The Faraday paradox was a once inexplicable aspect of the reaction between nitric acid and steel.
When he placed the iron in the concentrated nitric acid, he expected that it would dissolve at a higher rate, but no attack was observed.
However, the oxide film did not dissolve and the attack did not continue in the concentrated nitric acid.
When the acid is concentrated enough, and because concentrated nitric acid is an oxidizing agent, the electrochemical 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.
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.