A corrosion system can be regarded as a short-circuited electrochemical cell in which the anodic process is something like

• Fe(s) → Fe²⁺(aq) + 2 e^–
  

and the cathodic steps can be any of

• O₂ + 2 H₂O + 4e^– → 4 OH^–
• H⁺ + e^– → ½ H₂(g)
• M²⁺ + 2 e^– → M(s); where M is a metal.

Corrosion often begins at a location (1)[Fig- 1], where the metal is under stress (at a bend or weld) or is isolated from the air, where two pieces of metal are joined or under a loosely-adhering paint film. The metal ions dissolve in the moisture film and the electrons migrate to another location (2), where they are taken up by a depolarizer. Oxygen (O2) is the most common depolarizer; the resulting hydroxide ions react with the Fe2+ to form the misture of hydrous iron oxides known as rust.

Which parts of the metal serve as anodes and cathodes can depend on many factors, as can be seen from the irregular corrosion patterns that are commonly observed. Atoms in regions that have undergone stress, as might be produced by forming or machining, often tend to have higher free energies, and thus tend to become anodic. If one part of a metallic object is protected from the atmosphere so that there is insufficient O2 to build or maintain the oxide film, this protected region will often be the site at which corrosion is most active. The fact that such sites are usually hidden from view accounts for much of the difficulty in detecting and controlling corrosion.

Pitting corrosion
Most metals are covered with a thin oxide film which inhibits anodic dissolution. When corrosion does occur, it sometimes hollows out a narrow hole or pit in the metal. The bottoms of these pits tend to be deprived of oxygen, thus promoting further growth of the pit into the metal.(Fig-2)

In contrast to anodic sites, which tend to be localized to specific regions of the surface, the cathodic part of the process can occur almost anywhere. Because metallic oxides are usually semiconductors, most oxide coatings do not inhibit the flow of electrons to the surface, so almost any region that is exposed to O2 or to some other electron acceptor can act as a cathode. The tendency of oxygen-deprived locations to become anodic is the cause of many commonly-observed patterns of corrosion.