Rust experiment. Image 1 of 2. Iron nails in test tubes under different conditions: water and air (far left), drying agent (left), distilled water topped with oil (right), and salty water (far right). Rust (not seen) is hydrated iron oxide, produced by a chemical reaction between iron metal and the water and oxygen in the air. This process can be accelerated by the presence of ions in solution, such as the electrolytic saltwater. For these nails after several weeks, see image T745/070.