When electrons from atoms at the surface of the metal are transferred to a suitable electron acceptor or depolarizer, then the electrochemical corrosion of metals occurs . Water must be present to serve as a medium for the transport of ions.

The most common depolarizers are oxygen, acids, and the cations of less active metals.

Because the electrons flow through the metallic object itself, the anodic and cathodic regions (the two halves of the electrochemical cell) can be at widely separated locations.

Contact with a different kind of metal, either direct or indirect, can lead to corrosion of the more active one.

Anodic regions tend to develop at locations where the metal is stressed or is protected from oxygen.

Corrosion of steel can be inhibited by galvanizing, that is, by coating it with zinc, a more active metal whose dissolution leaves a negative charge on the metal which inhibits the further dissolution of Fe2+.

Cathodic protection using an external voltage source is widely used to protect underground structures such as tanks, pipelines and piers. The source can be a sacrificial anode of zinc or aluminum, or a line-operated or photovoltaic power supply.

Concept Map of electrochemical corrosion