text: Oxygen combines with iron to form iron oxide (rust).  It begins immediately on contact and continues until either the oxygen or iron is consumed. Metal oxides are prevented (or at least reduced) by applying a coating that blocks the oxygen and does not itself contain available oxygen.  Some paints are specially formulated to do this.  Also some metals are self-passivating.  That is, they combine with oxygen to form a coating that prevents further oxygen contact . Aluminum, gold, silver, lead, and zinc are some examples.  I know of no practical method to reverse iron oxydation.