What causes steel to rust?
Electrochemical corrosion is the most important mechanism that causes Rust on Steel.
Four conditions must exist before electrochemical corrosion can proceed:
(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 steel.