<P> Rust is another name for iron oxide, which occurs when iron or an alloy that contains iron, like steel, is exposed to oxygen and moisture for a long period of time . Over time, the oxygen combines with the metal at an atomic level, forming a new compound called an oxide and weakening the bonds of the metal itself . Although some people refer to rust generally as "oxidation", that term is much more general; although rust forms when iron undergoes oxidation, not all oxidation forms rust . Only iron or alloys that contain iron can rust, but other metals can corrode in similar ways . </P> <P> The main catalyst for the rusting process is water . Iron or steel structures might appear to be solid, but water molecules can penetrate the microscopic pits and cracks in any exposed metal . The hydrogen atoms present in water molecules can combine with other elements to form acids, which will eventually cause more metal to be exposed . If chloride ions are present, as is the case with saltwater, the corrosion is likely to occur more quickly . Meanwhile, the oxygen atoms combine with metallic atoms to form the destructive oxide compound . As the atoms combine, they weaken the metal, making the structure brittle and crumbly . </P> <P> When impure (cast) iron is in contact with water, oxygen, other strong oxidants, or acids, it rusts . If salt is present, for example in seawater or salt spray, the iron tends to rust more quickly, as a result of electrochemical reactions . Iron metal is relatively unaffected by pure water or by dry oxygen . As with other metals, like aluminium, a tightly adhering oxide coating, a passivation layer, protects the bulk iron from further oxidation . The conversion of the passivating ferrous oxide layer to rust results from the combined action of two agents, usually oxygen and water . </P> <P> Other degrading solutions are sulfur dioxide in water and carbon dioxide in water . Under these corrosive conditions, iron hydroxide species are formed . Unlike ferrous oxides, the hydroxides do not adhere to the bulk metal . As they form and flake off from the surface, fresh iron is exposed, and the corrosion process continues until either all of the iron is consumed or all of the oxygen, water, carbon dioxide, or sulfur dioxide in the system are removed or consumed . </P>

What type of reaction is the rusting of iron