In the temporary fence industry, galvanized steel is one of the most commonly used materials. You’ve probably seen silver-coloured galvanized fencing around your city, and if you’re in the construction business, chances are you’ve used it on a project. But did you know that there are different kinds of galvanized steel, and how it is finished can affect the durability of your fence panel? Read on to learn the difference between pre-galvanized and hot-dip galvanized steel, and why one is better for temporary fencing.
Before we get to the difference between pre-galvanizing and hot dip galvanizing, we need to understand what it means to galvanize metal. Galvanization is a process that involves applying a protective coating of zinc to metals like steel or iron. The zinc coating offers protection against rust and corrosion, effectively increasing the durability or longevity of the metal.
Although galvanization won’t prevent corrosion indefinitely, galvanized steel will last much longer than unprotected steel when exposed to the same conditions.
The difference between pre-galvanizing (or “pre-gal”) and hot-dip galvanizing lies in the process. Let’s break the two down.
When steel components are used in the fabrication of a more complex finished product, they may be galvanized before the final product is assembled. This is what is known as pre-galvanization, or “pre-gal”. It may also be called “in-line”, “continuous”, or “mill” galvanizing. Essentially, the steel components – such as sheets or wire – are galvanized using an automated process before they are cut to size.
This automated process will differ depending on the type of component being galvanized. Generally speaking, the steel components are immersed in a “galvanizing bath” of molten zinc for a short period of time. Once the steel is removed from the bath, excess zinc is removed using a mechanical wiper, air knife, or steam. This leaves a relatively thin galvanized coating of zinc.
The batch hot-dip galvanization process involves immersing the entire finished steel product, or “steelwork”, in a bath of pure liquid zinc. This ensures complete coverage of the product, including the inside surfaces of any hollow areas (such as the inside of a tube). Hot dip galvanization also typically results in a thicker zinc coating than pre-galvanization. A metallurgical bond forms between the steel product and the zinc layer, providing protection against corrosion.
If you search for images of hot dipped galvanized steel, you’ll likely find photos of steel products with a unique surface pattern known as spangle. Spangling can develop when the molten zinc, once adhered to the steel, cools below its melting point. Once it reaches this temperature, the randomly arranged atoms of the liquid zinc begin to arrange themselves into an orderly, crystalline pattern.
Spangle can also be caused by trace metals either deliberately added to the liquid zinc, or unintentionally introduced to the galvanizing bath through the process of hot dipping. The idea that spangling is caused by a tainted or “dirty” galvanizing bath has led to the perception that galvanized metals with spangle are inferior than those without. However, spangle is not a reliable indication of the integrity of most galvanized steel products.
The winner in the pre-galvanized vs. hot-dip galvanized debate really depends on your unique requirements and the intended use of the steel. At Modu-Loc, we offer galvanized temporary fence panels for a variety of applications, including construction, special events, outdoor retail and storage spaces, and more.
When it comes to temporary fencing, which is fabricated from several different steel components and is expected to hold up to inclement weather, hot-dip galvanized steel is preferred. Hot-dip galvanized steel is best for temporary fence panels for a couple of reasons.
Dipping the entire steel fence panel in a hot zinc bath ensures that every nook and cranny is coated – including the areas most vulnerable to corrosion. For instance, molten zinc enters the steel tubes which form the frame of the fence, coating them inside and out. This is key as the square tubes experience a lot of wear and tear from the constant insertion and removal of fence bases. Hot dipping the finished fence panel also ensures all of the welds are coated, offering additional protection.
Compare this to the use of pre-galvanized components in a temporary fence structure. If pre-galvanized steel tubes are cut to length during fabrication of the fence panel, this results in an uncoated surface at the cut end which would be prone to corrosion. Similarly, the welds that hold any pre-galvanized wire mesh the fence frame would be unprotected.
As mentioned, pre-galvanized components are usually only immersed in the galvanizing bath for a very short time, resulting in a relatively thin coating. The thicker zinc coating produced by hot dip galvanization offers improved rust and corrosion protection versus pre-galvanization. This is an important quality in a temporary fence panel, as it is most often used outdoors and exposed to snow, ice, rain, and salt.
When steel is hot dip galvanized, the resulting zinc coating is actually comprised of four layers. The topmost layer is the pure zinc coating. The remaining three layers are alloys of zinc and iron, which are harder than the base steel and form a metallurgical bond between the steel and the zinc. This means they become an integral part of the steel instead of just a coating.
This metallurgical bond created by the process of hot dip galvanization holds up better to abrasion than the thin coating created by pre-galvanization. This is crucial given how much abuse and wear and tear a fence panel endures throughout its lifecycle as a rental item. A temporary fence panel is subjected to a significant amount of abrasion through transport, installation, and removal.
This improved durability and longevity is why Modu-Loc chooses to offer hot-dip galvanized steel fence over other kinds of galvanized fence. While we continue to offer our standard steel temporary fence with green powder coating, we have added hot-dip galvanized fence to our inventory for its improved corrosion resistance, especially for those markets that experience severe winter weather.
If you’re someone who rents temporary fencing for short periods of time, the long-term durability of the fence may be of no consequence to you. However, if you need the fence for a particularly long project, you’ll want to consider the galvanization process used on the fence. And if you are planning to buy temporary fencing, you should look for fence that is hot-dip galvanized to ensure optimum durability and longevity.
The following is a list of frequently asked questions about hot-dip galvanizing. Click on the question to be taken to the answer listed further down on the page. If you do not see your question listed here, try searching using the links above, the search engine on the top right of the page, or contact the AGA for assistance.
Zinc metal used in the galvanizing process provides an impervious barrier between the steel substrate and corrosive elements in the atmosphere. It does not allow moisture and corrosive chlorides and sulfides to attack the steel. Zinc is more importantly anodic to steel – meaning it will corrode before the steel, until the zinc is entirely consumed.
There are four steps:
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When compared with paint systems, hot-dip galvanizing after fabrication has comparable initial application costs and, almost always, lower life-cycle costs. In fact, the lower life-cycle costs of a hot-dip galvanized project make galvanizing the smart choice for today and tomorrow.
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Hot-dip galvanized steel resists corrosion in numerous environments extremely well. It is not uncommon for galvanized steel to last more than 70 years under certain conditions. To get a good idea of how long your project will last, see the service-life chart.
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The three intermetallic layers that form during the galvanizing process are all harder than the substrate steel and have excellent abrasion resistance.
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Zinc on newly galvanized steel is very reactive and wants to form zinc oxide and zinc hydroxide corrosion products that eventually become the stable zinc carbonate. When galvanized steel is tightly stacked or stored in wet boxes that don’t allow for free flowing air, the zinc forms excessive layers of zinc hydroxide, otherwise known as wet storage stain. Most wet storage stain can be easily removed with a cleaner or nylon brush. To prevent wet storage stain, store galvanized steel indoors or block it so that there is ample free flowing air between each galvanized article.
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The steel chemistry is the primary determinant of galvanized coating thickness and appearance. Continuously cast steel produced by the steel companies has a wide variety of chemistries, thus the different coating appearances.
There are several different additives that galvanizers may put in their zinc kettle to enhance the coating appearance by making it shiny, spangled or matte gray. The appearance of the coating (matte gray, shiny, spangled) does nothing to change the corrosion protection of the zinc coating.
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Constant exposure to temperatures below 390F (200C) is a perfectly acceptable environment for hot-dip galvanized steel. Good performance can also be obtained when hot-dip galvanized steel is exposed to temperatures above 390F (200C) on an intermittent basis.
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Called duplex coatings, zinc and paint in combination (synergistic effect) produce a corrosion protection approximately 2X the sum of the corrosion protection that each alone would provide. Additionally, duplex coatings make for easy repainting, excellent safety marking systems, and good color-coding. Painting over galvanized steel that has been in service for many years also extends the life of the zinc coating.
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Structural steel (plate, wide-flange beams, angles, channels, pipe, tubing) are galvanized to ASTM A 123/A 123M. Fasteners and small parts that fit into a centrifuging basket are galvanized to ASTM A 153/A 153M. Reinforcing steel is galvanized to ASTM A 767/A 767M.
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Depending on the product mix, square feet per ton, and condition of the steel surface, galvanizing is often less expensive on an initial cost basis. However, as with any purchase, the lifetime costs should be considered when making a project decision on the corrosion prevention system to utilize. And, with galvanizing, the life cycle cost, i.e. the cost per year to maintain, is almost always less than a paint system. Paint systems require maintenance, partial repainting and full repainting several times over a 30-year project life. The costs can be staggering, making the decision to paint a costly one in the long run.
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Galvanizers can progressively dip such a fabrication or article of steel. They dip one half in the molten zinc bath, remove it, turn it around or over and immerse the other half in the zinc. This method is often erroneously referred to as ‘double dipping’.
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Hot-dip fasteners generally have about 10 times as much zinc on the surface and are suitable for use in all exterior and interior applications. Zinc-plated fasteners will provide a disappointing performance if used outside, especially when used to connect hot-dip galvanized structural steel members.
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The corrosion rate of zinc and how long it will provide protection is a function of the coating thickness and the amount of corrosive elements in the atmosphere. For example, in rural settings where there is less automotive/truck exhaust and plant emissions, galvanized steel can easily last for 100 – 150 years without maintenance. Industrial and marine locations contain significantly more aggressive corrosion elements such as chlorides and sulfides and galvanized steel may last for 50 – 100 years in those cases. The relationship between coating thickness and atmospheric conditions is contained in a popular graph developed by the AGA. Please see the publication Hot-Dip Galvanizing for Corrosion Protection: A Specifier’s Guide on this web site.
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Yes. Specifically, fabricated steel must allow for easy flow of the cleaning chemicals and molten zinc metal over and through it. This means that gussets must be cropped, holes put in the proper location for draining and venting of zinc from tubular configurations, weld flux removed, overlapping surfaces must be seal-welded, and light gauge material temporarily braced. The details of design and fabrication are contained in the AGA publication The Design of Products to be Hot-dip Galvanized After Fabrication, found on this web site.
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First of all, the variety of things galvanized is broad. Structural steel (angles, channels, wide-flange beams, I-beams, H-beams), grating, expanded metal, corrugated sheets, wire, cables, plate, castings, tubing, pipe, bolts & nuts. The industries that utilized hot-dip galvanized steel range from bridge & highway (reinforcing steel for decks and column concrete, girders, stringers, light and signposts, guardrail, fencing), water & wastewater treatment plants (walkway grating/expanded metal, handrails) architectural (facades, exposed structural steel, lentils), parking garages (reinforcing steel for concrete decks, exposed structural steel columns and barriers), pulp & paper plants (structural steel, walkways, handrail), OEMs (motor housings, electrical cabinets, frames, heat exchanger coils), electrical utilities (transmission towers, distribution poles, substations, wind turbine poles), communication (cell towers), rail transportation (poles, switchgear, miscellaneous hardware), chemical/petro-chemical (pipeline hardware, manufacturing buildings, storage tanks, walkways), recreation (boat trailers, stadiums, arenas, racetracks), and many more.
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The hot-dip galvanizing process can accommodate various different shapes and sizes of steel. Kettle sizes vary in dimensions from one galvanizer to the next. You can view the online listing of all the galvanizers in North America and their kettle sizes.
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Numerous different fabrications for a variety of applications are galvanized each year. To view a list of the different types of products that have been hot-dip galvanized click here.
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The galvanized coating appearance may either be bright and shiny resulting from the presence of an outer layer of pure zinc, or duller, matte gray as the result of the coating’s intermetallic layers being exposed. Performance is not affected. Coating appearance depends on the amount of zinc in the coating.
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Coating thickness depends on the thickness, roughness, chemistry, and design of the steel being galvanized. Any or all of these factors could produce galvanized coatings of non-uniform thickness. Members of the American Galvanizers Association galvanize to ASTM standards, which define minimum average coating thickness grades for various material categories.
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Minimizing potential warpage and distortion is easily done in the project’s design stages by selecting steel of equal thicknesses for use in every separate subassembly that is to be hot-dip galvanized, using symmetrical designs whenever possible, and by avoiding the use of light-gage steel (<1/16” / 1.6 mm). Some structures may benefit from the use of temporary bracing to help maintain their shape and/or alignment.
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Galvanized coatings can be easily and effectively painted, not only for aesthetics but also to extend the structure’s service life. The age and extent of weathering of the galvanized coating dictate the extent of surface preparation required to produce a quality paint system over galvanized steel. ASTM D , Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting, should be consulted for suggested surface preparation methods for galvanized coatings of varying ages.
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As an average, the weight of the article will increase by about 3.5% due to zinc picked up in the galvanizing process. However, that figure can vary greatly based on numerous factors. The fabrication’s shape, size, and steel chemistry all play a major role, and other factors like the black weight, the different types of steel that get welded together, and the galvanizing bath chemistry can also have an effect.
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When galvanized parts are used for slip-critical connections, they must either be brushed, abrasive blasted, or painted with zinc-silicate paint to increase the surface roughness and, thus, the slip factor.
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Rebar is commonly fabricated after galvanizing. In order to minimize the possibility for coating damage, avoid bending the rebar at a radius of more than 8 times its radius. ASTM A 767, Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement, has a table that provides maximum bend diameters for various-sized rebar.
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No, the steel chemistry and surface condition are the primary determinants of zinc coating thickness. Leaving the steel in the molten zinc a little longer than optimal may have one of two effects: 1) it may increase the coating thickness, but only marginally; 2) it may significantly increase the coating thickness and cause a brittle coating.
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“Double-dipping” is the progressive dipping of steel that is too large to fit into the kettle in a single dip. Double-dipping cannot be used to produce a thicker hot-dip galvanized coating.
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The primary reason for vent holes is to allow otherwise trapped air and gases to escape; the primary reason for drain holes is to allow cleaning solutions and molten zinc metal to flow entirely into, over, and throughout the part, and then back into the tank or kettle.
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When stitch-welding is used, there is a possibility of gas release between gaps, which will prevent the galvanized coating from forming in these areas. By leaving at least a 3/32” (2.4 mm) gap between the contacting surfaces, gases are allowed to escape and cleaning solutions and molten zinc are allowed to flow in between the surfaces for a complete and uniform coating.
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“White rust” is the term mistakenly applied to wet storage stain, which actually is a milder corrosion product than white rust. Wet storage stain can be avoided by properly stacking freshly galvanized articles, avoiding unprotected exposure to wet or humid climates, or by using a surface passivation treatment after galvanizing. Wet storage stain typically weathers away once the part is in service. (True “white rust” is most commonly associated with galvanized cooling towers.)
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Yes, but because masking or stop-off materials may not be 100% effective, contact your galvanizer for suggestions.
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There are no known studies to suggest zinc corrosion products cause any harm to the environment. Zinc is a naturally occurring element (25th most abundant element in the earth), and necessary for all organisms to live. It is a recommended part of our diet (RDA 15 mg) and necessary for reproduction. It is used in baby ointments, vitamins, surgical instruments, sunscreens and cold lozenges.
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Zinc is a noble metal and will sacrifice itself (i.e. corrode, give up its electrons and create a bi-metallic couple) to protect most metals. So, it is recommended to insulate galvanized steel so that it doesn’t come in direct contact with dissimilar metals. Rubber or plastic, both non-conductive, are often used to provide this insulation.
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The process steps are similar but the production equipment is very different. After fabrication galvanizing is a more manual process where structural steel (fabricated plate, wide-flange beams, angles, channels, tube, pipe, fasteners) is suspended by wire, chain or hook from crane hoists and immersed in the cleaning solutions and zinc. Continuous sheet galvanizing involves uncoiling sheet, passing it through the cleaning steps and molten zinc bath at speeds up to 500 feet per minute, drying and recoiling.
The uses of after-fabrication galvanized steel are usually exterior in nature because the zinc coating is relatively thick (3.0 – 6 mils, 75 – 150 microns, 1.7 to 3.6 oz/sq. ft.) and will protect steel from corrosion in most atmospheric conditions for 50 to 100 years. Galvanized sheet is suitable for interior applications because of the relatively thin coating (0.45 oz on each side), unless it is painted after galvanizing.
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G90 is a grade of galvanized sheet produced to ASTM A653. It has 0.90 oz/sq. ft. of zinc overall or 0.45 oz/sq. ft. per side. A60 is also a grade, has 0.30 oz/sq. ft. per side, and has been annealed after galvanizing to produce a surface that promotes good adhesion of paint.
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In order for zinc to develop its protective patina of zinc carbonate that is very stable and non-reactive, it requires a wetting and drying cycle like that produced by nature. Salt spray tests keep the zinc wet and essentially wash the zinc corrosion products off as they develop, inflating the corrosion rate of zinc. This lab test is not reflective of real-world performance of zinc coatings.
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Constant exposure to temperatures below 390 F (200 C) is a perfectly acceptable environment for hot-dip galvanized steel. Good performance can also be obtained when hot-dip galvanized steel is exposed to temperatures above 390 F (200 C) on an intermittent basis.
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No, the steel chemistry and surface condition are the primary determinants of zinc coating thickness. Leaving the steel in the molten zinc a little longer than optimal may have one of two effects: 1) it may increase the coating thickness, but only marginally; 2) it may significantly increase the coating thickness and cause a brittle coating.
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There is no such thing as cold galvanizing. The term is often used in reference to painting with zinc-rich paint. Galvanizing by definition means a metallurgical reaction between zinc and iron to create a bond between the zinc and the steel of approximately psi. There is no such reaction when zinc-rich paints are applied and the bond strength is only several hundred psi.
If you want to learn more, please visit our website Hot-Dip Galvanized Welded Steel Pipe.