Brass, an alloy of zinc and copper, has been used for more than 3,000 years.
Reference: History of Brass
The above photomicrograph and accompanying figure represent a typical
cross section of a hot-dip galvanized coating. The outermost (Eta)
layer of the coating is formed as the material is withdrawn from the
molten zinc bath. This layer is a covering of pure zinc. The
underlying series of zinc–iron alloys in the coating are
the result of a metallurgical reaction between molten zinc and the
steel or iron material being galvanized. As the zinc–iron alloys form,
they will grow perpendicularly to the steel surface. The effect this
has on corners and edges of material is that the coating there is
generally thicker than the surrounding coating, as seen in the
micrograph below. This is in sharp contrast to other types of
protective coatings that tend to thin out at the edges and corners
of material.
The galvanized coating itself is considered to be a self inspecting system. This is because the reaction between the molten zinc and the steel will not occur unless the steel surface is chemically clean. In effect, a galvanized coating that appears sound and continuous is in fact sound and continuous. If a coating should become damaged however, the zinc will continue to provide cathodic protection to the exposed steel. Even if areas as much as 1/4 inch in length and/or width become exposed, the surrounding zinc will provide cathodic protection to this area as long as the coating lasts.
As for mechanical protection, the galvanized zinc coating literally becomes part of the steel substrate thus, an adhesion bond on the order of several thousand psi exists between the two. Also, since the zinc–iron alloys are harder than the underlying steel, they will provide excellent abrasion resistance to the galvanized article. The Eta layer is relatively ductile and also contributes to the protection of the galvanized material by providing good impact resistance. The combination of all these layers to produce the galvanized coating provides toughness and resistance to mechanical damage in transport, erection and service.
The thickness and appearance of the galvanized coating are affected by several factors. These include: