Rusting is an electrochemical process with two reactions happening at once:

• At the anode, iron atoms lose electrons and dissolve into the water as Fe2+ ions
• At the cathode, oxygen gains those electrons and joins with water to form hydroxide ions
• The dissolved iron then reacts with hydroxide and dries into the flaky orange rust we know

This sets up a tiny galvanic cell — essentially a battery — on the metal surface. Aluminum oxide forms a tight layer that protects the metal, but iron oxide is porous and flaky. It cracks and peels, exposing fresh iron again. Rust never stops because it never seals the surface.