Water speeds up the rusting process because iron can transfer its electrons to water, which helps speed the oxidation of iron. The water provides an easier pathway that is available with pure oxygen.

Iron is more electropositive than hydrogen, that is, iron is more prone to giving up its electrons to an anion than hydrogen is. For this reason, at high temperature, the reaction of oxygen giving up the hydrogens to bond with iron instead becomes energetically possible, producing hydrogen gas as a byproduct, releasing even more energy. At higher temperature yet, the reaction actually absorbs chemical energy, which causes it to run in the other direction.