Structural steel products after having been hot-dip galvanized for corrosion protection.

There are many different types of steel in use today, the most common of which by far is carbon steel. And clean carbon steel is black. Not grey. Not silver. Not blue. Not shiny. Black. Most people think steel is “grey”! Here’s why…

Steel is not a metal on the periodic table like gold, silver, or nickel. But it is an alloy made mostly of iron (around 95%), which is why it is naturally black (think of wrought iron). And the remaining 5% is a mixture of carbon, nickel, silicon, and/or other elements that affect its hardness, malleability, and other properties. See Figure 1 for steel products in their natural colour.

Figure 1: Bundles of steel pipe (left) and a close up of reinforcing steel (right) showing the natural colour of steel — black.

As awesome as steel is, when exposed to the elements and road maintenance chemicals, it tends to corrode (rust), which is why there are several different corrosion-resistant treatments for steel, such as paints, epoxies, and other coatings. The most common treatment for everyday outdoor steel is something called hot-dipped galvanizing. This treatment coats the steel in pure zinc, which makes it shiny or grey and highly corrosion-resistant.

Most of the steel you see on the streets and highways is hot-dipped galvanized (see Figures 2 and 3).

Figure 2: Traffic light poles, transmission towers, chain link fences are all common outdoor steel fabrications that are almost always galvanized for weather resistance and corrosion protection.

Figure 3: Guardrail, handrail, light fixtures, baseball diamonds, almost every piece of outdoor hardware made of metal is galvanized for longevity, reducing maintenance costs. Galvanized steel is all around us, everywhere!

What makes the hot-dipped galvanizing process better than paint is that paint is a coating applied to the surface of the steel. Although this provides some level of protection, it can be chipped, exposing the steel to the elements and allowing rust to develop, and once it starts, it doesn’t stop. Galvanizing, on the other hand, embeds the zinc into the steel. The process involves dipping the steel product into a bath of molten zinc, coating the steel with zinc, which is highly corrosion-resistant. It also creates a layer of steel-zinc alloy embedded into the surface of the steel — imagine the steel absorbing the corrosion resistant zinc into itself. This is called galvanizing. Corrosion protection and zinc galvanizing are sciences unto themselves, so let’s just focus on how the process works.

Welcome to AZZ Galvanizing’s facility in Acton, Ontario. AZZ is the largest galvanizing company in North America with facilities across Canada and the U.S. They process tens of thousands of tons of steel every year in this local facility alone. See Figure 4.

Figure 4: The steel is shipped by truck to AZZ from steel fabricators across Ontario to be galvanized.

The hot-dip galvanizing process requires the steel to be clean of rust, oil, grease, dust, paint markings, welding slag, and any other debris that may affect the galvanizing process. For this, the steel is inspected and brushed clean as needed before being brought into the facility to start the process (see Figure 5).

Figure 5: The steel is inspected by hand for anything that may need to be cleaned off by grinding, sandblasting, or other mechanical means.


After the manual cleaning, the steel is sent through a series of open tanks removing other impurities that can impede the galvanizing process. The tanks contain different solutions for preparing the steel’s surface through caustic cleaning (removal of organic impurities, such as dirt grease and oil), acid pickling (removal of scale and rust), and fluxing (priming the steel to receive the final zinc galvanizing). See Figure 6.

Figure 6: The facility is equipped with several overhead cranes (left) to handle the steel products and to lower them into the different tanks for the cleaning process starting with the caustic alkali tank to remove organic impurities on the steel. The second step involves submerging the steel in a hydrochloric acid solution to remove scale and rust (right).

After rinsing the steel in water, it is then primed in a tank of diluted zinc ammonium chloride, which coats the steel, preventing further rust from developing, and primes it for receiving its final zinc coating. Next stop: the galvanizing tank. In this step, the steel is submerged in a bath of almost pure molten zinc heated to 840oF. The zinc metal reacts with the iron on the steel surface to form a zinc-iron alloy coating embedded into the steel surface, making it unremovable — unlike paint. This alloy coating will protect the steel for decades (see Figure 7 and 8).

Figure 7: The capacity of a galvanizing facility is governed by the size of its tank holding the zinc, called a “kettle.” AZZ’s facility in Acton, Ontario is 46 feet long, seen here full of molten zinc kept at 840oF. The technician here is clearing any byproducts floating on the surface generated during the galvanizing process,
Figure 8: The operator lowering a bundle of steel bars into the kettle for galvanizing (left). A rack full of highway guardrail segments cooling down right after a hot dip in the kettle (right).

A galvanizing facility goes through a lot of zinc, adding over 5% to the weight of all the steel that gets galvanized. The zinc is delivered in large blocks weighing around 2400 lb. A small percentage of nickel is also added to the mix in the kettle to help stabilize the chemical reaction with the steel fabrications getting galvanized. See Figure 9.

Figure 9: Individual 2400 lb zinc blocks ready to be added and melted into the kettle (left). Nickel is added in smaller amounts as needed (right).

Almost any type of steel product can be galvanized, as long as it can fit in the kettle. See Figures 10 and 11.

Figure 10: Before and after — steel trusses for pre-engineered buildings delivered to AZZ’s galvanizing facility (left). The same trusses after galvanizing, banded together ready for shipping back to the builder (right).
Figure 11: Galvanized steel products ready to be shipped back to AZZ’s clients.

The zinc on the outside of the galvanized steel over time reacts with the oxygen and carbon dioxide in the air to form zinc carbonate (which gives it that dull grey colour). Think of zinc carbonate as the steel’s bullet proof vest against corrosion. So, the grey colour is the zinc galvanizing, protecting the black steel from corrosion. It might not last forever, but most of it will outlast you and me!

And a special thanks to Duane Bickers, plant manager at AZZ Galvanizing in Acton, Ontario, for taking the time out of a busy day to give me a tour of his operations, contributing to the protection and longevity of countless steel products for many communities across Ontario!

Have you seen an interesting building or piece of infrastructure in or around Southern Ontario that you’d like Eric Chiasson, your personal engineer, to write about?

Send us your suggestions, comments, or questions to articles@local-news.ca and we’ll see what Eric can find out!

Sources:
AZZ Galvanizing website. Url: https://www.azz.com/galvanizing/ (access Sept. 12 , 2022)

Galvanize It: Online Seminar. Url: https://galvanizeit.org/inspection-course/galvanizing-process (accessed Sept. 12, 2022).

For more information on the author: https://www.linkedin.com/in/eric-chiasson-10601082