Iron Corrosion

When iron is immersed in water, or when humidity from the air condenses on an iron surface, the oxygen dissolved reacts with iron causing the formation of iron oxides.

There are always two distinct chemical reactions in a corrosion process:

1) Anodic Dissolution of 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

Fe(OH)₂  will then reacts with oxygen to give iron oxides: Fe₂O₃ (red) and Fe₃O₄ (black)

The final product (when dry) has the reddish-brown flaky character we associate with rust.

On a metal surface in contact with water an infinite number of cathodic and anodic sites are established
uniformly distributed over the metallic surface.

The result is a uniform attack on the metal surface.

The rate of attack, corrosion rate, is a function of different factors like temperature, fluid velocity, pH.

Low pH, acid solution, are more corrosive than neutral solution.