5. Corrosion

Corrosion

Alloys

Alloys are homogeneous mixtures of metal with other metals or nonmetals. Alloy formation enhances the desirable properties of the material, such as hardness, tensile strength and resistance to corrosion.

Examples of a few alloys:

Brass: copper and zinc

Bronze: copper and tin

Solder: lead and tin

Amalgam: mercury and other metal

Corrosion

Gradual deterioration of material usually a metal by the action of moisture, air or chemicals in the surrounding environment.

Rusting:

4Fe(s)+3O2(from air)+xH2O(moisture)→2Fe2O3. xH2O(rust)

Corrosion of copper:

Cu(s)+H2O(moisture)+CO2(from air)→CuCO3.Cu(OH)2(green)

Corrosion of silver:

Ag(s)+H2S(from air)→Ag2S(black)+H2(g)

Prevention of Corrosion

Prevention:

1. Coating with paints or oil or grease: Application of paint or oil or grease on metal surfaces keep out air and moisture.

2. Alloying: Alloyed metal is more resistant to corrosion. Example: stainless steel.

3. Galvanization: This is a process of coating molten zinc on iron articles. Zinc forms a protective layer and prevents corrosion.

4. Electroplating: It is a method of coating one metal with another by the use of electric current. This method not only lends protection but also enhances the metallic appearance.

CORROSION :-

The surface of metals is corroded when they are exposed to moist air for a long period of time. This is called corrosion

For example 

• Silver becomes black when exposed to air and form a coating of silver sulphide.
• Copper react with  moist Co_{2 and form a green coat of copper carbonate.}
• Iron acquires a coating of brown floky substance called rust.

 Prevention of corrosion. 

The rusting of iron can be prevented by painting , oiling, greasing, galvanizing, chrome plating anodizing and making alloys.

• Galvanisation:-  It is a method of protecting steel and iron from ressting by coating them a thin layer of zinc.
• Alloying is a very good method of improving the properties of a metal an alloy is a homogenous mixture of two or more metals or a metal and a non- metal
• Iron  →    it is mixed with small amount of carbon
• Steel →   iron + nickel and chromium.
• Brass  → copper  + tin
• Solder   →  lead  +  tin ( used for welding electric wire together )