In some literature, it is mentioned that the closer the corrosion potential is to the negative direction, the lower the tendency for corrosion. However, in other literature, it is stated that the closer the corrosion potential is to the positive direction, the lower the tendency for corrosion. How can this be explained?
Dear Dr. Niu Ziyang ,
the more negative the standard electrode potential, the greater the tendency to form metal ions and hence to corrode. Corrosion potential indicates the tendency of metals to corrode, but it does not provide the rate of corrosion. Rate of corrosion is proportional to the rate of electron transferred between electrode and electrolyte. Rate of electron transfer is represented as current (I).
High corrosion potentials are good. The system will rather take up electrons than lose electrons, so a reduction is more likely. If the corrosion potential is dropping, it means that the sample is oxidizing and negative charges are accumulating in it.
For more details, please see the source:
-THE ELECTROCHEMISTRY OF CORROSION Edited by Gareth Hinds from the original work of J G N Thomas
Available at: http://resource.npl.co.uk/docs/science_technology/materials/life_management_of_materials/publications/online_guides/pdf/the_electrochemistry_of_corrosion.pdf
and
Corrosion Potential by Palmsens
Available at:
https://www.palmsens.com/knowledgebase-article/corrosion-potential/#:~:text=High%20corrosion%20potentials%20are%20good,charges%20are%20accumulating%20in%20it.
My best regards, Pierluigi Traverso.
metal corrosion enhances at an environment of greater positive oxidation potential, or greater negative reduction potential. The question is the potential you have meant- is it oxidation potential or reduction potential?
Let us see, Suppose connecting a Sodium-sodium ion (hypothetical) electrode with SHE. There would be oxidation on Sodium-sodium ion electrode sponteniously, that would make the electrode negative. Since oxidation occurs simultaneously, w.r.t. SHE, sodium-elctrode's oxidiation potential is positive, reduction potential negative
Now, if the equal amount of negative potential is applied on sodium electrode w.r.t. SHE, the reaction stops, and the electrode is at equilibrium. So, when we report equilibrium potential of Na-electrode w.r.t. SHE, we are reporting reduction potential.
But for a strong oxidizer-rich environment, the oxidizer is hungry to oxidize others and get itself reduced, i.e. hungry for electrons. A SHE connected to the environment would begin oxidation reaction (and electrode immersed into that environment reduction reaction), and electrode immersed in that oxidizing environ would report a large positive potential w.r.t. SHE. To drag electrons away from the environment, the electrode immersed in that environment needs to be strongly positive (w.r.t.) SHE. So, the oxidizer-rich environment has a large positive potential, in which metals would corrode fast (unless we end up passivation). The highly oxidizing environment has a large positive reduction potential, because, if an electrode immersed in that environment is connected with SHE, reduction would strongly occur in that environmental electrode sponteniously
Yup, these linguistics are pretty confusing- and oftentimes I also get confused. Hope you can find your answer, as well as source of your error.