A Deep Dive Into the Corrosion Prevention Capabilities of Hot-Dip Galvanizing
Peer Perspective: Alana Fossa, senior corrosion engineer with the American Galvanizers Association
Q: Tell us a little bit about your role and a little bit about your background.
A: I’m the senior corrosion engineer for the American Galvanizers Association (AGA). My job is twofold. The primary part of my role is helping architects, engineers, DOTs, other types of specifiers, and even fabricators and galvanizers with any sort of technical issues they may have with using galvanized steel. I manage a hotline at the AGA where specifiers can call in and we help them with questions they might have about specifying galvanized steel, the performance of galvanized steel, and then any sort of processing needs or design help. We really want to make their job of specifying and designing with hot-dip galvanized steel easier.
The second part of my role is helping with the management of studies and research programs that the AGA does related to the performance of hot-dip galvanizing, the applications of it and the processing of the galvanized steel. Within that, I participate in different committees and standard organizations that deal with specifications for hot-dip galvanizing. I’m the vice chair of the ASTM standard subcommittee called A05.13, and this subcommittee manages a lot of the specifications related to hot-dip galvanizing.
Q: Could you talk to us a little bit about the basic galvanizing process for HSS members and how that works?
A: When the parts get to the galvanizing facility, if it is HSS material, the very first thing they do is inspect that material and make sure that it’s properly vented for immersion in the galvanizing kettle. The galvanizer will go in and determine if there are any areas where any essential fabrication steps were missed, check for any places that zinc might build up, or maybe there’s a bare spot that could be left uncoated if it were to be immersed as is.
Then the parts are going to be hung on lifting frames, suspended, and placed in a series of cleaning or pretreatment tanks. We want to make sure that the steel is perfectly clean before it enters the galvanizing bath. If any part of the surface is not clean, it does not get galvanized. It goes through a decreasing step that’s typically a caustic solution that removes dirt, oils and other types of organic residues. It then goes through a dilute acid pickling step using a dilute hydrochloric or sulfuric acid solution. That removes mill scale and surface oxides, like rust. Then it goes through a flux pretreatment tank. That’s a solution containing a mixture of ammonium chloride and zinc chloride that further cleans the steel and also puts on a protective layer to prevent any oxidation from occurring.
Once you’re at the actual galvanizing step of the process, this is when you have material fully immersed in a bath of at least 98% pure molten zinc. The temperature is typically somewhere between 820 to 850 degrees Fahrenheit. When we’re immersing the kettle, the zinc is going to react with the iron in the HSS members to form a metallurgical reaction that creates the coating. As the parts are withdrawn from the bath, that coating is complete.
Q: Are there any adverse effects to galvanizing steel members, specifically HSS, that engineers or architects should know about?
A: I wouldn’t call them adverse effects; I would just say there are some materials and design and fabrication methods that could lead to steel members that are susceptible to a couple of different concerns. The first one is designing HSS with proper venting and drainage. The size and placement of drainage and vent holes are critical to any sort of hollow piece that’s going to be galvanized. That molten zinc is extremely dense — five times denser than maple syrup. We need to have holes in this product to have the zinc enter hollow parts of these structures so that it can successfully be immersed and withdrawn from the bath and not cause any sort of aesthetic or quality issues. On top of that, the vent and drain holes are also important because if you were just to take a hollow item with an air pocket that can’t be accessed because it’s been welded or sealed shut, if this enters the galvanizing bath, that air will superheat and expand and can cause an explosion in the galvanizing kettle. Obviously, that can cause damage your parts, but it’s also a safety issue at the galvanizing plant that can lead to serious injury and damage to the equipment at the galvanizing plant. That’s why those careful inspections are crucial.
Another issue that comes up is liquid metal assisted cracking. There are a number of steps that we utilize in the galvanizing process to minimize our risk, but liquid metal assisted cracking is very infrequent and not considered a root cause of cracking during galvanizing. It’s a challenging topic because there are additives in some zinc baths that are known to be associated with cracking of HSS members or other types of products. However, it’s a complicated and multivariable issue. To protect against liquid metal assisted cracking, specifiers should follow the practices of ASTM A384 and A143 to avoid distortion and embrittlement while galvanizers can minimize or avoid the use of tin in the galvanizing bath.
Finally, ASTM A500 square and rectangular HSS have an inside corner radius below the recommended value of 3T used to avoid strain-age embrittlement when galvanizing. These HSS members are successfully galvanized all the time, but the tight corner radii can lead to corner cracking in a small percentage of the parts. Increased visual inspection of the corners after galvanizing is a helpful method to identify HSS members that need repair. You can also perform a heat treatment before galvanizing to relieve internal stresses from bending to minimize this concern. The good news is ASTM A1085 increased the lower bound bend radius, so corner cracking of round and rectangular HSS is much less frequent and the need for heat treatment is much less common.
Q: Why do you think it’s so important to have information on galvanizing and zinc coating so widely available in a format that’s digestible for people?
A: Hot-dip galvanizing has been a proven corrosion protection system for over 150 years, and there are still so many people who are not familiar with it. The AGA has access to over 100 years of corrosion data, and the amount of research that has gone into solving the technical issues or concerns people have related to hot-dip galvanizing is astounding. It’s very important to convey this wealth of knowledge in that digestible format because at the end of the day, this knowledge, the whole point of it, is to save engineers, architects, fabricators and even the galvanizer time, money and potential headaches related to the use of these materials. And over time, that makes specifying hot-dip galvanizing much more cost-effective and easy.
I truly believe in hot-dip galvanizing as a sustainable construction material. It’s recyclable, it’s sustainable and it lasts for generations. And we see firsthand the solutions it provides in terms of these properties. There are possibilities for reuse. We can strip and regalvanize existing steel products. It lasts for decades and decades — over 100 years in some environments. We can really have a positive impact on our communities for many decades with this knowledge without burdening future generations.
Q: Looking at some key differences between painting, galvanizing and metallizing, when might one of these processes be preferred over another?
A: With paint, we have a mechanically bonded surface coating. It protects steel by providing a barrier between the steel and the environment, so if the steel is not exposed to the environment, then rust cannot occur. Hot-dip galvanizing also provides barrier protection, but that coating is applied through a metallurgical reaction, and that essentially makes the coating a part of the steel itself. With zinc metallizing, it’s a spray application of semi-molten zinc droplets onto the surface of blast-cleaned steel and applied at any thickness. Hot-dip galvanizing and zinc metallizing also provide cathodic protection where the zinc coating preferentially corrodes to protect the base steel. These two coatings also develop a passive layer of corrosion products on the surface called the “zinc patina,” which slows down the zinc corrosion rate to achieve decades of maintenance-free longevity. Because hot-dip galvanizing is an immersion process, we get full coating of the interior of any sort of surface like HSS, while metallizing and paint are not going to be able to enter the internal surfaces of many tubular products.
I would say when we look at one over the other, cost and the application environment are going to be factors for any type of project. Metallizing is more expensive than hot-dip galvanizing, but it can be applied at any coating thickness to increase longevity and is used for steels parts which cannot be successfully galvanized. Hot-dip galvanizing, depending on the paint system used, is either comparable in terms of cost or slightly more expensive than an industrial or marine grade paint system. Some paint formulations have good resistance to chemical or marine environments in comparison to hot-dip galvanizing, while hot-dip galvanizing gives you greater abrasion resistance, durability and longevity in many atmospheric environments.
Q: Can you give a quick overview of best design practices for galvanized HSS?
A: There’s a list of standards that have been established but a lot of specifiers are not aware that these standards exist. They know the primary standard for galvanizing is A123, but they don’t realize that there are a lot of standards referenced within that govern design and fabrication best practices. One of these is ASTM A385, which has design and fabrication best practices that will help you achieve high-quality galvanized coatings. This is where we see the vent and drain hole sizes and placement and information on recommended steel chemistry for galvanizing.
Specifiers should also be aware of ASTM A143, which identifies methods to avoid embrittlement, and ASTM A384, which contains industry practices to minimize or avoid warpage and distortion. There is also a design guide from the AGA called “Design of Products to Be Hot-Dip Galvanized After Fabrication.” It summarizes and gives more pictures and examples of all the information contained in those three standards I just mentioned — 385, 143 and 384.
With the AGA, I worked hard to publish this guide in 2019. It’s called “Zinc and Dissimilar Metals in Contact.” It has a chart you can look at where you look at different metals in direct contact with zinc, look at the type of service environment, and look at the surface area of zinc compared to the other metal — the zinc-to-metal ratio — and a color map will give you an idea of whether there is a risk of galvanic corrosion on your project.
Q: What is your favorite or most notable project when working with galvanized HSS?
A: One of my favorite HSS projects is the Salvador Dali Museum in St. Petersburg, Florida. This museum has a beautiful glass structure called the Enigma, and it extends out from the side of the museum. It’s really a framework of HSS that was hot-dip galvanized and then also powder coated to blend in with the rest of the building. They were concerned about corrosion protection because they’re in a coastal environment. The powder coating not only helps the HSS blend aesthetically with the building but it also gives an additional layer of protection against environmental factors.
The good news about powder coating or painting over galvanizing is that there is a synergy between these two coatings that actually allows them to last 50% longer than they would over that same structure if it was just painted over steel. So that is a unique benefit that we call the synergistic effect.
This project shows an innovative way to use hot-dip galvanizing in a corrosive environment, but there’s also a large aesthetic requirement with this project. People think that, “Oh, galvanizing, I’m just going to get all gray.” That’s not true. There’s really a lot of options if you want corrosion protection, but you also want a different type of aesthetic; you’re not limited with galvanizing.
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