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.

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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 of 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.

References

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