by Alana Hochstein
Structural steel has been the material of choice in the building market for decades because of the numerous benefits it provides. While steel offers an effective and efficient framing system for the building envelope, the hot-dip galvanizing of both interior and exterior elements is one method to provide a durable and maintenance-free corrosion protection system for generations.
Around the world, hot-dip galvanized steel is considered aesthetically pleasing, and the contemporary, industrial appearance of hot-dip galvanized steel is rapidly becoming a popular architectural choice. Further, hot-dip galvanizing’s recyclability, cost effectiveness, and low environmental impacts make it a sustainable choice today and for the future.
Batch hot-dip galvanizing
Batch hot-dip galvanizing is the process of dipping fabricated steel into a kettle containing molten zinc. While the steel is in the kettle, the iron in the steel metallurgically reacts with the molten zinc to form a tightly bonded alloy coating, thereby providing corrosion protection to the steel. Continuous hot-dip galvanizing (pre-galvanized sheet products) and batch hot-dip galvanizing are often confused because they are both “hot-dip” products with a soft gray or shiny metallic appearance, but the two differ in terms of coating thickness, longevity, and types of product able to be coated using each process. Continuous hot-dip galvanizing provides a smooth and relatively thin coating, and can be specified for products such as sheet, pipe, and tube. These products are produced and galvanized on continuous production lines and then fabricated into final products (e.g. corrugated roofing and siding, duct work, and culvert pipe). Batch hot-dip galvanizing is performed after fabrication by immersing steel products (e.g. poles, beams, plate, frames, and other assemblies) into cleaning solutions and zinc.
The metallurgical bond created during the hot-dip galvanizing process provides a number of benefits. Not only does the galvanizing process create a barrier coating resistant to abrasion because it is harder than the substrate steel, but it also protects the steel cathodically. Unique in comparison to concrete, wood, stone, or corrosion-resistant steels, this cathodic protection property means the zinc sacrificially corrodes to protect exposed base steel.
Another aspect of hot-dip galvanizing differentiating it from other building materials is its longevity in atmospheric environments. Using real-world corrosion data, it is possible to estimate the life of hot-dip galvanized coatings in atmospheric conditions by referring to the time-to-first maintenance (TFM) chart (Figure 1). Time-to-first maintenance has a linear relationship to the hot-dip galvanized coating thickness, and is defined as 5 percent rusting of the base steel surface, meaning 95 percent of the zinc coating is still intact. For example, structural steel pieces 6.4 mm (0.25 in.) thick or greater are required to have at least 100 µm (3.9 mils) of coating thickness, according to the primary batch hot-dip galvanizing specification ASTM A123, Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products. As the TFM chart shows, this would equate to a minimum of 72 years before maintenance in an industrial environment, or greater for marine and suburban atmospheric environments.1
Hot-dip galvanizing also provides environmental and economic benefits to achieve a sustainable design. The primary components of hot-dip galvanizing—zinc and steel—are natural, abundant, and safe. Since the coating is the result of a metallurgical reaction between steel and zinc, the coating does not contain volatile organic compounds (VOCs), and the only products introduced into soil and water environments are natural zinc corrosion products.2 Though taking in excess zinc is possible, a number of studies show the small amounts of additional zinc from hot-dip galvanizing added to the environment over decades of service is not enough to harm any organisms or exceed criterion levels.3 Both zinc and steel are also 100 percent recyclable without the loss of any physical or chemical properties.4 This means rather than being down-cycled into other, lesser products, zinc and steel can be reused as zinc and steel building products again and again without compromising their integrity.5 Further, steel is the most recycled material in the world with 98 percent of all steel currently being recycled, and zinc has a high reclamation rate of 80 percent overall in North America and 95 percent in building projects.6
For Leadership in Energy and Environmental Design (LEED) projects, the American Galvanizers Association (AGA) has published an industry wide (generic) environmental product declaration (EPD) for hot-dip galvanized steel including hot-rolled sections, plate, and hollow structural sections (HSS). The EPD can be obtained from Underwriters Laboratories (UL) Environment or through the website galvanizeit.org/epd. Additionally, health product declarations (HPDs) are available based on the type of zinc used by the galvanizer. HPDs for high grade/special high grade or prime western grade zinc can be found through HPD Collaborative, or downloaded from galvanizeit.org/hpd.
In addition to being environmentally friendly, sustainable building structures must also be economically responsible for future generations. Hot-dip galvanized steel can provide economic savings both initially and throughout the life of a project, freeing up money for new construction rather than costly maintenance. Although often competitive on an initial basis with other building materials and steel corrosion protection systems, the low life-cycle cost of hot-dip galvanizing provides significant cost savings due to minimal or zero maintenance over the design life.7 The AGA life-cycle cost calculator is available at lccc.galvanizeit.org to input specific project details and analyze the initial and life-cycle cost of hot-dip galvanizing in comparison to more than 30 other corrosion protection systems, based on published cost data and calculated in accordance with ASTM A1068, Standard Practice for Life-Cycle Cost Analysis of Corrosion Protection Systems on Iron and Steel Products.8
Although traditionally hot-dip galvanizing is selected for durable and sustainable corrosion protection, the functional use of hot-dip galvanized steel can also allow a greater amount of design freedom and become part of the exposed beauty of a project. Steel has a high strength-to-weight ratio, meaning structural steel requires less extensive and costly foundations in comparison to concrete designs and other heavy materials.9 Specifically, the American Institute of Steel Construction (AISC) estimates approximately 0.9 t (1 ton) of steel provides the same strength as 7.2 t (8 ton) of concrete.10 Requiring less supports, steel designs are often more open and provide endless options as the lightness of steel members, and the ability to easily curve, arch, and manipulate the pieces, can be used to achieve visual interest. These design elements are evident in structures like the Cincinnati Children’s Hospital Parking Garage, Ohio (Figure 2), where the exposed hot-dip galvanized bracings were curved and waved to achieve a striking exterior façade. Hot-dip galvanizing offers further flexibility in the design of structural connections. Welding can be successfully performed before hot-dip galvanizing to maximize corrosion protection or afterward to accommodate pieces which must be fitted at the jobsite.11 When welding at the jobsite, all areas requiring coating removal for field welding must be repaired in accordance with ASTM A780, Standard Practice for Repair of Damaged and Uncoated Areas of Hot-dip Galvanized Coatings. For building designs featuring exposed bolted connections, the elements are individually galvanized and assembled after hot-dip galvanizing to ensure corrosion protection throughout. Though the hot-dip galvanized coating does not affect the design of bearing connections, roughened hot-dip galvanized slip-critical connections have Class A friction properties according to AISC’s, Specification for Structural Steel Buildings. This means hot-dip galvanized surfaces have a lower slip-coefficient than connections with Class B paint coatings, and therefore galvanized structures may require more bolts, holes, and joints. Alternatively, the slip coefficient for hot-dip galvanized structures can be significantly increased by applying zinc-rich paints to the galvanized faying surface to achieve slip properties similar to Class B.12
In addition to providing design flexibility and corrosion resistance, hot-dip galvanized steel is specified more and more for its appearance. Although it can be difficult to control the initial appearance, the contemporary, industrial appearance of hot-dip galvanized steel is considered aesthetically pleasing. The San Diego Central Library (Figure 3) in California provides an example where hot-dip galvanizing provides durable and sustainable corrosion protection in addition to a modern, industrial aesthetic. Looking to achieve an overall image of great opulence reflecting the lofty status of San Diego, hot-dip galvanized steel was specified for the outside façade of the building, including the three-story dome, which has become an architectural achievement for the city.
The natural, gray finishes of hot-dip galvanized steel can also compliment and blend in with rural, wooded, wildlife-sensitive areas or where a neutral appearance is important. For the design of the Morris Arboretum “Out on a Limb Tree Adventure” in Philadelphia (Figure 4), the architect was looking to incorporate a structure that would add to the greenness of the trees, rather than detract from it. The resulting design includes hot-dip galvanized steel walkways weaving throughout the expansive canopy platform to provide a non-intrusive path to the treetops for nature enthusiasts.
Where natural earth tones are desired to blend the steel components with the surroundings instead of a soft metallic look, a proprietary color technology can be applied to provide a rustic brown or aged appearance. For example, the Boulevard Brewery Co. visitor center in Kansas City, Missouri (Figure 5), was constructed from a historic facility using repurposed wood from an old brewery building and a six-generation farm. To showcase and preserve the old-school charm of the building, hot-dip galvanizing with a proprietary rustic brown colorant was specified for structural supports and wheat motif railings to provide durability and a sense of historical ambiance.
Applications of hot-dip galvanizing in residential projects include balcony or terrace railings, staircases, carports, conservatories, or steel window grills. Residential garden structures such as furniture, fences, playground equipment, and greenhouses also benefit from the aesthetics and corrosion protection hot-dip galvanizing provides. In buildings with modern or classical architecture, hot-dip galvanizing displays its natural metallic charm to advantage, especially when used in combination with wood, stone, glass, painted components, or greenery to achieve a unique and attractive contrast. Incorporating this idea is the home of Hélène Guimont (Figure 6), beautifully situated on the lower peninsula of the Saint Lawrence River in Rimouski, Québec. One of the design objectives was to bring an air of “lightness” to the feel of the home inside and out. The resulting design includes a striking blend of classic and contemporary design cues combining hot-dip galvanized steel structures and beam posts with stone and wood components.
Where a certain color other than a natural, gray metallic is preferred, painting or powder coating over hot-dip galvanizing, known as a duplex system, offers additional options. The use of a duplex system ensures the desired aesthetic can be achieved without sacrificing corrosion protection. In fact, the synergistic effect of combining the two coatings is superior to either protection system used alone. Combined, the overall corrosion protection can last from 1.5 to 2.3 times the combined lifetimes of both systems without any maintenance.13 The Salvador Dali Museum in St. Petersburg, Florida (Figure 7), is an example where powder coating over hot-dip galvanizing provides the needed aesthetic while protecting the structural tubing inside and out from the highly corrosive coastal environment. By powder coating the galvanized surface, the designers were able to select the perfect color to highlight the geometry of the abstract 23-m (75-ft) glass structure and provide a juxtaposition of classical and fantastic elements.
Maximizing coating quality
There is a common misconception it is not possible to obtain batch hot-dip galvanized steel of the quality and aesthetics required for building and architecture projects involving architecturally exposed structural steel (AESS). This is because, unlike painted or powder-coated steel, stainless steel, concrete, or metal plating, the initial appearance of hot-dip galvanized steel is hard to predict for a variety of reasons, such as steel chemistry, cooling rate, and stress induced during processing. Regardless of initial appearance, all galvanized steel parts take on a uniform matte gray appearance upon exposure to the environment within six months to two years. In addition to concerns over initial appearance, ASTM A123 does not have strict requirements governing the aesthetic properties of the coating unlike other protective coating systems over steel. Despite these challenges, it is possible to achieve AESS quality hot-dip galvanizing when specifying design, inspection, and storage requirements beyond those referenced within ASTM A123 to maximize aesthetics.
Utilizing ASTM A385, Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip), to identify any design and fabrication practices allows design professionals to obtain a hot-dip galvanized coating that can meet AESS expectations. The recommendations in the standard are not necessary when working with corrosion-resistant steels and/or other corrosion protection systems, but are specific to maximizing quality for batch hot-dip galvanizing. For example, the foundation of ASTM A385 is proper steel selection. Steels beyond recommended ranges for elements such as silicon and phosphorus may react with zinc to produce matte and/or rough coatings, or excessively thick coatings potentially susceptible to delamination.
ASTM A385 also contains design details for recommended sizes and placement of things like vent and drain holes and cropped corners, necessary to maximize drainage of the zinc into, around, and off the parts. Since hot-dip galvanizing is an immersion process and involves dipping the steel through a series of pretreatment process tanks before entering the galvanizing kettle, it is critical for steel designs to accommodate the free flow of cleaning solutions, fluxes, air, and zinc to ensure the formation of a smooth and uniform coating. Incorporating the
design details listed within ASTM A385 will effectively minimize spattering, drips, runs, bare spots, and other surface blemishes.
Even after optimizing the design for batch hot-dip galvanizing, designers should be aware hot-dip galvanizing does not provide a perfectly smooth, shiny, or uniform appearance such as chrome, aluminum, mechanical plating, a finished paint system, or stainless steel. As a result, inspection requirements above and beyond what is stated in ASTM A123 are often needed to achieve AESS quality galvanizing. For example, additional surface smoothing or the removal of dross, skimmings, or excess zinc may be required to achieve the final desired appearance. It is critical to inform all parties of any elevated quality or inspection requirements, as well as establish responsibilities and clarify these requirements prior to hot-dip galvanizing to reduce additional time, materials, and cost for the overall project.
If specifying a duplex system, proper surface preparation is critical to ensure successful adhesion of the paint or powder coating system over the hot-dip galvanized surface. This is because different preparation steps are required when applying the same coating system over black steel vs. a galvanized surface. Further, different procedures for cleaning are recommended depending on the initial condition (weathering) of the galvanized surface. One can refer to ASTM D6386, Preparation of Zinc (Hot- Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting, for the process and procedures for preparing hot-dip galvanized steel for painting. Meanwhile, ASTM D7803, Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder Coating, has been established to detail the best practice for preparing hot-dip galvanized surfaces for powder coating. The galvanizer must be informed ahead of time if the parts are to be top coated so galvanizing post-treatments or surface roughnesses, which affect adhesion of the paint or powder coating system, can be avoided or remedied.
Should the hot-dip galvanized coating need to be repaired at the galvanizing facility or at the jobsite, selection of repair material should be considered for maximum aesthetics. Materials suitable for the repair of batch hot-dip galvanized coatings per ASTM A780, Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings are:
- zinc-rich paint;
- zinc-based solder; or
- zinc-spray metallizing.
In terms of appearance, zinc-based solders and zinc-spray metallizing more closely match the appearance of hot-dip galvanized steel, but there are limitations to applying both materials which must be considered in addition to cost. Zinc-rich paints can economically provide suitable aesthetics if a matte gray or weathered color repair paint is utilized. Although the appearance may at first be a mixture of gray paint and shiny hot-dip galvanizing, all hot-dip galvanized coatings weather to a matte gray and closely match the repair paint color over time. The use of shiny or silver repair paint results in permanent mixed appearance once the base galvanizing weathers.
After acceptance of the material, hot-dip galvanized articles should not be stored tightly in bundles and stacks like other building materials. It is important to ensure proper storage to prevent the formation of wet storage stain. Often present in the form of white or gray powdery corrosion products on the surface of the coating, wet storage stain develops on closely packed articles if deprived of freely moving air and exposed to standing water or other moisture such as rain or high humidity. Where applicable, one must follow AGA guidelines on preventing and removing wet storage stain.14 Additional time and labor may be required to provide adequate airflow during transportation and storage, either through the addition of spacers or sheltering until the parts can be installed.
When specifying hot-dip galvanized steel for building and architecture projects, following established design and fabrication guidelines for optimizing coating quality ensures maximum aesthetics can be achieved for ornamental and/or AESS components. Whether trying to achieve a delicate and bright look, a striking appearance, or a structure that will blend in with the surroundings, hot-dip galvanizing can provide a natural gray appearance or be top coated with a variety of colors to achieve a sustainable, corrosion-resistant design leaving an impression for generations.
1 Read the 2014 publication The Performance of Hot-dip Galvanized Steel Products in the Atmosphere, Soil, Water, Concrete and More by the American Galvanizers Association (AGA).
2 Refer to AGA’s 2017 publication Hot-dip Galvanizing for Sustainable Design.
3 Consult AGA’s 2013 publications Hot-Dip Galvanized Steel’s Contribution to Zinc Levels in the Soil Environment and Hot-Dip Galvanized Steel’s Contribution to Zinc Levels in the Water Environment for more information.
4 See Note 2.
5 Consult the 2004 research report “Life-cycle Assessment Study for Hot-dip Galvanized Balcony System Compared with Painted Balcony System” by S. Vares, K. Tattari, and T. Hakkinen.
6 For more information, read Zinc: A Sustainable Material Essential for Modern Life, published by the International Zinc Association (IZA) in 2017.
7 Read “Steel-Framed Parking Garages Take Off at JFK and Newark International Airports” by J. Englot and R. Davidson, published in the April 2001 issue of Modern Steel Construction.
8 Refer to the National Association of Corrosion Engineers (NACE) paper “Expected Service Life and Cost Considerations for Maintenance and New Construction Protective Coating Work” and the AGA’s “Hot-Dip Galvanizing–American Galvanizers Association National Survey” of 2009.
9 Read the American Institute of Steel Construction’s (AISC’s) 2009 presentation on “Sustainability and Steel.”
10 See Note 9.
11 Refer to Slip Resistance of HDG Faying Surfaces with Zinc-Rich Paints, published in 2017 by AGA.
12 See Note 11.
13 Consult Duplex systems: Hot-Dip Galvanizing Plus Painting by J.F. Eijnsbergen, published in (1994) Amsterdam: Elsevier.
14 Refer to Wet Storage Stain: A Guide to Minimizing and Treating Wet Storage Stain on Hot-Dip Galvanized Steel, published in 2014 by AGA.
Alana Hochstein is the corrosion engineer for the American Galvanizers Association (AGA). She provides assistance to architects, engineers, fabricators, owners, and other specifiers regarding technical issues and the processing of hot-dip galvanized steel. She also manages AGA studies and research on performance, application, and processing of hot-dip galvanized steel, including the selection of abrasives to prepare hot-dip galvanized steel for painting or powder coating. Hochstein is a member of both the National Association of Corrosion Engineers (NACE) International and the Society for Protective Coatings (SSPC), and is a certified NACE Level 1 coating inspector. She can be reached at firstname.lastname@example.org.