The reaction of the molten zinc with the steel during the hot dip galvanizing process results in a coating that is metallurgically bonded to the steel. The coating is not a simple zinc coating over the steel substrate but, rather a series of hard iron/zinc alloys that become more zinc rich towards the outer surface of the coating.
These alloys provide for an adherent coating that is harder than the underlying steel providing very good resistance against mechanical damage. The coating also thickens over corners providing excellent protection over areas that tend to be thinly coated and poorly protected using alternative coatings.
Should an exposed area of steel occur, the zinc surface will preferentially corrode at a slow rate and protect the steel. This preferential protection is termed cathodic protection and, where damage is small, the protective corrosion properties of zinc will fill the exposed area arresting attack.