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 figure below is a photomicrograph of a galvanized steel coating cross-section and shows a typical coating microstructure consisting of three alloy layers and a layer of pure metallic zinc.
Progressing from the underlying steel surface outward, these are:
- The thin Gamma layer composed of an alloy that is 75% zinc and 25% iron
- The Delta layer composed of an alloy that is 90% zinc and 10% iron
- The Zeta layer composed of an alloy that is 94% zinc and 6% iron
- The outer Eta layer that is composed of pure zinc
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Below the name of each layer in the figure above (magnified cross-section of hot-dip galvanized coating) appears its respective hardness, expressed by a Diamond Pyramid Number (DPN). The DPN is a progressive measure of hardness; the higher the number, the greater the hardness. Typically, the Gamma, Delta and Zeta layers are harder than the underlying steel. The hardness of these inner layers provides exceptional protection against coating damage by abrasion. The Eta layer is quite ductile, providing the coating with some impact resistance. 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 as in service. The toughness of the galvanized coating is extremely important since barrier protection is dependent upon the integrity of the coating.
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 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. The figure below is a photomicrograph showing a cross-section of a corner of a galvanized piece of steel.
Because the galvanizing process involves total immersion of the material, all surfaces are coated. Galvanizing provides protection on both exterior and interior surfaces of hollow structures. Hollow structures that are painted (but not galvanized) have no corrosion protection on the inside.
The inspection process for galvanized items is simple and 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 takes place in a factory regardless of weather or humidity conditions. Most brush- and spray-applied coatings depend 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 of two or three days for galvanizing is common.
The American Society of Testing and Materials International (ASTM), the Canadian Specification Association (CSA) and the American Association of State Highway and Transportation Officials (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 the figure below 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” in the galvanizing industry, tends 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 a bright appearance.
It is difficult to provide precise guidance in the area of steel selection without qualifying all of the grades of steel commercially available. The guidelines discussed below usually result in the selection of steels that provide good galvanized coatings.
- Levels of carbon less than 0.25%, phosphorus less than 0.04%, or manganese less than 1.35% are beneficial
- Silicon levels less than 0.04 % or between 0.15% and 0.25% are desirable
Even though it is not a part of the controlled composition of the steel, silicon may be present in many steels commonly galvanized. This occurs primarily because silicon is used in the deoxidization process in steel making and is found in continuously cast steel. The phosphorus content should never be greater than 0.04% for steel that is intended for galvanizing. Phosphorus acts as a catalyst during galvanizing, resulting in rapid growth of the zinc-iron alloy layers. This growth is virtually uncontrollable during the galvanizing process.
As the galvanizing reaction is a diffusion process, higher zinc bath temperatures and longer immersion times generally will produce somewhat heavier alloy layers. Like all diffusion processes, the reaction proceeds rapidly at first and then slows as layers grow and become thicker. However, continued immersion beyond a certain time will have little effect on further coating growth. When galvanizing reactive steels, the diffusion process proceeds at a faster rate, producing thicker coatings.
The thickness of the outer pure zinc layer is largely dependent upon the rate of withdrawal from the zinc bath. A rapid rate of withdrawal causes an article to carry out more zinc and generally results in a thicker coating.
ASTM, CSA and AASHTO specifications and inspection standards for galvanizing recognize that variations occur in both coating thickness and compositions. Thickness specifications are stated in average terms. Further, coating thickness measurements must be taken at several points on each inspected article to comply with ASTM A 123/A 123M for structural steel and A 153/A 153M for hardware.
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