In the Rough Science programme 'The Reef', the Rough Scientists constructed a burglar alarm to protect the delicate coral reef off the island of Chumbe. They strung out a wire across the surface of the water to mark out a no-go region for boats. If a boat crossed the line the circuit was completed and the alarm sounded. Being an important nature reserve this had to be powered by an environmentally friendly source – wave power or sunlight. Rough Scientist Mikey B was given the challenge of stopping the wire rusting over time. He decided to use a lump of magnesium as a sacrificial electrode.

To find out more about how magnesium stops iron rusting and how it is used to protect oil drilling platforms and boats, read this extract from the second level OU course Our Chemical Environment (ST240).

According to this table aluminium should react with oxygen more readily than iron, which is easily oxidized in the presence of water and oxygen. How is it then that aluminium can be used for aeroplanes which are in constant contact with water and oxygen?

In reality, aluminium does react with oxygen very rapidly and forms aluminium oxide, Al₂O₃. The oxide has the ability to adhere to the surface of the underlying metal with great tenacity .

This thin but robust layer of oxide then protects the underlying metal from further oxidation

If the oxide becomes scratched and aluminium metal exposed, more oxide is formed and the protected layer renewed. Even if aluminium metal is cleaned with emery paper, a protective oxide layer would form before the metal could be put into the solution containing Fe²⁺(aq) ions.

The situation with iron is very similar in the sense that iron oxidizes and forms the familiar red–orange rust. However, rust takes up a greater volume than the iron from which it is formed and consequently splits and cracks. It is also porous to water and oxygen so that more rust can be formed from the underlying metal and eventually the oxidation process is complete.

Moisture, in addition to oxygen, is also necessary for the rusting of iron. Rust is not the simple oxide Fe₂O₃ but is a compound that actually contains water molecules. It can be represented by Fe₂O₃.nH₂O where n can vary according to the conditions under which the rust forms. The water molecules are incorporated into spaces within the crystal structure of iron oxide.

Iron (and steel) is not weather-resistant yet it does find widespread use because of its relative cheapness and its mechanical properties. Perhaps the most obvious way in which steel can be prevented from rusting is to protect the metal surface with a coating of something that does not rust. The most widely used coating is paint and it is very effective provided that the paint does not become damaged. If this happens, or the metal surface is not completely free of rust when it is painted, then further rusting can occur .

The situation is made worse because rusting can spread under the paintwork and is often at an advanced stage when signs of blistered paint become apparent. Where a hard paint surface is not required (such as the inside of car body panels), waxes and paints that do not set completely are useful. If such a surface suffers a small scratch, the paint will flow a little and effectively self-heal the scratch.

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