Aluminium sheets do not corrode easily as aluminium when exposed to atmosphere, aluminium oxide is formed at the surface of the metal. Aluminium oxide is not porous and hence inhibit any further reaction of the metal with the surrounding.

Copper on the other hand react with the carbonic acid present in the atmosphere to form copper carbonate and hence it imparts greenish color to the copper vessels. During the rains carbonic acid get easily dissolved in the rain water and hence react with copper vessel to give a green coating on the metal.