On Air

Investment

Buy this Domain?
Do you interesting about this domain and the running project?
Feel free to send your offer to webmaster.
pay with Paypal

Advertising

Faraday paradox (electrochemistry)

The Faraday paradox was a once inexplicable aspect of the reaction between nitric acid and steel. Around 1830, the English scientist Michael Faraday found that diluted nitric acid would attack steel, but concentrated nitric acid would not. The attempt to explain this discovery led to advances in electrochemistry.

Passivation

The key to resolving the paradox is passivation. When the acid is concentrated enough, and because concentrated nitric acid is an oxidizing agent, the potential of the metal is raised to the point that a layer of metastable Fe3O4 forms on the surface and protects it from further corrosion, even though the pH is so low that stable Fe3O4 cannot exist. This explanation is supported by the observation that scratching the surface causes a burst of bubbles. Diluted nitric acid is not as strong an oxidizing agent and hence does not raise the potential of the metal to the extent that metastable Fe3O4 forms on the surface. In this case, the metal freely corrodes.

Further reading, Sources

  • Digby D. Macdonald, Passivity - the key to our metals-based civilization, IUPAC 1999 (PDF here)
Strong, Oxidizing acids

References

"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/Faraday_paradox_(electrochemistry), the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Faraday_paradox_(electrochemistry)&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany