Corrosion of a metal is oxidation: the metal reacts with oxygen to form a metal oxide, so oxidation is the gain of oxygen. How easily a metal oxidises depends on its position in the reactivity series. The higher a metal is in the series, the more reactive it is, the more readily it reacts with oxygen and the less resistant it is to oxidation. The lower a metal is, the more resistant it is.
Potassium and sodium tarnish within seconds in air, and magnesium burns brightly to form magnesium oxide. Iron is lower in the series and rusts slowly when exposed to oxygen and water. Copper is lower still and corrodes much more slowly, forming a green layer over many years. Gold is at the bottom of the series, so it does not oxidise in air and stays shiny, which is why it is used for jewellery and electrical contacts.
Aluminium is an exception that is often tested. It is high in the series, but it reacts with oxygen at once to form a thin layer of aluminium oxide that sticks firmly to the surface and stops the metal beneath from reacting further.