General theory of passivity

Abstract

Iron corrodes vigorously in dilute nitric acid, but its corrosion virtually stops in concentrated nitric acid. It was as early as 1790 that iron was found to become chemically inert or passive when immersed in concentrated nitric acid. Faraday noted in 1836 "the surface is oxidized to form a superficial film of oxide so thin as not to be sensible but enough to prevent corrosion of iron". An alternative explanation was that the iron is transformed into an inert allotropic modification when it becomes passive and hence behaves as a noble metal.