Why does iron rust when exposed to air and water?

Iron rusts through an electrochemical process called oxidation. When iron is exposed to both oxygen and water, the water acts as an electrolyte that allows electrons to move between different areas of the metal's surface. Iron atoms lose electrons and form iron ions, which then react with oxygen and water to produce iron oxide, commonly known as rust. The presence of water is essential because it enables the electron transfer that drives the reaction. Salt or acids in the water speed up the process by increasing electrical conductivity. Unlike aluminum oxide, which forms a tight protective layer on aluminum, iron oxide is porous and flaky, so it does not protect the underlying metal and allows corrosion to continue deeper into the material.