When a piece of iron is placed in a diluted nitric acid solution, it will vigorously dissolve and emit red fumes, which indicates corrosion of the iron. However, if concentrated nitric acid is slowly added to increase the solution's concentration above 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 on the surface of the previously dissolving active metal (formation of an oxide film or adsorbed film). Due to this abrupt change, the laws governing the anode dissolution process undergo a qualitative transformation, and the dissolution rate of the metal drops sharply. This abrupt change in the state of the metal surface is referred to as passivation.
