Natural Environmental Process: "Rusting"

A layer of reddish cover the surface of the steel or iron-Rust.


Rusting, is oxidation of the metal to form a metal oxide.

Conditions for rusting:
Present of water and oxygen.

The corrosion of iron can be explained with theory of voltaic cell.
Process of Rusting:

First
Dents on the surface of iron is no longer protected and contact with water are the typically sites of iron loss. These sites is known as anodic regions and Fe undergoes oxidation donor 2 electron to form Fe2+ ion.

On the regions of high concentration of oxygen where near the water surface;Oxygen react with H ions and electron from Fe to form water. This region is so called cathodic region because oxygen is undergoes reduction.
Then the overall red ox reaction is completed but the rust still haven't form.

Next, the Fe2+ions form from the anodic region disperse through the surrounding of water and react with oxygen.

Finally
The rust is formed at some distance from the pit.

The following diagram show the process of rusting:

Situations that make the rate of rusting increases:

~At low pH value(high concentration of H ion)
~In contact with ionic solution.
~In contact with a less active metal (like Copper)
~The presence of corrosive agents, such as sea water, salt air, acids, acid precipitation, soils, and some sulfur compounds present in the atmosphere, which act as catalysts in the oxidation process.


Corrosion of iron and steel due to rusting is responsible for millions of pounds of damage each year. Rust does not firmly adhere to the surface of the metal allowing it oxide further. The oxide causes damage to the surface of the metal known as pitting which, over time, reduces these structural integrity of the metal.

Methods to protect steel or iron from rusting:-

Barrier methods: introduce a layer that prevent steel in contact with moiture and air .
Ex: a layer of grease, paint, plastic, or an unreactive metal, such as tin, copper, or chromium.
Sacrificial protection is the iron covered by more active metal (stronger reducing agent) such as zinc and magnesium which loss electron more ready than Fe.



Case:Eiffel Tower Maintance


The Eiffel is built by the cast iron, a material will last longer if it is painted regularly.


Maintenance on the tower includes applying 50 metric tons of three graded tones of paint every 7 years to protect the 200,000 square meters of iron lattice work from rust. The darkest paint is used at the bottom and the lightest shade at the top.Each repainting took time for 15 months with 25 painter and used up 1500 brushes. On occasion, the color of the paint is changed. The tower is currently painted to a shade of brown.


The 17th repainted- the application of an anticorrosion treatment lasts one year, so that the tower can stay open and continue to greet visitors.