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outermost (Eta) layer of the coating is formed as a covering of pure zinc that is carried out on the material as it is withdrawn from the molten zinc bath. 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:

The chemical composition of the steel The surface condition of the steel Bath immersion time Bath withdrawal rate Steel cooling rate

Please contact us at galvanize@metalplate.com with any questions or comments you may have.