Question

The shining surface of metals may become dull due to formation of

Answer 1

When some metals get exposed to air there is an oxidation process occurring on the outer layer of the metal. Due to the oxidation; a new layer covers the existing metal; hiding its original view; giving it a dull appearance. Almost all metals combine with oxygen to form metal oxides.

Metal + Oxygen → Metal oxide

Answer 2

its due to corrosion....
when some metals are exposed to air, ..due to moisture and oxygen metals corrode ....