The metal surface is converted to a state difficult to be oxidized, and the corrosion rate of a metal delay method. A reactive metal or alloy, wherein the chemical activity is greatly reduced, and the phenomenon of the noble metal becomes a state called passivation. Metal is passivated to protect metals against corrosion, but sometimes in order to ensure the normal reaction of metal dissolution can participate, and the need to prevent passivation, such as electroplating and chemical power supplies. 

When the metal dissolved in a passive condition, the generated surface of the dense, solid material having good coverage, which is formed from a separate phase, known as a passivation film or said phase film, the metal surface of the film and mechanically isolated from the solution, the dissolution rate of the metal is greatly reduced, and showed a passive state. Experimental evidence is that on some metal surface passivation can be seen with respect to the presence of the film, and to measure its thickness and composition. Such as the use of a metal capable of dissolving the oxide film does not work with the reagents dissolve and remove carefully the metal film under the passivation film can be separated can be seen. 

When the anode metal dissolved in the solution surrounding the vicinity of layer component changes. On the one hand, dissolved metal ions due to diffusion down fast enough (dissolution speed) and has accumulated. On the other hand, the interface layer is also hydrogen ions migrate to the cathode, the anion solution (including OH-) migrate to the anode. As a result, there OH- ions and other ions concentration near the anode. With the continuation of the electrolysis reaction, the solution at the interface adjacent to the anode layer, the electrolyte concentration may be developed into a saturated or supersaturated state. Thus, the solubility product of a metal hydroxide or a small deposition of salts is necessary to form a layer on the metal surface and insoluble film, which film is often very loose, it is not sufficient to lead to passivation of the metal directly, but only hinder dissolution of the metal, but it covers the electrode surface contact area between the solution and the metal is greatly reduced. Thus, it is necessary to increase the current density of the electrode, the potential of the electrode will become corrected. This makes it possible to cause a discharge in the electrode OH- ions, and its product (e.g. OH) and metal atoms on the electrode surface reaction to form the passivation film. Analysis showed that most of the passivation film (e.g. iron Fe3O4) made of a metal oxide, but a few have the hydroxides, chromates, phosphates, silicates, sulfates, and chlorides and insoluble components.