The corrosion protection of hot dip galvanizing results from the formation of a dense insoluble corrosion product on the surface of the coating. Newly galvanized steel is bright and shiny as the surface comprises pure zinc. Over a period of time this surface oxidises to zinc oxide (ZnO) in air. This is rapidly converted to zinc hydroxide (Zn(OH)2) by the moisture in the atmosphere. Both these corrosion products are soluble and are easily washed off the surface by rain or condensation. The final protective coating on the zinc surface is the conversion, in free flowing air, of ZnO and Zn(OH)2 to insoluble dense zinc carbonate (ZnCO3).

It is, therefore, of paramount importance that a freshly galvanized surface be allowed to "breathe" in order to form the protective corrosion product. Although the ZnCO3 is slowly removed, it is replaced by the continual supply of corrosion products as outlined above.

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 which 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 good resistance against mechanical damage. Also the coating thickens over corners providing excellent protection over areas that tend to be thinly coated and protected using organic coatings.

Micrograph showing the slightly thicker galvanized coating at corners

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 products of zinc will fill the exposed area arresting attack. Finally, in addition to the barrier protection afforded, zinc protects against underfilm creep (attack under the coating) should mechanical damage be so severe as to expose the underlying steel. Underfilm creep is a common failure mechanism with organic coatings where rust spreads from any damaged area. This is highlighted below.

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