Metallurgical Bond

Galvanizing forms a metallurgical bond between the coating and underlying steel or iron . Galvanizing creates a barrier that is a part or extension of the metal itself . During galvanizing,the molten zinc reacts with the surface of the steel or iron article undergoing galvanizing to form a series of zinc/iron alloys. A typical galvanized coating consists of three alloy layers and a layer of metallic zinc from the underlying metal surface outward, these are:

• The thin Gamma layer composed of an alloy that is 75 percent zinc and 25 percent iron
• The Delta layer composed of an alloy that is 90 percent zinc and 10 percent iron
• The Zeta layer composed of an alloy that is 94 percent zinc and 6 percent iron
• The outer Eta layer that is composed of zinc

 

 

Photomicrograph of Hot Dipped Galvanized Coating

    Each layer is characterized by a measure of hardness the Diamond Pyramid Number (DPN). The DPN is a progressive measure of hardness. Typically, the Gamma, Zeta, and Delta layers are harder than the underlying steel . The hardness of these inner layers provides exceptional protection against coating damage through abrasion. The Etta layer of the galvanized coating is quite ductile, providing the galvanized coating with good resistance to damage by abrasion . The galvanized coating is adherent to the underlying steel on the order of several thousand pounds per square (psi) inch. Other coatings typically offer adhesion rated at several hundred psi at best.

These three factors, hardness, ductility, and adherence combine to provide the galvanized coating with unmatched protection against damage by rough handling during transportation to the job site. The toughness of the galvanized coating is extremely important since barrier protection from corrosion 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) must be permeable to some degree. Correctly applied galvanized coatings are impermeable. If the coating is physically damaged, the galvanizing will continue to provide cathodic protection to the exposed steel. If individual areas of underlying steel or iron of length or width of as much as 1/4" become exposed, the surrounding zinc will provide these areas with cathodic protection for as long as the coating lasts.

  The galvanizing process naturally produces coatings that are at least as thick on the corners and edges as the coating on the rest of the article. As coating damage is most likely to occur at the edges, this is where added protection is needed most. Brush- or spray-applied coatings have a natural tendency to thin at the corners and edges. Because the galvanizing process involves total immersion of the material, it is a complete process; all surfaces are coated.