When a piece of iron is placed in a diluted nitric acid solution, it will dissolve vigorously and emit red fumes, which indicates the corrosion of iron. However, if concentrated nitric acid is slowly added to increase the solution's concentration beyond 40%, the corrosion rate will suddenly drop to one-four-thousandth of its original rate. This phenomenon is known as passivation.
Under certain conditions, when the potential of a metal shifts positively due to an applied anode current or localized anode current, a certain abrupt change occurs in the state of the originally dissolved active metal surface (formation of oxide film or adsorption film). Due to this abrupt change, the regularity governing the anode dissolution process undergoes a qualitative transformation, and the dissolution rate of the metal drops sharply. This abrupt change in the state of the metal surface is termed passivation.
