Galvanizing forms a metallurgical bond between the zinc and the underlying steel or iron, creating a barrier that is part of the metal itself. During galvanizing, the molten zinc reacts with the surface of the steel or iron article to form a series of zinc/iron alloy layers.
The galvanized coating is adherent to the underlying steel on the order of several thousand pounds per square inch (psi). Other coatings typically offer adhesion rated at several hundred psi at best.
Hardness, ductility, and adherence combine
to provide the galvanized coating with unmatched protection against damage
caused by rough handling during transportation to and/or at the job
site, as well in service. The toughness of the galvanized coating is
extremely important since barrier protection is dependent upon the integrity
of the coating.
Other coatings damage easily during
shipment or through rough handling on the job site. Experts will argue
that all organic forms of barrier protection (such as paint) by their
nature are permeable to some degree. Correctly applied galvanized coatings
If the galvanized coating is physically damaged, it will continue cathodic protection to the exposed steel. If individual areas of underlying steel or iron become exposed as much as 1/4”, the surrounding zinc will provide these areas with cathodic protection for as long as the coating lasts.
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The galvanizing process naturally produces
coatings that are at least as thick at the corners and edges as the
coating on the rest of the article. As
Because the galvanizing process involves
total immersion of the material, it is a complete process~ all surfaces
are coated. Galvanizing provides both outside and inside protection for
hollow structures. Hollow structures that are painted have no corrosion
process for galvanized items is simple, fast, and requires minimal labor.
This is important because the inspection process required to assure the
quality of many brush and spray applied coatings is highly labor intensive
and uses expensive skilled labor.
Galvanizing continues at the factory under any weather or humidity conditions. Most brush and spray applied coatings are dependent upon proper weather and humidity conditions for correct application. This dependence on atmospheric conditions often translates into costly construction delays.
The galvanizer’s ability to work in any type of weather allows a higher degree of assurance of on-time delivery. Working under these circumstances, galvanizing can be completed quickly and with short lead times. A turnaround time o~f two or three days for galvanizing is common.
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ASTM, CSA, and AASHTO specifications establish minimum standards for thickness of galvanized coatings on various categories of items. These minimum standards are routinely exceeded by galvanizers due to the nature of the galvanizing process. Factors influencing the thickness and appearance of the galvanized coating include chemical composition of the steel, steel surface condition, cold working of steel prior to galvanizing, bath temperature, bath immersion time, bath withdrawal rate, and steel cooling rate.
The chemical composition of the steel being galvanized is very important. The amount of silicon and phosphorus in the steel strongly influences the thickness and appearance of the galvanized coating. Silicon, phosphorous, or combinations of the two elements can cause thick, brittle galvanized coatings. The coating thickness curve shown in Figure 10 relates the effect of silicon in the base steel to the thickness of the zinc coating. The carbon, sulfur, and manganese content of the steel also may have a minor effect on the galvanized coating thickness.
The combination of
elements mentioned above, known as “reactive steel” to the galvanizing
industry, tend to accelerate the growth of zinc-iron alloy layers. This
may result in a finished galvanized coating consisting entirely of
zinc-iron alloy. Instead of a shiny appearance, the galvanized coating
will have a dark gray, matte finish. This dark gray, matte coating will
provide as much corrosion protection as a galvanized coating having the
common bright appearance.
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