Why surface of iron or copper gets tarnished when it is exposed in moist air?

The surface of iron or copper tarnishes when exposed to moist air due to the formation of oxides or other compounds through chemical reactions with moisture and oxygen. In iron, this leads to rusting, primarily iron oxide, while copper develops a green patina (copper carbonate) due to the reaction with carbon dioxide and moisture. These reactions are accelerated in the presence of electrolytes such as salts found in the air, leading to more rapid corrosion. Ultimately, this tarnishing alters the metal's appearance and can compromise its structural integrity over time.