Rust is usually used to refer to a very common, reddish-brown compound called iron oxide (Fe2O3). Iron oxide is formed when iron and oxygen react in the presence of water or moisture in the air. Iron oxide is very common, because iron reacts easily with oxygen in the air. In fact, it reacts so readily with oxygen that it's actually very rare to find pure iron in nature. Rust can also refer to the reaction of iron and chloride in an underwater environment.