CORROSION AND CORROSION -RESISTANT METALS

 . Stat what is corrosion 

. State the causes of corrosion 

. Name the metals resistant to corrosion 

. State the properties of corrosion -       resistant metals



Corrosion is the slow and continuous eating away of metalic components by chemical or electrochemical action.Corrosion affect the service conditions and accuracy of the components. It is very essential to understand the causes for corrosion and to know the metals that resist corrosion.

Atmospheric corrosion 

Any metal exposed to normal atmospheric conditions becomes covered with a thin film of moisture. The metals under cover chemically reacts with the oxygen in the atmosphere and forms and oxide on the surface of the metal.

The most common example of corrosion due to the moisture is the rapid surface formation of 'RED REST' resulting from the exposure of iron and steel to the atmosphere.

METALS THAT RESIST CORROSION 

Metals like copper, zinc, aluminium,lead, stainless steel, chromium etc. are corrosion resistant.

Copper 



When copper is exposed to atmospheric pollution for a long Time, the surface develops a green coating or 'PATINA'

This coating caused by the action of the acids in the atmosphere protects the surface of the copper.

Zinc



A carbonate coating forms on the surface after a period of exposure, and this acts as a protective film that gradually strengths with time. This coating is grey in colour like the colour of the parent metal itself .

This coating does not crack or peel off due to variation in temparature. For this reason zinc is an excellent exterior building material. It gives excellent protection when coated on steel.

Aluminium 



Aluminium and its alloys have a great affinity for oxygen. Aluminium surfaces quickly develop a thin, transparent Film of aluminium oxide or 'ALUMINA' which prevents further oxidation and retains bright appearance. However exterior use of aluminium result in the thickening of the oxide film. This film becomes grey in colour and protects the parent metal from further attack. The oxide film on aluminium and its alloys can be artificially thickened by a process called anodising.

Lead 



Lead is one of the most corrosion resistant of all metals. A large quantity of lead is used as sheathing materials for underground telephones and power cables. The WHITE OXIDE film resulting from exposure to the atmosphere prevents further attack 

Stainless steel 




It has high structural strength as well as resistance to corrosion. Stainless steel are not confined to application requiring resistance to atmosphere corrosion. They are used extensively for chemical plant and food processing equipment where they combine corrosion resistance at elevated temperatures.

Nickel 



Nickel is used extensively for NICKEL PLATING as it has high resistance to chemical attack. When allowed with copper in the proportion of 2:1 ( nickel two third) 'MONEY METAL ' is produced which is extremely resistant to corrosion, particularly to sea water and acid.

Chromium 



One of its most important uses is for electroplating metalic surface. It is high resistance to the influence of corrosion and it remains its high polish and colour for a long period 



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