Details
Hot-dip galvanizing is also known as batch hot dip galvanizing. Hot-dip galvanizing is a process of applying a protective zinc coating by dipping the product in bath of molten zinc. Continuous galvanizing is a form of hot-dip galvanizing for steel sheets. Galvanizing is a favored method of protective coating due to its low cost, ease of application, and long maintenance-free service life. Generally applied at 830 to 870 degrees, the zinc will bond with the steel. It is the most common outdoor protective coating in use. When used on bolts, due to the thickness of the zinc coating, all galvanized nuts must be tapped oversize. Zinc coatings are primarily for protection against corrosion. Hot-dip galvanizing is a process of applying to steel a hot-dipped zinc coating in which the zinc and steel form a metallurgical bond. The thickness of a hot-dipped coating can vary from a thin zinc/iron alloy layer to heavy applications suitable for extended outdoor exposure. Inline galvanized coatings are applied during the manufacturing process of the hollow or open section, with the cleaned steel section exiting the mill and passing into the galvanizing bath. This coating is usually measured as thickness or as coating mass in grams per square meter and ranges from a minimum of about 100 g/m2 upwards, with an average around 175 g/m2.
This method involves dipping steel in a hot bath of liquid zinc. In some cases, steelmakers mix other metals, such as aluminum, antimony, cadmium, and tin, with the zinc. Manufacturers add such metals in order to improve the appearance or the protective qualities of the coating. Metal pails and similar small objects are dipped in the zinc bath and galvanized one at a time.
ASTM A123 is the specification for hot-dip galvanizing. This specification covers the standard requirements for hot-dip galvanized zinc coatings on iron and steel products made from rolled pressed and forged shapes, castings, plates, bars, and strips. This specification deals with unfabricated products and fabricated products, for example, assembled steel products, structural steel fabrications, large tubes already bent or welded before galvanizing, and wire work fabricated from uncoated steel wire.
The Process
The galvanizing process consists of three basic steps: surface preparation, galvanizing, and inspection.
SURFACE PREPARATION
Surface preparation is the most important step in the application of any coating. In most instances incorrect or inadequate surface preparation is generally the cause of a coating failing before its expected service lifetime. The surface preparation step in the galvanizing process has its own built-in means of quality control in that zinc simply will not metallurgically react with a steel surface that is not perfectly clean. Any failures or inadequacies in surface preparation will immediately be apparent when the steel is withdrawn from the molten zinc because the unclean areas will remain uncoated and immediate corrective action must be taken.
Once a job has been delivered and accepted at the galvanizer’s plant, there is one point of responsibility for ensuring that the material leaves the plant properly galvanized. That point of responsibility is the galvanizer. On-site painting or other field-applied systems of corrosion protection may involve the use of different subcontractors and/or work groups to prepare the surface and to apply the coating. Paint can only be applied under certain weather conditions. This can result in problems in coordinating activities, that lead to costly and time-consuming delays, errors, and disputes concerning responsibility and financial liability.
Surface preparation for galvanizing typically consists of three steps: caustic cleaning, acid pickling and fluxing.
Surface preparation also can be accomplished using abrasive cleaning as an alternative to or in conjunction with chemical cleaning. Abrasive cleaning is a process whereby sand, metallic shot or grit is propelled against the steel material by air blasts or rapidly rotating wheels.
The method for applying the flux depends upon whether the galvanizer uses the wet or dry galvanizing process.
In the dry galvanizing process, the steel or iron is dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. The material is then dried prior to immersion in molten zinc. In the wet galvanizing process, a blanket of liquid zinc ammonium chloride is floated on top of the molten zinc. The iron or steel being galvanized passes through the flux on its way into the molten zinc.
GALVANIZING
In this step, the material is completely immersed in a bath consisting of a minimum of 98% pure molten zinc. The bath chemistry is specified by the American Society for Testing and Materials (ASTM) in Specification B 6. The bath temperature is maintained at about 840 F (449 C).
Fabricated items are immersed in the bath until they reach bath temperature. The zinc metal then reacts with the iron on the steel surface to form a zinc/iron intermetallic alloy. The articles are withdrawn slowly from the galvanizing bath and the excess zinc is removed by draining, vibrating and/or centrifuging.
The metallurgical reactions that result in the formation and structure of the zinc/iron alloy layers continue after the articles are withdrawn from the bath, as long as these articles are near the bath temperature. The articles are cooled in either water or ambient air immediately after withdrawal from the bath.
Because the galvanizing process involves total material immersion, 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 interior corrosion protection.
Galvanizing is performed at the factory under any 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. Two- or three-day turnaround times for galvanizing are common. Galvanizing can also take up to a week or more, depending on market conditions.
INSPECTION
The two properties of the hot-dip galvanized coating that are closely scrutinized after galvanizing are coating thickness and coating appearance. A variety of simple physical and laboratory tests may be performed to determine thickness, uniformity, adherence and appearance.
Products are galvanized according to long-established, well-accepted, and approved standards of ASTM, the Canadian Standards Association (CSA), and the American Association of State Highway and Transportation Officials (AASHTO). These standards cover everything from minimum required coating thicknesses for various categories of galvanized items to the composition of the zinc metal used in the process.
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.
Physical Properties
THE METALLURGICAL BOND
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 iron in the steel to form a series of zinc/iron alloy layers. Figure 5 is a photomicrograph of a galvanized steel coating’s cross-section and shows a typical coating microstructure consisting of three alloy layers and a layer of pure metallic zinc.
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.
IMPACT AND ABRASION RESISTANCE
| Layer Name | Composition | DPN Hardness
|
| Eta | 100% Zn | 70
|
| Zeta | 94% Zn 6% Fe | 179
|
| Delta | 90% Zn 10% Fe | 244
|
| Gamma | 75% Zn 25% Fe | 250
|
| Base Steel | | 159
|
From the above table, DPN stands for Diamond Pyramid Number. 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 through abrasion. The Eta layer of the galvanized coating is quite ductile, providing the coating with some impact resistance.
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 during its service life. The toughness of the galvanized coating is extremely important since barrier protection is dependent upon coating integrity.
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 are impermeable.
If the galvanized coating is physically damaged, it will continue to provide cathodic protection to the exposed steel. If individual areas of underlying steel or iron become exposed by up to 1/4"-diameter spot, the surrounding zinc will provide these areas with cathodic protection for as long as the coating lasts.
Hot dip galvanized strut products are fabricated from steel and then completely immersed in a bath of molten zinc. A metallic bond occurs resulting in a zinc coating that completely coats all sufaces, including edges and welds. Another advantage of this method is coating thickness. Strut products that are hot dip galvanized after fabrication have a minimum thickness of 1.50 ounces per square foot on each side, or a total 3.0 ounces per square foot of steel, according to ASTM A123. The zinc thickness is controlled by the amount of time each part is immersed in the molten zinc bath as well as the speed at which it is removed. The term "double dipping" refers to parts too large to fit into the galvanizing kettle and, therefore, must be dipped one end at a time. It does not refer to extra coating thickness. The layer of zinc which bonds to steel provides a dual protection against corrosion. It protects first as an overall barrier coating. If this coating happens to be scratched or gouged, zinc's secondary defense is called upon to protect the steel by galvanic action. Hot-Dip Galvanized After Fabrication is recommended for prolonged outdoor exposure and will usually protect steel for 20 years or more in most atmospheric environments and in many industrial environments. For best results, a zinc rich paint (available from B-Line) should be applied to field cuts. The zinc rich paint will provide immediate protection for these areas and eliminate the short time period for galvanic action to “heal” the damaged coating.
Distortion and Warping
When the decision is made to hot-dip galvanize, the design engineer should ensure that the pieces can be suitably fabricated for highest-quality galvanizing. Best practice suggests steel to be galvanized should be symmetrical and of similar thickness. With proper consideration and understanding of how the hot-dip galvanizing process affects steel, asymmetrical designs or structures containing sections of unequal thickness can be successfully galvanized, as can fabrications where cold-working techniques (bending, hole-punching, rolling, shearing) are employed.
Steel being galvanized progresses through a temperature cycle upon immersion into and withdrawal from the galvanizing bath. Because parts are immersed at an angle, uneven heating occurs, creating a temperature profile along the part being galvanized. This temperature profile allows the steel’s internal stresses to be relieved at different times in the immersion cycle. These stresses may cause changes in shape and/or alignment (distortion and warping).
The following steps can be taken to minimize this risk:
▪ Where possible, use symmetrically rolled sections in preference to angle or channel frames. I-beams are preferred to angles or channels.
▪ Use parts in an assembly of equal or near equal thickness, especially at joints.
▪ Use temporary bracing or reinforcing on thin-walled and asymmetrical designs.
▪ Bend members to the largest acceptable radii to minimize local stress concentration.
▪ Accurately pre-form members of an assembly so it is not necessary to force, spring, or bend them into position during joining. Continuously weld joints using balanced welding techniques to reduce uneven thermal stresses. Pinholes from welding are very dangerous in items to be galvanized and must be avoided. Staggered welding techniques to produce a structural weld are acceptable. For staggered welding of 1/8-inch (4 mm) or lighter material, weld centers should be closer than 4 inches (10 cm).
▪ Avoid designs that require progressive-dip galvanizing. It is preferable to build assemblies and subassemblies in suitable modules so they can be immersed quickly and galvanized in a single dip. In this way, the entire fabrication can expand and contract uniformly. Where progressive dipping is required, consult your galvanizer.
Consult your galvanizer regarding the use of temporary bracing or reinforcing. The guidelines for safeguarding against warping and distortion during hot-dip galvanizing of steel assemblies are outlined in ASTM A384.
Relevant ASTM Standards
A 123/A 123M-2000 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products
A 143-74 Standard Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement
A 153/A 153M-98 Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
A 384-76 Standard Practice for Safeguarding Against Warpage and Distortion During Hot-Dip Galvanizing of Steel Assemblies
A 385-98 Standard Practice for Providing High Quality Zinc Coatings (Hot-Dip)
A 767/A 767M-97 Standard Specification for Zinc Coated (Galvanized) Steel Bars for Concrete Reinforcement
A 780-93A Specification for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings
B 6-98 Standard Specification for Zinc
D 6386-99 Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting
E 376-89 Standard Practice for Measuring Coating Thickness by Magnetic Field or Eddy-Current (Electromagnetic) Test Methods