The principal method of making steel resist
is by alloying it with another metal, zinc. When steel is
submerged in melted zinc, the chemical reaction permanently
bonds the zinc to the steel through galvanizing.
Therefore, the zinc isn't exactly a sealer, like paint, because
it doesn't just coat the steel; it actually permanently becomes
a part of it.
The zinc goes through a reaction with the iron molecules
within the steel to form galvanized steel. The most external
layer is all zinc, but successive layers are a mixture of
zinc and iron, with an interior of pure steel. These multiple
layers are responsible for the amazing property of the metal
to withstand corrosion-inducing circumstances, such as saltwater
Zinc also protects the steel by acting as a "sacrificial
layer." If, for some reason, rust does take hold on the
surface of galvanized steel, the zinc will get corroded first.
This allows the zinc that is spread over the breach or scratch
to prevent rust from reaching the steel.