Faraday paradox (electrochemistry)
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The '''Faraday paradox''' was a once inexplicable aspect of the reaction between [[nitric acid]] and [[steel]]. Around [[1830]], the [[England|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]].
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 [[Iron(II,III) oxide|Fe<sub>3</sub>O<sub>4</sub>]] forms on the surface and protects it from further corrosion, even though the [[pH]] is so low that stable Fe<sub>3</sub>O<sub>4</sub> 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 of an oxidizing agent and hence does not raise the potential of the metal to the extent that metastable Fe<sub>3</sub>O<sub>4</sub> forms on the surface. In this case, the metal freely corrodes.
==References==
*Digby D. Macdonald, ''Passivity - the key to our metals-based civilization'', [[IUPAC]] 1999 [http://www.iupac.org/publications/pac/1999/71_06_pdf/macdonald.pdf (PDF here)]
[[Category:Electrochemistry]]
[[Category:Paradoxes]]