Rusty nail. Coloured scanning electron micrograph (SEM) of the tip of a rusty metal nail. Rusting is an electrolytic process in which iron reacts with water and oxygen to form a hydrated iron oxide. Rust is more brittle, porous and bulky than iron, so it tends to flake off. It is often thought that being injured by a rusty nail causes tetanus (lockjaw). In fact, the disease is caused when a wound is infected by Clostridium tetani bacteria. This is found in soil, not rust, although rusty metal found outdoors may well have been in contact with infected soil. Magnification unknown.