Page 36 - Curriculum Visions Dynamic Book

Protecting with magnesium
Many metals are subject to corrosion when placed in damp air or damp soil. The most vulnerable of all are iron and steel structures. Small structures can be protected by covering them in a protective coating of, say, paint. However, some iron and steel structures are too big for such treatment. Instead, they are coupled to blocks of metals such as magnesium.
By connecting the metals together in a moist environment, a natural battery is formed. In such a battery, one of the electrodes (the anode) always corrodes, while the other (the cathode) remains undamaged.
The list below shows how magnesium can be used to protect steel. The metals shown in the list (called a reactivity series) will always protect any metal that comes below in the series. Those below act as cathodes, those above act as anodes. Thus magnesium will protect exposed iron because it is more reactive, but tin will not.
REACTIVITY SERIES
Element
Reactivity
potassium sodium calcium magnesium aluminum manganese chromium zinc
iron
cadmium
tin
lead copper mercury silver gold platinum
most reactive
least reactive
Also...
You may be familiar with the protective role of tin as a plating over steel. Here the tin is used as a kind of paint. However, if the tin plating becomes scratched, you will find the iron corrodes rapidly. This is one reason tin plating is used less today than in the past.
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