the free surface of a metal sheet gets covered with a butter brown coloured layer .when this metal sheet is coated with another metal it doesnot have any such layer why ? name the brown coloured metal substance? mention the phenominon responsible for the brown coloued layer? identify the metal which when coated on the metal sheet does not permit the formation brown coloured layer on it give the reason fo the action?

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Solution

Brown colour layer is called rust and the metal is iron. Iron is more reactive and undergoes immediate reaction with oxygen forming hydrated iron oxide called rust when exposed to air.
4Fe + 3O₂+ moisture ------> 2Fe₂O₃ . x H₂O

When this metal is coated with zinc or magnesium which is more reactive than iron they form the oxide film which acts as a protective layer.

2Zn + O₂ ----> 2 ZnO

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