Posted by: Kash Farooq | August 20, 2013

Why don’t oil rigs rust?

Reinforced steel is used for large structure construction projects as it has a number of properties that make it a perfect choice. For example, oil platforms have steel legs.

Iwafune-oki - Japan's only offshore oil and gas field.

Iwafune-oki – Japan’s only offshore oil and gas field. (Image: Wikimedia Commons)

Steel is an alloy of iron and (mainly) carbon. But we all know that iron rusts. Iron combines with oxygen to form iron oxide – i.e. familiar red-orange coloured rust. (This is the same reason why Mars is red).

Rusty barbed wire. (Image by Wikimedia Commons user Waugsberg)

Rusty barbed wire. (Image by Wikimedia Commons user Waugsberg)

Rust takes up a greater volume than the iron from which it is formed. It is also porous – so if your steel gets a coating of rust, water can still get through it and continue to create rust underneath the rust layer. These two properties will split and crack the metal, weakening it.

So, if we don’t do something, the steel legs of an oil platform will rust, weaken and subsequently collapse – we clearly need to somehow protect the steel.

To prevent small steel objects from rusting we galvanize them: we give them a coating of zinc. We literally dip the objects in molten zinc. Zinc also reacts with oxygen to form zinc oxide but, unlike iron oxide, zinc oxide is waterproof. Once this outer layer of zinc oxide is created, the metal underneath – the steel coated with zinc – is protected.

But oil platform legs are too large to use this technique – we can’t dip them in a massive vat of molten zinc. A different technique is needed.

Instead of galvanizing, large steel structures are protected by attaching a sacrificial metal to them.

A sacrificial metal is a metal that is easily oxidised. A metal such as magnesium is often used. Magnesium readily reacts with oxygen and the following equation shows what happens:

Mg = Mg2+ + 2e

The equation shows a magnesium atom oxidising to form a magnesium Mg2+ ion.

However, the important part of the process, in terms of protecting our oil rig’s steel legs, are that 2 electrons are released during the reaction (shown in bold in the equation). These electrons flow along a steel cable connecting our sacrificial metal to our oil rig and give the whole oil rig a small negative charge.

Preventing rusting of an oil rig by attaching a sacrificial metal (Image: Open University, course S104)

Preventing rusting of an oil rig by attaching a sacrificial metal (Image: Open University, course S104)

So how does giving an oil rig (and its legs) a negative charge stop iron oxidising to form rust? Well, oxidation is a process that produces positively charged ions by releasing electrons – as can be seen in the magnesium oxidation equation above. To oxidise the iron in steel a similar reaction would need to take place. Iron atoms would need to form iron ions by releasing electrons into the structure – a structure that is already negatively charged. The negatively charged structure forms a sort of charged barrier – the structure can’t become even more negatively charged. It prevents iron from forming ions and hence stops it oxidising to form rust.

Eventually, the sacrificial metal block “runs out”. It becomes completely corroded and has to be replaced, which is relatively cheap. Attaching a sacrificial metal is an ingenious way to cheaply protect large steel structures exposed to water.


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