Passivity

When iron is immersed in concentrated nitric acid, it is seen to immediately react with the solution, as evident by a burst of evolution of gaseous nitrous oxides, but the reaction stops almost immediately, and henceforth the metal remains bright and uncorroded indefinitely. It is ‘passivated’ with respect to the thermodynamically strongly favored corrosion reaction, thus showing ‘similar-to-noble metal behaviour’. The explanation — already proposed by Faraday — is that the oxidizing power of HNO₃ is sufficient to produce an thin, nonporous oxide film which, while easily soluble, happens to dissolve into the acid at only a negligible rate. The film thickness is below the wavelength of visible light and the film, therefore, transparent.