Rust is ferric oxide Fe₂O₃. The reaction is ferrous iron or native iron combines with oxygen to make ferric oxide. It is simply a chemical reaction that runs spontaneously at room Temperature and Pressure. 2Fe + 3/2 O₂= Fe₂O₃= rust or the mineral hematite

Iron combines with oxygen to form a mineral called hematite (Fe₂O₃). When the metal corroding is artificial, we usually refer tot he hematite as rust.

It should be noted that hydrogen bonds less strongly to oxygen than iron does. This means that iron will, albeit slowly, take the oxygen right out of water to become rust - producing hydrogen gas.

Rust occurs when certain types of metals come in contact with oxygen over long enough periods to undergo oxidation - a chemical reaction in which the oxygen combines with the metal at an atomic level, forming a new compound called an oxide. Metal-oxides have weaker bonds than the metal itself, which is why rusty chunks of metal tend to crumble easily relative to the original material. If the original metal is iron or steel, the resulting rust is called iron oxide. Copper becomes copper oxide, etc.