What is Rust?

Rust is a generic term to describe a series of 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.

Rust forms due to a reaction between the iron and water; either water condensing from air or rain.

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 in 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