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What Causes Rust on Metals

 
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What Causes Rust on Metals

Electrochemical corrosion is the most important mechanism that causes  Rust on Metals.

Four conditions must exist before electrochemical corrosion can proceed and metal rust:

(1) there must be something that rusts , the metal anode, where the oxidation reaction takes place

(2) there must be a cathode, where the reduction reaction takes place

(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.

see also:

what is rust?

what is corrosion?

corrosion types

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