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Pre-Galvanized vs. Hot-Dip Galvanized Steel: Which is Better?

May 12, 2021

Galvanized temporary fencing surrounds a construction site.

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.

What is Galvanization?

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.

What’s the Difference Between Pre-Galvanized and Hot-Dip Galvanized Steel?

The difference between pre-galvanizing (or “pre-gal”) and hot-dip galvanizing lies in the process. Let’s break the two down.

Pre-Galvanization

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.

Hot-Dip Galvanization

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.

A close-up photo shows the crystalline spangle pattern visible on a hot-dip galvanized steel fence panel

A silver powder-coated fence panel is seen on the left. On the right, another fence panel shows the spangle pattern common in hot-dip galvanized steel.

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.

Which is Better: Pre-Galvanized or Hot-Dip Galvanized Steel?

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.

Coating Continuity

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.

Coating Thickness

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.

Metallurgical Bond

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.

Hot-Dip Galvanized Steel and Temporary Fencing

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.

Author: Joanna Bieda

Joanna Bieda is the Director of Marketing and Communications at Modu-Loc Fence Rentals, and has been with the company since 2014. She loves writing and is a self-professed data nerd. She thoroughly enjoys teaching customers about all things fence via Modu-Loc's blog.
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