Page 25 - Curriculum Visions Dynamic Book

Aluminium as a sacrificial anode
Aluminium is bolted on to the keels of many ships to protect the main steel hull of the vessel.
When steel is placed in salt water it behaves like part of an electrical battery. In any battery there are two electrodes (a negative electrode or cathode, and a positive electrode or anode). As the battery works, one of the electrodes
(the positive electrode, or anode) is corroded (used up).
There is a danger that the steel of a ship’s hull will behave as the electrode that is used up and this can cause severe corrosion to the ship. However, if pieces of aluminium
are placed on the hull below the water line, the aluminium
is sacrificed instead, so protecting the hull.
Scientists call this effect “cathodic protection” because
the hull of the ship is the cathode of the natural battery.
It is far easier to replace chunks of aluminium bolted to the steel hull than to have to replace the whole hull! The secret to how this works lies in the fact that aluminium is much more reactive than iron. Other common cathodic protectors are zinc and magnesium.
cladding: a surface sheet of material designed to protect other materials from corrosion.
corrosion: the slow decay of a substance resulting from contact with gases and liquids in the environment. The term is often applied to metals. Rust is the corrosion of iron and steel.
oxide: a compound that includes oxygen and one other element.
rust: the corrosion of iron and steel.
 The hulls of ships are usually fitted with sacrificial anodes. As they are below the water line they are normally obscured except when the ship is in dry dock.
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