Preventing corrosion
The second use is in applying techniques to prevent the corrosion of metals.
The technique
Keeping iron (Fe) in contact with magnesium (Mg) or zinc (Zn), metals more reactive than iron, in order to prevent it from rusting is an example.
Why a more reactive metal?
Magnesium and zinc stand above iron in the activity series. So they react before the iron does — that is, they corrode in the iron's place.
The metal that is sacrificed
The more reactive metal corrodes away gradually while the iron is protected. When it is used up it can be replaced, and the protection continues.
A common mistake
This cannot be done with a metal less reactive than iron — copper, for instance. Then it is the iron that corrodes.
The third and fourth uses
Electrochemical cells
Selecting suitable metals for making electrochemical cells. Two metals far apart in the series give a larger voltage.
The extraction method
Deciding the method suitable for extracting a metal. That is the subject of the next section.
Separating a metal from the natural ore that contains it is called metal extraction.