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What causes Corrosion?

 
     
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                                                What Causes Corrosion
Electrochemical corrosion is the most important classification of corrosion.
Four conditions must exist before electrochemical corrosion can proceed:

(1) there must be something that corrodes , the metal anode, where the oxydation 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 corrosion.
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.

Therefore, no corrosion will occur as long as the coating is unbroken.