GCSE OCR Gateway Chemistry C6.1p Mitigation of Iron Corrosion

C6.1q To be able to explain how mitigation of corrosion is achieved by creating a physical barrier to oxygen and water and by sacrificial protection

How to stop iron rusting

Protective methods

One obvious way of protecting iron and steel from rusting is to coat the metal with a material that water and air cannot penetrate.

Traditionally paint such as Hammerite has been used fairly successfully to protect iron from rusting.  

Similarly, oiling the iron gives a more flexible coating that water cannot penetrate. 

These traditional methods work but are not suitable for all conditions particularly when iron is exposed to sea water a much more vigorous and corrosive environment. 

So how can iron and steel be protected from corrosion especially since most boats and ships are steel hulled?

The answer is in what’s called sacrificial protection.

Sacrificial methods

Because corrosion is an oxidative process connecting iron to a more reactive metal will protect it from rusting.

What happens is that the more reactive metal will oxidise instead of the iron it protects

Typically, magnesium and zinc are used to protect iron .  Its called sacrificial since the other more reactive metal is eventually consumed and sacrificed to protect the iron,

You can see blocks of magnesium bolted to ships hull for this very purpose

You can also set up an interesting and colourful experiment to show sacrificial protection happening. 

You need to make up a hot agar solution containing traces of both potassium hexacyano ferrate(III) (K₃Fe(CN)₆)  and the acid base indicator phenolphthalein.

Pour this solution into test tubes that contain iron nails, one on its own, another wrapped in copper wire, another wrapped in zinc plate and a fourth wrapped in magnesium ribbon. 

Here is the set up:

And in photo:

The blue colour is due to the formation of Prussian Blue a distinctive blue colour that shows the presence of iron (III) (Fe³⁺) ions in rust. 

The magenta colour shows how the magnesium or other reactive metal is protecting iron from corrosion.  The magnesium has reacted to form an alkaline solution hence the phenolphthalein has turned magenta. 

The next photo shows how a boat hull is being protected from corrosion using blocks of magnesium or zinc bolted to the hull. 

The other photo shows a corroded zinc block on a ships hull.