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To begin to understand corrosion, and the role that water plays, we have to understand a little about how metals are constructed at the atomic level (don't be scared - this is easy to understand). Remember that rusting is a type of corrosion that only refers to iron. Copper or aluminium do not rust, they corrode. All metals can corrode, only iron can rust.

Atoms in brief: Atoms are the microscopic building blocks that make up all substances. Each atom has a nucleus in the centre, and electrons that spin around it. The nucleus has a positive charge, and electrons have a negative charge, but the atom itself has no charge because the positives and negatives cancel eachother out. An atom with a nucleus which has a charge of "5 positive" will have 5 electrons spinning around it ("5 +" and "5 -" means that the atom has a zero charge). This is called charge neutral, and all atoms like to be charge neutral.
Different substances are made of different types of atom (Gold is made of gold atoms, Carbon Dioxide is made of carbon atoms and oxygen atoms joined together). The smallest piece of material will contain billions of atoms.
It is only the central nucleus of atoms that is different. All electrons are the same (a carbon atom and a gold atom will each have a different nucleus, and although gold has more electrons than carbon, all their electrons are identical. There is a difference between a gold nucleus and a carbon nucleus, but not between a gold electron and a carbon electron. (If you want to learn all about atoms, read our section on Atomic Structure).
Metals in brief: Metals, like all substances, are made of atoms. Metal atoms are arranged in a very regular pattern, like a grid. However, metals are special in the way that they are bonded. Most atoms cling onto their electrons tightly, but metal atoms don't - allowing some of them to float around. Because all electrons are the same, metals do not care if they lose one, as long as it is replaced by another (Remember that all atoms like to be charge neutral, and electrons cancel out the positive charge of the nucleus. If a metal lost electrons and they were not replaced, it would build up a positive charge). This means that electrons in a metal are free to flow anywhere: as long as the metal has the same number of electrons, it doesn't mind if they move around from atom to atom.

These free electrons give metals a special ability - they can conduct electricty. When an electric current travels along a piece of metal, such as a copper wire, the current is actually a stream of electrons flowing along the wire. The atoms of the metal stay in the same place (so you don't see the metal changing in any way), but the electrons are moving. As long as electrons are pushed into one end of the wire, they are free to be pumped out at the other end, the metal must keep the same number of electrons overall.


We know from Experiment 4 that a current flowing through metal (therefore electrons flowing through metal) has a direct effect on corrosion. At the negative side of the current (the place that electrons are flowing to) there was no corrosion. But at the positive side of the current (the place that the electrons are flowing from) there is corrosion.

Initial Conclusion: Electrons flow through the metal away from areas where corrosion is happening. They travel to other areas where corrosion does not happen.

Corrosion page 4