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Rust forms due to the reaction of oxygen dissolved in water with iron. Rust is a generic term to describe different oxides, Fe(OH)2, Fe(OH)3, FeO(OH), Fe2O3.H2O that form when iron corrodes.

The common form of Rust is a Red products:Fe2O3.

The oxygen in the air dissolves in the water and causes rust to form. There are always two distinct chemical reactions in a corrosion process, the basic chemistry of corrosion is:

1) Anodic Dissolution of Metal (Iron) that goes into solution (water)

Fe  ----->   Fe2+  +   2e-

2) Cathodic Reduction of Oxygen dissolved into water

O2  + 2H2O + 4e- ---->  4OH-  

The final reaction is:  Fe2+ + 2OH-  ----->  Fe(OH)2

This oxide will then further reacts with oxygen to give the final red product:Fe2O3.H2O


Corrosion of Metals

The corrosion of metals is an electrochemical process. That is, it is an electrical circuit where the exchange of electrons (electricity) is conducted by chemical reactions in part of the circuit. These chemical reactions occur at the surface of the metal exposed to the electrolyte. Oxidation reactions (corrosion) occur at the surface of the anode and reduction reactions occur at the surface of the cathode.


Corrosion metal definition

There are different corrosion metal definition, but the basic principle is the same: an interaction of the metal with its environments. These are some example of corrosion metal definition:

Corrosion of metals can be defined as the destructive attack of a metal through interaction with its environment  
Corrosion is a process that occurs when a material deteriorates due to its interaction with the surrounding environment in which an electrochemical reaction consumes the material through oxidation.  

What causes corrosion

As seen above a Corrosion process is the results of the interaction of a metals with its environments. Usually what causes corrosion, at least what we can see in our daily life, is what is called "wet" corrosion. "Wet" corrosion means that there is a water phase, humidity, rain, seawater..., in contact with the metal surface. So what causes corrosion is the interaction of the water, or better the oxygen dissolved in the water, through the reactions seen above, with the metal.

see also:

what causes rust?

corrosion types

what is corrosion?

what is rust?

how to stop rust?