Rust is a process of metal or metal alloys combining with oxygen atoms to produce an oxide. Rust is also seen as tarnish on some metals. All metals will rust but at differing rates. Even gold, platinum and silver will rust but the process is very slow and produces various colors of tarnish. Gold and platinum oxides are very rare because not only will it take many years for these metals to rust, but the amount will be very insignificant. Although all metals rust, there are processes and alloys (metal compounds) that will resist rust and slow the oxidation process.
Iron will rust very quickly. If iron is allowed to get wet and exposed to air noticeable brown rust can develop in only a few hours. Iron Oxide is the natural state for the element and removing the oxygen to produce pure iron is part of the smelting process.
Iron will also rust quickly if exposed to intense heat. The heating process alters the Iron's chemical makeup and makes it very susceptible to recombining with oxygen in the air. Allowing an empty iron skillet on the stove to overheat will produce a quick brown tinge of rust over the metal's surface.
Aluminum is also mined out of the ground as an oxidized compound called Bauxite. Aluminum oxide takes a tremendous amount of energy to smelt and release the oxygen content. For this reason aluminum was considered a very precious metal until the 20th century. It was so precious the builders for the Washington Monument capped the spire with an aluminum ignot.
Once smelted and converted into pure aluminum, the metal will slowly rust when exposed to air and heat. Aluminum oxide is often seen on aluminum automobile parts as a white dust. Wiping away the dust will again expose the aluminum.
Copper rusts from its metallic natural brown shade to a bright green. Rusted Copper is considered tarnished but the process is the same as the metal recombines with oxygen in the air.
For most practical applications copper is allowed to rust and have a thick green coat called patina. The green copper rust works as a seal to protect the untarnished copper underneath. This process allows copper to work as a roofing, structural or cosmetic building material that will age and turn green with time. The metal, however, remains strong as the unexposed copper retains its integrity while protected by the patina.
With time, however, even the patina will fade in the face of enduring weather and corrosion. Eventually, the copper will rust through and need replacing.