Improving the appearance of hot-dip galvanized steel

by brittney_cutler_2 | March 4, 2022 8:00 pm

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By Alana Fossa and John Krzywicki

The appearance of batch, hot-dip galvanized (HDG) steel can be an enigma. Variations in initial coating appearance from one project to the next and the natural weathering of HDG steel often cause confusion and misaligned expectations within the design community. Design professionals may expect all galvanized steel to consistently maintain a shiny, smooth, or spangled finish common to galvanized sheet metal (think HVAC ductwork or corrugated panels) and end up with a matte grey coating once the HDG steel is installed at the jobsite. This can lead to frustration on projects where a certain esthetic is expected.

It is difficult for the batch, hot-dip galvanizer to precisely control the metallurgical reaction between iron and zinc which forms the HDG coating. Producing consistent coating appearances on the varying steel articles they process each day is an impossible task. Design, specification, steel chemistry, and fabrication practices all have an influence on the HDG coating appearance. Although appearance does not affect the durability or corrosion resistance of HDG steel, it can impact the intended use of the product. With proper communication between the design professional, fabricator, and galvanizer, most appearance concerns can be mitigated. However, when projects demand a particular esthetic, post-treatments can be applied to the galvanized surface to alter the final appearance. These post-treatment options essentially provide unlimited final appearances for HDG steel.

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Initial HDG appearance

As the zinc coating is exposed to natural wet-dry cycles in the environment, it begins to form a passive layer of corrosion products on the surface. After a period of six months to two years, a zinc patina will develop fully. The zinc patina formation creates a matte, uniform, and weathered grey appearance—evening out the differences which may have existed originally (Figure 1). Initial inconsistencies in the coating weather away after a short time and the appearance for the bulk of steel’s service life will be uniform. This property of galvanized steel is important to consider for projects desiring an immediate matte, uniform appearance because the weathering process cannot be practically accelerated.

Design and specification

The initial appearance of HDG steel is difficult to predict and control for various reasons, including design, specification, steel chemical composition, stress induced during steel processing, and cooling rates after galvanizing. In fact, HDG steel is known to take on a variety of initial appearances from bright and shiny, dull, spangled, mottled, or even matte grey (Figure 2, page 18). These differences can occur between individual pieces and even within sections of the same piece.

Many do not realize the galvanizer has limited control over the surface appearance and finish of the HDG coating. Instead, a combined effort and open communication between the design professional, fabricator, and galvanizer are required for optimal corrosion protection and esthetics. Specifiers should consult ASTM A385, Standard Practice for Providing High-quality Zinc Coatings (Hot-dip), for design and fabrication practices to optimize steel parts for improved hot-dip galvanizing. The foundation of the specification is proper steel selection, as it has the greatest impact on the initial appearance, smoothness, structure, and thickness of the galvanized coating. The specification also identifies design issues, such as overlapping surfaces, different thickness of material in an assembly, moving parts within an assembly, and through-holes, which require special attention to deliver a coating. Of utmost importance in this specification are details for venting and drainage holes to accommodate the free flow of cleaning solutions, and air and zinc necessary for a smooth and consistent coating. These details, along with galvanizing best practices, will not only produce high-quality galvanized coatings, but also increase esthetics, reduce costs, and improve turnaround.

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Case study: Cliffwalk at the Capilano Suspension Bridge

Reaching out over a misty expanse of evergreen foliage, the Cliffwalk at Capilano Suspension Bridge in Vancouver, British Columbia, dares visitors to tread out over the open canyon below and take in the scenic view from the unique perspective of a rock climber. The Cliffwalk’s location makes it difficult to maintain or repair, as the arc hangs 90 m (295 ft) above the river below. Further, the attraction is located only a few miles from the Pacific coastline surrounding Vancouver, making corrosive moisture in constant contact with the exposed steel elements. HDG steel was blended with other materials, such as wood, glass, aluminum, duplex coated steel, and weathering steel to create a light, airy feel. The natural, matte grey esthetic of HDG steel blends easily into the cliff face and forest environment without detracting from the beautiful natural surroundings. Esthetics and sustainability were of key importance to this project, and hot-dip galvanizing delivered on both.

Refining the appearance of HDG steel

The engineer and/or architect often have the greatest influence on the appearance of the HDG coating; much more than the galvanizer. Specification and design practices are highly influential on the initial and final appearance of the coating as well as the overall quality. This makes it important for the design professional to communicate the intended use of the product with the galvanizer and fabricator early in the project to ensure expectations are set and responsibilities are clearly defined.

The final appearance and finish requirements for HDG coatings are provided in ASTM A123, Standard Specification for Zinc (Hot-dip Galvanized) Coatings on Iron and Steel Products. Prior to the 2018 revision, Canadian standard CSA G164, Hot Dip Galvanizing of Irregularly Shaped Articles, provided an alternative specification for general galvanizing but the standard now applies only to products intended for use in electrical and communication systems. As ASTM A123 was not originally intended to provide an esthetic finish, many natural and common surface conditions (i.e. runs, inclusions, roughness, or excess zinc) are acceptable under this standard if they do not affect the corrosion protection of the coating or intended use of the product. These allowances cause many specifiers to believe elevated esthetics cannot be achieved for artistic designs and architectural members. In reality, these surface conditions can be reasonably ground, sanded, or filed after galvanizing to minimize their appearance, improve uniformity, and achieve a desired look for featured or showcase elements (i.e. architecturally exposed structural steel [AESS], façades, etc.). Further, designers have the option to specify additional design, fabrication, and coating requirements known to improve overall appearance while minimizing time and cost for smoothing after galvanizing.

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For architects, the Canadian Institute of Steel Construction (CISC) Category Matrix for Specifying Architecturally Exposed Structural Steel (AESS) is recommended to communicate the desired esthetic for AESS members requiring heightened esthetics for surfaces and connections. Any combination of exceptions and/or increased requirements for hot-dip galvanizing should be defined using the Custom Elements (C) category based on viewing distance, function, and cost. To assist specifiers with defining Custom Elements for hot-dip galvanized members within the CISC AESS Category Matrix, AGA offers a free publication, Hot-Dip Galvanized Architecturally Exposed Structural Steel Guide, available at galvanizeit.org/AESS.

If a project’s design requirements go above and beyond ASTM A123 appearance and finish mandates, they must be clearly communicated to both the fabricator and galvanizer. Opening direct lines of communication early in the design process provides the best chance for improved appearance. For example, design professionals can work directly with galvanizers to optimize placement, quantity, and size of venting/drainage holes beyond the minimum requirements to avoid runs and clogs of zinc (Figure 3).

For any welded assemblies, esthetics near the weld area similarly benefit from a greater attention to detail. Welding electrodes containing high silicon (> 0.25 per cent Si) can cause thick and dark coatings in the area even if the weld is ground flat before galvanizing. Low-silicon welding electrodes can be specified to avoid a swollen appearance, but availability may be uncommon for some welding processes (e.g. gas-shielded flux cored arc welding [FCAW-G]). The additional effort to grind and file welds, hollow structural section (HSS) seams, mill markings, and other raised surface conditions will minimize their appearance after hot-dip galvanizing, but are unlikely to eliminate them (Figure 4).

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At an additional cost, design professionals may also recommend a commercial blast cleaning prior to hot-dip galvanizing to improve overall aesthetics, reduce the occurrence of thick and brittle coatings, and promote a more uniform appearance across assemblies containing different steel compositions and initial surface conditions.

Touch-up and repair materials

If the coating is damaged after hot-dip galvanizing, it is possible to specify a repair method to maximize esthetics. ASTM A780, Practice for Repair of Damaged and Uncoated Areas of Hot-dip Galvanized Coatings, lists the following three methods for repair:

∞ zinc-based solder;

∞ zinc-rich paint; and

∞ thermal-sprayed zinc (zinc metallizing).

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Zinc-based solders and thermal-sprayed zinc more closely match the appearance of galvanizing, but there are some limitations in addition to a higher investment. Zinc-rich paints economically provide suitable esthetics if a matte grey one is utilized because the base galvanizing will weather to a similar colour over time. A shiny or silver repair paint provides a more uniform appearance initially, but results in a permanent mixed appearance upon natural weathering of the galvanizing.

Although not an exhaustive list of additional methods to improve the appearance of batch, HDG products, the above tend to have the greatest impact beyond the design details available in ASTM A385. To help specifiers navigate other design and fabrication best practices for galvanizing, the American Galvanizers Association (AGA) provides the free publication, Design of Products to be Hot-Dip Galvanized After Fabrication[7].

Altering the appearance of HDG steel

Although it can be difficult to control the initial appearance of hot-dip galvanizing, design professionals continue to turn to it for a modern industrial appearance or to complement the surrounding environment with a neutral grey. Though HDG coatings provide a unique esthetic, there are ways to alter its appearance for projects where a desired sheen or colour is an important attribute of the overall design.

Newly galvanized parts will dull slowly as they weather naturally in the environment, but a prematurely matte surface is achieved by applying a zinc-phosphate conversion coating. This procedure is typically used for rural environments or wilderness areas where surface reflectivity must be reduced prior to installation for safety or environmental concerns. Some galvanizing plants can dull the coating using an immersion process, but a spray or immersion application may also be performed by a third party.

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To alter colouration of the galvanized coating while preserving other interesting visual aspects of the metallic coating, metal surface treatments can provide a distinctive alternative. These post-galvanizing treatments do not affect the corrosion protection and are applied either by immersion in solution or spray application. For example, a natural earth tone can be achieved to blend galvanized components within a natural landscape by applying a colour treatment to react with the zinc surface to develop a mottled and rustic, brown finish often associated with a historical esthetic. The final effect is often compared to weathering steel. Where a more vibrant colour is preferred, other passivation technologies, such as polymer-based, waterborne products specifically designed for use over zinc surfaces, can be spray applied. They dry within seconds to offer long-lasting translucent colours in a variety of hues (Figure 5). Due to the variety of initial appearances that are possible, it is impossible to guarantee spangles or other features will be visible underneath a translucent surface treatment.

A duplex system: Painting or powder-coating over galvanizing

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Where a certain colour other than a natural, grey metallic is preferred, the HDG coating may be painted or powder-coated, known as a duplex system. In addition to a specified hue and texture, duplex systems provide improved longevity, 1.5 to 2.3 times the combined lifetimes of both systems, as the result of a synergistic effect between the coatings. The paint or powder-coating should be specified based on the intended use of the part, application method, environmental concerns, and the desired appearance. For assistance in evaluating the available options designed to work well over hot-dip galvanized surfaces, it is best to consult a paint manufacturer directly or reference the Society for Protective Coating (SSPC) Guide 19, Selection of Protective Coatings For Use Over Galvanized Substrates. SSPC Guide 19 is a valuable resource to guide specifiers in selecting an appropriate liquid coating system for the duplex system.

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Surface preparation is the most important step in ensuring the appearance and overall performance of painted or powder-coated HDG steel meets the desired expectations. The following standards clarify the steps necessary to prepare the galvanized surface for a wide range of liquid coating systems:

ASTM D6386, Practice for Preparation of Zinc (Hot-dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting; and

ASTM D7803, Practice for Preparation of Zinc (Hot-dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder Coating.

After coating selection and preparation of the hot-dip galvanized surface, the paint or powder-coating should always be mixed, applied, and cured in accordance with the manufacturer’s instructions.

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Case Study: Woodwards Building W Tower

Reaching high into the Vancouver sky, the intricately etched decorative panels of the Woodwards Building W Tower are a great example of the synergistic effect of hot-dip galvanizing and paint/powder coating (a.k.a. “duplex system”). For projects requiring a particular colour scheme, duplexing is a good way to get the corrosion-resistant protection of galvanizing, while allowing a structure to incorporate any colour desired.

Duplex systems also have the added advantage of extending the time-to-first maintenance for the paint. Galvanized coatings are as a better substrate for paint than bare steel because zinc corrodes at approximately 1/30^th the rate of bare steel slowing underfilm corrosion. When using these two systems in tandem, the typical repetitive, scheduled maintenance for paint on bare steel will be significantly reduced, and no unsightly paint peeling or rust bleeding will occur. The duplex system will protect the steel panels from the inside out, while allowing the bright colouring consistent with the architect’s vision to exist free of corrosion.

Conclusion

Appearance concerns regarding after-fabrication, batch, hot-dip galvanizing can be avoided with proper communication between the design professional, fabricator, and galvanizer. Depending on the final intended use of the HDG steel, a strategy can be developed between the parties to ensure the highest coating quality is achieved while preserving the superior corrosion protection provided by HDG steel.

Authors

[12]Alana Fossa is the senior corrosion engineer for the American Galvanizers Association (AGA) and the vice-chair of the ASTM Subcommittee A05.13 which authors and edits specifications on hot-dip galvanizing of steel articles. Fossa provides assistance to architects, engineers, fabricators, owners, and other specifiers regarding technical issues and the processing of hot-dip galvanized (HDG) steel. She also manages AGA studies and research on performance, application, and processing of HDG steel. She can be reached at afossa@galvanizeit.org.

[13]John Krzywicki has worked for the American Galvanizers Association (AGA) for six years, starting as a corrosion engineer and eventually being promoted to marketing director. His strong engineering background provides him with the technical knowledge necessary to effectively communicate design, performance, and sustainability considerations when using hot-dip galvanized (HDG) steel. He can be reached at jkrzywicki@galvanizeit.org.

Source URL: https://www.constructioncanada.net/improving-the-appearance-of-hot-dip-galvanized-steel/

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