Tuesday, June 1, 2010

Corrosion of metallic Materials

Corrosion problem is the chemical, electrochemical or metal physical reaction of a metallic material with its environment, leading to a change in its properties.

Materials are affected above all by the air round them. The air contains oxygen, water vapour, smoke gases with sulphur and phosphorus compounds, combustion products such as carbon dioxide, sulphur dioxide, dilute acids such as carbonic acid, sulphurous acid and hydrochloric acid. Most metals have combined chemically with oxygen, water, sulphur, phosphorous or carbon to form ores. Even after these compounds have been analysed with the expenditure of a great deal of energy in smelting, they strive to return to their original states as ores. This is the nature of most forms of damage to metals (corrosion).

If a bright metal strip – iron (Fe), copper (Cu) or zinc (Zn) is held in the flame of a burner, it loses its surface lustre and it covered by an oxide layer. The oxide coat is often impervious and prevent the oxidation of the metal underneath it.

Copper form copper oxide iron forms iron oxide:

2 Cu + O2   → 2 CuO
2 Fe + O2  → 2 FeO

If iron is heated red-hot in air, hammer scale forms (Fe2O4 or FeO + Fe2O3)

Zinc form zinc oxide:  2 Zn + O2 → 2 ZnO
Lead form lead oxide: 2 Pb + O2 →  2 PbO
Aluminum form aluminium oxide: 4 Al + 3 O2 →  2 Al2O3
Magnesium form magnesium oxide: 2 Mg + O2 → 2 MgO

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