rotting pots

rotting pot

If you put a piece of copper into water, it releases positively charged fragments of atoms (known as cations) into solution. This leaves the copper with an overall negative charge due to a surplus of negatively charged electrons.

dissociation of copper in water

Aluminium is less stable than copper, so when you put a piece of aluminium into water, it releases comparatively more cations and the metal is left with a larger negative charge.

dissociation of aluminium in water

If you put a piece of copper into electrical contact with a piece of aluminium and put both of these into water, the surplus electrons on the aluminium flow into the copper to try and correct the charge imbalance between the two metals. This means there is electrical flow in the metals.

In order to complete the electrical circuit, electricity is carried through the water by flow of cations:

galvanic corrosion

This flow of electricity electrolyses the water, splitting it up into its components of hydrogen (released at the aluminium) and oxygen (released at the copper). The hydrogen results in bubbles of gas, whereas the oxygen reacts with the aluminium cations to form aluminium oxides. One of these is alumina, Al2O3, which is insoluble in water and can be seen as a white deposit.

Meanwhile the aluminium stopper is continually losing material as cations are released to keep the electricity flowing. The result we see is that it corrodes much faster than it would if it was in the water without copper.

By making the surface area of the copper much larger than that of the aluminium, this drives the reaction faster still.

And the twist? The vessels were electro-formed using a controlled version of this reaction. So, the same process used in their creation will ultimately be responsible for their destruction…

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