.org/science-f ... p010.shtml

I hope this answers your question. Have fun!\

Best regards,

Barrett L Tomlinson

[karlapfeiffer]

Thanks for this expeditious reply. It is a bit more clearer now. But I still have one more question. Now related to Tafel extrapolation method (TEM).

According to the literature OCP is the same as corrosion potential (Ecorr), is that right? Well, why then, when I record the polarization curves (cathodic and anodic) with TEM, tangents on the curves cross each other at the Ecorr which is then higher then the previously recorded OCP? I really have some issues figuring these 2 terms and what do they mean. Please help me...

Best
Karla

[barretttomlinson]

Hi,

Here are resources on the Tafel Extrapolation Method:

http://www.che.sc.edu/faculty/popov/drb ... ements.pdf

http://www.gamry.com/App_Notes/DC_Corro ... ements.htm

These sutes duscus both the theory and practice of the Tafel Extrapoation method amd I hope they will answer your question. Yiu need to be sure your measurements are done after voltages have stabilized, and that tangents are measured over curve regions that are linear over a tenfold current change to get valid results.

Best regards,

Barrett L Tomlinson