Rust is the mortal enemy of iron and the destroyer of nuts, bolts and bicycle handles around the world. Just a touch of water and any metal that contains iron is slowly corroded, transformed into a brittle orange ghost of its former self. On the surface, rust is simply the formation of iron oxide. It’s created in an oxidation reaction, where iron and oxygen bond together. However, the reaction needs a little help from water to get things going.
When water and atmospheric carbon dioxide combine, they create a very weak carbonic acid. When carbonic acid sits on iron metal, it begins to dissolve it, and the iron atoms are then free to bond with oxygen atoms in the acid to form rust.
Did you know? Five per cent of Earth’s crust is iron
The best way to prevent rust from forming is to keep iron away from any moisture and prevent iron and oxygen from finding each other. However, for things like pipelines and ship hulls, staying dry isn’t an option. Instead, an anti-corrosive method called ‘cathodic protection’ is often used to fight off the effects of rust. During the formation of rust, electrons move around the metal and create two electrically charged points called the anode (positively charged) and the cathode (negatively charged). Corrosion occurs at the anode side of the metal. One way to stop iron corroding at the anode site is to introduce a sacrificial metal, such as zinc. The zinc dummy anode is more reactive than iron – the cathode – and free from rust and corrosion. Ships with iron hulls can also pass an electrical current through the metal as a form of cathodic protection. The electrical current interrupts the movement of electrons between the anode and cathode and prevents rust from forming.