Corrosion
Some of the metals in contact with air undergo corrosion, e.g. Green coloured salt is formed on copper and brass vessels when remain in contact with air. Iron rusts because of the reaction of this metal with oxygen of air. Rust is mainly iron oxide ((Fe203 . jcH20 ) which is red in colour. Corrosion is a process to form rust on metal surface when exposed to air, water or moisture. Oxygen of air, carbon dioxide and moisture are responsible for corrosion.Billions of rupees are wasted due to corrosion. Simple method to prevent corrosion is to cover the iron surface with paint, but it can not last long, because when the paint is removed corrosion starts again. Iron should be prevented to come in contact with air so that oxidation reaction will not occur. For this, a thin layer of zinc is applied on the surface of iron. This iron is known as galvanized iron and coating the layer of zinc on iron is known as galvanizing. If a small amount of zinc from galvanized iron is removed, eventhen iron will not be corroded because more active zinc metal will spread and will prevent corrosion of iron.
Methods to prevent corrosion
To prevent the corrosion of iron plates of steamers, blocks of more reactive metals like magnesium or zinc are combined with iron plates and then allowed to keep in contact with sea water. Iron plates act as cathode and magnesium or zinc blocks act as anode because oxidation potential of zinc or magnesium is more than that of iron. These blocks undergo continuous corrosion in sea water and hence required to change periodically.
Corrosion can also be prevented by using suitable chemical substances known as inhibitors. Inhibitors will form chemical bond with metal surface and will prevent corrosion.
To prevent corrosion, layer of enamel is applied on the metal.
Alloys
Metals used in the preparation of machine parts, weapons, household utensils etc. properties like hardness, conductivity, ductility and resistance to corrosion must be increased. No metal in general contain all useful properties. Hence in practice, instead of using a pure metal, an alloy made up of two or more metals is used.
One metal is melted with other metal till homogeneous liquid is obtained in form an of alloy. Thus alloy is a solid solution. Some of the physical properties Of original metals in an alloy are changed. An alloy is a homogeneous solid mixture of metal into metal. An alloy of metal is prepared to get desired property. Brass alloy is obtained by preparing homogeneous mixture of copper and zinc. Hard and corrosion resistant stainless steel alloy is prepared by mixing nickel. Sometimes melting point of the alloy is lower than their constituents e.g. solder which is an alloy of lead and tin. Its melting point is lower than its constituents and hence this alloy is used in soldering of electrical wires. Following table gives constituents, properties and uses of alloys.
Some common alloys, their constituents, properties and uses
Alloying of gold
Pure gold is 24 carat gold and is soft. Hence ornaments deform their shapes when a little pressure is applied. Therefore copper or silver is added to form an alloy. Ornaments containing 22 parts of gold and 2 parts of copper or silver, are 22 carat gold ornaments.