Iron Corrosion

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

There are always to distinct chemical reactions in iron corrosion. In the most common scenario, i.e. the iron corrosion in contact with aerated water:

1) Anodic Dissolution of Iron that goes into solution, water (corrosion of iron)

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 of iron corrosion 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 and the corrosion of iron is generalized over the entire surface, uniform attack on the metal surface.

The rate of corrosion of iron, defined as corrosion rate, is a function of different factors like temperature, fluid velocity, pH and content of oxygen.

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

The image below shows the corrosion of a metal, iron, in contact with aerated water.