What Causes Rust on Iron
Electrochemical corrosion is the most important mechanism that causes Rust on Iron.
Four conditions must exist before electrochemical corrosion can proceed and Iron rust:
(1) there must be something that rusts , the metal anode, where the oxydation reaction take places
(2) there must be a cathode, where the reduction reaction take places
(3) there must be continuous conductive liquid path (electrolyte, usually condensate and salt or other contaminations), for example: water, seawater, condensing water, humidity..
(4) there must be a conductor to carry the flow of electrons from the anode to the cathode.
This conductor is usually in the form of metal-to-metal contact such as in bolted or riveted joints.
The elimination of any one of the four conditions will remove the conditions that causes rust on metal.
An unbroken (perfect) coating on the surface of the metal will prevent the electrolyte from connecting the cathode and anode so the current cannot flow and rust on metal can't form.
Therefore, no rust will occur on metal as long as the coating is unbroken.