Harness design freedom with hot-dip galvanized steel

by nithya_caleb | May 27, 2019 10:00 am

by Melissa Lindsley and Alana Hochstein

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Most architects were drawn to the profession because of a desire to design impressive, unforgettable structures. In practice, they learn, not every project will be a masterpiece or even scratch the surface of their creativity. Often in these projects, they will still inject some unique elements, no matter how small, to put their signature on the design. However, when an architect is provided design freedom on a project, their creativity seems to have no limits. These are the projects that led them to the profession and where they leave their mark. It is where the bounds of a square box are too limiting. This is where architecturally exposed structural steel (AESS) shines brightest, allowing their imaginations to become reality.

AESS is designed to meet the primary needs of a building, canopy, or ancillary structure, while remaining exposed to view. It can be exposed on the interior or exterior of a building structure (this article will primarily focus on external elements). Exterior AESS is not only critical to the structural integrity of the building, but also provides a unique esthetic. When considering the importance of maintaining esthetics and integrity, architects should ensure the AESS is corrosion-resistant for as long as possible.

Hot-dip galvanizing

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Hot-dip galvanizing (HDG) is the process of dipping fabricated steel into a bath of molten zinc. While immersed in the kettle (bath), the iron in the steel reacts with the zinc to form a durable, uniform, corrosion-resistant coating harder than the base steel. Hot-dip galvanized steel is primarily specified to provide long-lasting corrosion protection with little to no maintenance.

More and more, specifiers are looking to hot-dip galvanize exterior AESS to protect their designs for generations to enjoy. However, this practice can lead to friction between the architect, steel fabricator, and galvanizer, as there are some common misconceptions about the appearance of hot-dip galvanized steel. When many architects think about galvanized steel, they picture the shiny, smooth, and often spangled finish of galvanized sheet metal used in ductwork, hinges, and other similar applications, but most fabricators and galvanizers know batch (or after-fabrication) hot-dip galvanizing structural steel elements do not often yield this type of appearance.

Unlike many other coating systems (paint, powder coating, metal plating) or materials (stainless steel, weathering steel), the initial appearance of hot-dip galvanized steel is difficult to predict for a variety of reasons, including steel chemistry, cooling rate, and stress induced during steel processing and fabrication. Further, surface conditions normally acceptable under ASTM A123/A123M-17, Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, (i.e. runs, skimmings, roughness, excess zinc) may be unacceptable for showcase elements of AESS without additional detailing by the galvanizer and/or fabricator.

Despite these challenges, it is possible to achieve high-quality HDG coatings meeting the esthetic requirements of AESS if there is clear, direct communication between all parties (architect, fabricator, and galvanizer) throughout the design process. The following guidelines will explain how to apply the categorical approach of AESS and best design practices to minimize the cost premium and maximize appearance of hot-dip galvanized steel for AESS projects.

The categorical approach

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The Canadian Institute of Steel Construction (CISC) updated their guidelines for specifying AESS in 2011 to assist architects in communicating the additional esthetic requirements for galvanized AESS. These guidelines clarify the requirements and assign responsibilities between the architect, general contractor (GC), fabricator, and galvanizer. CISC’s guide established the following AESS categories for the steel based on viewing distance, type/function of the structure, and cost increase to achieve the sequentially higher appearances:

• AESS 1: Basic elements;
• AESS 2: Feature elements viewed at a distance greater than 6 m (20 ft);
• AESS 3: Feature elements viewed at a distance less than 6 m;
• AESS 4: Showcase elements with special surface and edge treatment beyond fabrication; and
• AESS 5: Custom elements with characteristics described in contract documents.

A summary of requirements for each category can be found in Table 1 of the CISC Guide for Specifying AESS. The key takeaway from this categorical approach is, requirements do not need to be equally applied across a project as AESS 1 or 2 are more suitable for elements that will not be closely seen by the public or will be covered by fireproofing, while artwork, columns, or showcase elements likely to be viewed up close or touches may require AESS 3 or 4. This allows one to optimize the quality and inspection requirements for galvanized steel to avoid unnecessary and expensive detailing/smoothing.

Assigning responsibilities beyond ASTM A123/A123M

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CISC’s guide to AESS primarily dictates additional requirements related to fabrication, but when specifying galvanized AESS elements, it is important to familiarize oneself with the finish and appearance requirements in ASTM A123/A123M. This will allow one to discuss and successfully assign responsibility to the fabricator and galvanizer for additional work required to meet the esthetic characteristics for the AESS categories in the project. As previously noted, there are surface conditions allowable under ASTM A123/A123M that do not affect corrosion protection, but will likely be unacceptable for the esthetic of an AESS project.

If these responsibilities are not clearly outlined in the project specifications, it is unlikely the fabricator, GC, and/or galvanizer would include the additional costs in their bid for the level of detailing required, leaving all parties unwilling to take on the additional work (and significant cost). The result of poorly written specifications is often galvanized steel delivered to the jobsite do not meet expectations or the specified AESS category. To avoid this discrepancy, it is best to include all the required information in specifications—at least call out the project is AESS and highlight areas where discussion may be necessary. It is also advisable to plan a pre-job meeting with the galvanizer, fabricator, and GC to explicitly review and assign requirements that are beyond ASTM A123/A123M to ensure they are included in the bid and contract award.

Additionally, it is best practices to set out in the specifications a prefabrication meeting to review the specifications, as well as a requirement for a mockup, to be approved by the architect, in order to establish an acceptable quality.

Although there are requirements in the AESS guide for visual samples to be used to demonstrate capabilities of meeting AESS 3 or 4, architects should use caution in expecting this to replace a pre-job meeting with all parties as previously suggested. These samples will demonstrate whether the galvanizer and fabricator can provide a galvanized part with suitable corrosion protection, smoothness, installation tolerances, and overall quality. However, it is not practical to expect the esthetic finish of all galvanized articles in the project will exactly match the sample pieces.

As previously noted, the initial appearance of hot-dip galvanized steel is difficult to predict. In fact, it is possible to galvanize identical pieces at the same time resulting in different finishes or overall appearance. However, regardless of the initial appearance, all galvanized steel parts will take on a uniform matte grey appearance on exposure to the environment (wet/dry cycles) and develop the zinc patina necessary for long-term corrosion protection. The evolution of the initial HDG appearance to this soft grey typically takes place within six months to two years. If some variation in the metallic grey finish does not meet one’s expectations, it may be prudent to consider a duplex system (paint or powder coating over galvanized steel).

Practices to maximize HDG esthetics

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Maximizing the esthetics of hot-dip galvanized steel is a team effort. Communication throughout the process is a critical step to achieving a successful AESS finish with hot-dip galvanizing. Practices followed by the designer/architect, fabricator, and galvanizer will have a significant impact on the final finish. The following are a summary of best practices that should be employed throughout the production process.

Design and specification

There are a few considerations in the overall design of the steel before fabrication that can have a significant impact on the overall galvanizing finish. These decisions are often made without considering the significant impact it may have on the appearance and without the galvanizer’s knowledge or input. Communicating early in the project with the galvanizer and referencing ASTM A385, Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip), are a great start for the architect and fabricator. However, it is likely additional design optimizations may be necessary to maximize HDG esthetics for AESS.

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A few areas to pay special attention are:

• material selection;
• placement of vent/drain holes and lifting points and practical limitations of the galvanizer’s facility and kettle;
• weld rod chemistry and weld cleanliness; and
• thermally cut edges.

Material selection has a major influence on the hot-dip galvanized coating appearance. Combining steels with different thicknesses, chemistries, or surface conditions can result in a mixed appearance after galvanizing. If unavoidable, it is best to send the parts to the galvanizer separately and connect after galvanizing to match appearances or abrasively blast the parts before HDG to encourage the potential for a uniform appearance.

Direct communication with the galvanizer about the size, quantity, and placement of venting and drainage holes to optimize handling through the galvanizing process is another opportunity to improve the final appearance. This can minimize the potential for zinc buildup and other visual imperfections. To take it a step farther, discussing cropped corners and temporary or designated lift points can also minimize excess zinc buildup, runs, and the appearance of chain/wire marks.

Similarly, designers should discuss and keep in mind practical limitations of the galvanizing plant and kettle. It may be difficult to process large parts that come close to the dross line of the kettle when dipped, or are larger than the galvanizing kettle and require progressive dipping. AESS requirements may still be achieved, but it is paramount to discuss and work through the concerns with the galvanizer to understand what limitations the facility or kettle may have.

Two other common design and fabrication practices that can lead to different finishes are welding and thermal cutting. The CISC guide to AESS has specific requirements for weld contouring and grinding of sharp edges based on the category. However, there are additional considerations in both of these areas to maximize the HDG coating appearance.

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Just as steel chemistry can have a profound effect on the appearance and thickness of the hot-dip galvanized coating, the chemistry of the weld rod can also impact the appearance. If the weld material chemistry is significantly different than the chemistry of the surrounding steel, particularly in terms of silicon content, it is common to see a thicker and/or darker HDG coating present in the weld area. Therefore, it is recommended to optimize the welding material used and discuss the potential for additional smoothing after HDG, which may be necessary to meet the weld contouring and blending requirements of the AESS category.

Thermally cut edges can also present quality and appearance issues after hot-dip galvanizing. Even if the fabricator performs grinding of sharp edges in cut areas, as outlined in the AESS guide, it is often not enough to prevent the known issues of thin and/or flaking coatings. To achieve a high-quality finish along thermally cut edges and avoid field repair of the galvanized coating, it is important to designate responsibility to ensure all affected surfaces are ground up to 1.5 mm (1/16 in.) even though it is not explicitly stated within the AESS category requirement.

This is not necessarily an exhaustive list of additional design practices to maximize appearance, but the noted ones are intended to highlight some common areas and demonstrate the importance of communication amongst the parties involved. The American Galvanizers Association (AGA) has a number of free resources as well as free technical support to assist the designer and specifier to optimize their design for maximum HDG appearance.

Process controls

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Although there are many design and fabrication considerations outside of the galvanizers purview impacting the overall HDG finish and appearance, there are process controls the galvanizer can implement to minimize surface smoothing and improve final appearance for AESS projects. A few controls the galvanizer can consider and possibly implement are:

• immersion/withdrawal controls (quick as possible immersion, suspension of parts above dross line, highest possible dip angle, minimize dip time, and slow withdrawal);
• elemental additions to the galvanizing bath;
• optimizing selection of touchup and repair materials; and
• proper storage.

Although the galvanizer can control these items, most require a team effort or shared responsibility. Many of these controls relate to the design practices discussed (optimizing vent/drain holes, discussing practical limitations, etc.) For example, the galvanizer can only immerse the material as quickly as is safe based on the vent holes provided. If the part is designed to optimize venting for quick immersion, then the galvanizer can speed up the immersion to help with overall appearance. Further, if the galvanizer employed all of these noted process controls, but the steel chemistry of the material used is outside of the recommended compositions for HDG, these controls will have a negligible effect on the final appearance.

There is also a lot of shared responsibility when considering the use of touchup and repair materials and proper storage. In-plant repair and storage are the responsibility of the galvanizer. However, field touchup and storage are the responsibility of the fabricator and/or the GC. The touchup and repair of hot-dip galvanized steel both in the plant and field is controlled by ASTM A780, Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings, which contains three acceptable materials: zinc rich paint, zinc solder, and zinc metallizing. Each is acceptable for touchup (effective), but they all have different appearances. So, if esthetics is critical, it should be discussed and added to the bid documents.

[9]It is important to note, the requirements within ASTM A780 are to ensure proper corrosion protection is in place and appearance is not really a consideration. The use of a preferred touchup material should be outlined in the project specification, and is another area where direct communication is beneficial to ensure the method used maximizes the esthetic to meet the appropriate AESS category. Some touchup materials that are a closer match to the initial appearance may not be the closest match to the galvanized coating once it weathers. Making sure the galvanizer, fabricator, and GC are in agreement and aware of esthetic requirements and preferences in regard to touchup at the plant and field is critical.

Proper storage of hot-dip galvanized steel is necessary to prevent the formation of wet storage stain (commonly referred to as white rust). Wet storage stain can develop on the HDG surface if the parts are stored where they can have a buildup of moisture without the presence of free-flowing air. The presence of light or medium wet storage stain does not negatively impact the corrosion protection afforded and, therefore, does not require remediation. However, it can lead to an undesired appearance. Similar to all hot-dip galvanized surfaces, differences in appearance due to wet storage stain will become uniform over time as the zinc coating weathers. However, if the appearance is unsuitable, the AGA has instructions on cleaning wet storage stain. Similar to touchup and repair, proper storage is a shared responsibility of the galvanizer and fabricator/GC.

Maximizing esthetics by utilizing duplex systems

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Some fabricators and architects who do not believe hot-dip galvanized finishes can meet various AESS requirements or client expectations think simply upgrading to a duplex system (paint/powder coating over HDG) is the solution to their concerns. Although duplex systems can provide unlimited colour options and cover differences in HDG coating appearance, if proper design practices are unemployed for HDG and for preparing the HDG surface for paint are unfollowed, it is likely the duplex coating will not meet the visual requirements for AESS.

When planning a duplex system, it is still imperative to design the steel to avoid excessive zinc buildup, roughness, and other surface imperfections that will be accentuated (not hidden) by the top coat. Similar to AESS requirements going beyond those of ASTM A123/A123M, non-AESS galvanized items that will be duplexed also have more strict guidelines on finish that must be discussed, and responsibility assigned to ensure success. Communicating through clearly written specifications the intent to duplex coat the steel to both the galvanizer and painter is needed to ensure critical surface preparation steps are accounted for in the bid and performed to ensure proper adhesion and maximize esthetics.

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When planning for duplex coated AESS, there is even more importance to clarifying and agreeing to responsibilities for surface smoothing of the HDG coating, and potential additional practices that may be necessary to achieve finishes meeting AESS 3 or 4. For example, it may be necessary to specify filler material to even out divots or indentations on the surface from vent/drain plugs, weld areas, etc. to provide a uniform, smooth surface for the paint.

When planning to duplex AESS steel, it is important to reference and understand ASTM D6386, Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting, and D7803, Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder Coating, to ensure proper surface preparation of the HDG coating. Additionally, AGA has resources and instructional guides and videos available for specifiers and painters to ensure successful duplex coatings.

Conclusion

Architecturally exposed structural steel (AESS) allows architects almost limitless design freedom to bring their creativity to life. Protecting exterior AESS with HDG allows these designs to transcend time without the worry of unsightly rust forming or expensive maintenance. Although hot-dip galvanized steel’s initial appearance is difficult to predict, there are a number of design practices and process controls that can be employed to maximize the appearance. By communicating early and often with all parties involved through a well-written specification, it is quite possible to eliminate headaches and achieve successful hot-dip galvanized AESS projects.

[12]Melissa Lindsley is the American Galvanizers Association’s (AGA’s) marketing director. She is recognized as a leading voice for the galvanizing industry in North America. Lindsley educates architects, engineers, fabricators, owners, and specifiers about the technical aspects and benefits of hot-dip galvanizing. She can be reached via e-mail at mlindsley@galvanizeit.org[13].

[14]Alana Hochstein is the American Galvanizers Association’s (AGA’s) corrosion engineer. She provides assistance to architects, engineers, fabricators, owners, and specifiers on technical issues and processing of hot-dip galvanized steel. Hochstein also manages AGA studies and research on performance, application, and processing of galvanizing steel, including selection of abrasives to prepare hot-dip galvanized steel for painting/powder coating. She is a member of both 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 ahochstein@galvanizeit.org[15].

 

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