Did you know?

Ted Lister, chemical education consultant, shares anecdotes and 'did you knows' to help you add that 'wow' factor to your lessons.  
In this issue: copper bottoms


In the 18th century, it was common for wooden ships to have their hulls sheathed with copper to protect them against barnacles and other marine organisms which could slow the ship and attack the wood below the waterline. This is where the expression 'copper-bottomed', meaning 'reliable' originates. 

corroded rivets

© istockphoto

However, there was a problem with this technique. Iron bolts were used to hold the wooden hulls together and if these came into contact with the copper sheathing immersed in sea water, the ideal conditions for bimetallic corrosion were in place. Here the more reactive metal, iron, corrodes faster than normal when in contact with a less reactive metal. This led to rapid deterioration of the hull bolts, with disastrous consequences. There are stories (uncorroborated) of ships at sail whose copper bottom fell off, leaving the ship top heavy and causing it to capsize. It is unlikely that this phenomenon was ever solely responsible for the loss of a ship but there is one well-documented instance where it might have been a contributory factor. Around 1782 the Royal George warship sank with little warning; some reports indicated that a loud crack was heard at the time. Apparently part of the ship's frame collapsed, which might have been caused by corroded bolts. 

Another example of bimetallic corrosion in sea water is that of the sacrificial protection of steel hulls of ships (and also oil rigs and pipelines). Here blocks of zinc (or sometimes magnesium) are attached to the steel (ie mostly iron) hull of the ship. The more reactive zinc corrodes away faster and protects the iron. The reaction: 

Zn(s) + aq  double arrow Zn2+(aq) +2e-  

proceeds to the right and the electrons that are released force the equilibrium below to the left: 

Fe(s) + aq  double arrow Fe2+(aq) +2e- 

The zinc corrodes away and has to be regularly replaced.