<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> <P> When iron rusts, the oxides take up more volume than the original metal; this expansion can generate enormous forces, damaging structures made with iron . See economic effect for more details . </P> <P> The rusting of iron is an electrochemical process that begins with the transfer of electrons from iron to oxygen . The iron is the reducing agent (gives up electrons) while the oxygen is the oxidising agent (gains electrons). The rate of corrosion is affected by water and accelerated by electrolytes, as illustrated by the effects of road salt on the corrosion of automobiles . The key reaction is the reduction of oxygen: </P> <Dl> <Dd> O + 4 e + 2 H O → 4 OH </Dd> </Dl>

The reaction of iron metal with oxygen to form iron (iii) oxide