by Katie Daniel | May 27, 2016 11:13 am
By Alana Hochstein
As more owners and design/construction professionals look toward constructing steel bridges that can last a century, it is imperative to evaluate the corrosion protection systems used to ensure these structures meet the long design life without significant deterioration, and without significant cost.
Hot-dip galvanizing (HDG) and zinc spray metallizing are often specified to protect bridges from the ravages of corrosion for their high recyclability, low environmental impact, and ability to offer long-term protection in harsh environments. To provide economic efficiencies and overcome the unique constraints of each application method, hot-dip galvanizing and zinc spray metallizing can be used in tandem on large or complex structures without sacrificing corrosion protection. To understand how HDG and metallizing come together to provide these benefits, the characteristics, costs, performance, and limitations of the two zinc coatings should be viewed separately.
Batch hot-dip galvanizing
Batch hot-dip galvanizing, also commonly called ‘general’ or just ‘hot-dip’ galvanizing, produces a zinc coating by completely immersing the iron or steel product in a bath (kettle) of molten zinc. Prior to dipping the steel in the molten zinc bath, the steel is chemically cleaned to remove organic contaminants, mill scale, and oxides. The final cleaning step also leaves a protective layer of zinc chloride on the surface to prevent oxidation before immersion in the kettle. This layer will evaporate when the steel hits the surface of the molten zinc bath. The zinc bath consists of at least 98 per cent pure zinc heated to approximately 443 C (830 F).
While the steel is in the galvanizing kettle, the zinc metallurgically reacts with the iron to form the hot-dip galvanized coating. This consists of a series of zinc-iron alloy layers and a surface layer of pure zinc. These intermetallic layers become tightly bonded (24,281 kPa [3600 psi]) to the steel, transforming into an integral part of the steel rather than just a surface coating. The zinc-iron alloy layers are harder than the base material, and provide excellent abrasion resistance.
Another unique characteristic of batch hot-dip galvanized coatings is its uniform, complete coverage. During the diffusion reaction in the kettle, the intermetallic layers grow perpendicular to all surfaces, ensuring edges, corners, and threads have coating thicknesses equal to or greater than flat surfaces.
Additionally, because hot-dip galvanizing is a total immersion process, all interior surfaces of hollow structures and difficult to access recesses of complex pieces are easily coated.
While the steel articles are in the zinc bath, they float, move, and vibrate to ensure areas in contact with racks or wires are able to be coated. However, it is common to see strictly cosmetic ‘touch marks’ when the steel is withdrawn from the galvanizing kettle. These can be fixed up for esthetic purposes.
The governing specifications for the hot-dip galvanizing of structural steel are:
They contain minimum coating thickness requirements based on steel type and thickness. The hot-dip galvanized coating is thicker and/or denser than other zinc coatings. The time-to-first-maintenance (TFM) chart shows the linear relationship between hot-dip galvanized coating thickness and maintenance-free service life in atmospheric exposure. For example, ASTM A123 requires structural steel 15 mm (5⁄8 in.) thick or greater to have a minimum coating thickness of 0.1 mm (3.9 mils), which in an industrial environment would provide a time to first maintenance of 72 years.
Hot-dip galvanizing is a complete immersion process, which means the parts must fit in the zinc bath to be coated. Size can be a limitation when hot-dip galvanizing large structures such as bridge components. However, the average kettle length in North America is 12 m (40 ft), and 16 to 18-m (55 to 60-ft) kettles are common. If an item is too large for total immersion in the kettle, progressive dipping can be utilized to immerse each end of the article sequentially to coat the entire item. In other words, if half of the piece can fit in the bath, the part can be galvanized on one side, rehung, and then dipped on the remaining surface. In essence, this would nearly double the size of the part that can be coated.
Progressively dipped pieces often have an overlap area visible on the piece. The line or darker area is purely cosmetic, and will fade over time as the coating naturally weathers. However, the overlap area will most likely have a thicker coating, so it is important to consider this if the area will be an important connection point with other pieces.
Zinc spray metallizing
Zinc spraying, or metallizing, is accomplished by feeding zinc powder or wire into a heated gun where it is melted and sprayed onto the part using combustion gases and/or auxiliary compressed air. While there are automated metallizing processes, the coating is still most often applied by a skilled operator. Prior to metallizing, the steel must be abrasively cleaned to a near white metal. As the steel surface must remain free of oxides, it is common to clean smaller areas, coat the area, and then move to the next section of the part, which adds time and cost to the coating process. The coating is 100 per cent zinc and is often sealed with a low-viscosity polyurethane, epoxy-phenolic, epoxy, or vinyl resin. Metallizing does not provide the same bond strength as galvanizing to steel, but still has a bond strength of approximately 10,300 kPa (1500 psi)—much stronger than the 2000 to 3500 kPa (300 to 500 psi) of paint coatings.
The metallized coating is rough and slightly porous. However, as the metallized coating is exposed to the atmosphere, zinc corrosion products tend to fill the pores, providing consistent cathodic protection. If the metallized coating is applied properly, and is thick enough, the corrosion performance will be similar to hot-dip galvanized steel. The metallized coating can be applied to meet any thickness requirement (with a density about 80 per cent of hot-dip galvanizing), but excessively thick coatings can have adhesion issues. The metallized coating cannot be applied to interior surfaces and tends to thin at corners and edges as they are harder to consistently spray than flat surfaces. Additionally, the complexity of the structure and accessibility are important to consider for the operator.
Zinc spray can be applied to materials of any size, and is often used as an alternative to, or in conjunction with, batch hot-dip galvanizing when a part is too large for immersion in the galvanizing kettle. Although the coating is more commonly applied in the shop, it is possible to apply in the field—making it a great option for extending the life of a steel structure already in place. The biggest limitations to metallizing applications are availability (i.e. skilled operator and equipment) and the significant cost premium.
The difference in initial cost for a typical project can be substantial between hot-dip galvanizing and metallizing (e.g. $24.98 vs. $125.88 per m2 [$2.32 vs. $11.70 per sf]). Metallizing is even more expensive when performed in the field, and also for smaller pieces, as the material handling increases. The lifecycle cost for metallizing is also greater because hot-dip galvanizing remains maintenance-free over the life of the project, while metallizing may require maintenance depending on the project and environment. To analyze the initial and lifecycle costs of galvanizing and metallizing for a specific project, the project data can be placed into the Lifecycle Cost Calculator (LCCC) tool located on the American Galvanizers Association (AGA) website. (Find the lifecycle cost calculator online by visiting lccc.galvanizeit.org).
When HDG and metallizing work together
Although HDG and metallizing are leading and sustainable choices in the battle against corrosion, there are certain circumstances where it may be difficult or impossible to galvanize, or too expensive to metallize. For instance, what should one do if a piece of steel is too large to be accommodated by either traditional or progressive dipping? What if the steel is located in the field, in a place too difficult to access? There is a surprisingly simple solution to help maximize corrosion protection capabilities—galvanize whatever possible and metallize the rest.
An example where galvanizing and metallizing can be used to overcome the limitations of either coating application method alone is in the construction of bridges containing large girder designs. Oversized girders that cannot be successfully or easily galvanized due to length or size can be metallized while the remaining bridge articles are hot-dip galvanized. It is possible to find even further efficiencies for oversized structures by immersing each end sequentially through hot-dip galvanizing, and metallizing only the remaining mid portion.
Use of the two zinc coatings in tandem is also applicable for bridge rehabilitation projects, where portions of the construction cannot be easily removed and transported from the job site for galvanizing. In such cases, metallizing can be performed in the field while more easily transportable items are hot-dip galvanized. This practice can also provide indirect cost savings in road and bridge closures, where it is not required to take critical steel members out of service to perform metallizing.
Another application for the combined use of HDG and metallizing is for structures with the potential to be negatively affected by exposure to the galvanizing process temperature. Hot-dip galvanizing does not change the mechanical properties of the steel, but there are certain fabrication and design practices (e.g. severe cold-working or bending, asymmetrical design) which may leave steel articles susceptible to concerns such as distortion. It is important to note galvanizing process temperature concerns can be greatly reduced, eliminated, or remedied by following the guidelines provided within ASTM A384/A384 M, Standard Practice for Safeguarding Against Warpage and Distortion During Hot-dip Galvanizing of Steel Assemblies and ASTM A143/A143M, Standard Practice for Safeguarding Against Embrittlement of Hot-dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement. However, for critical applications where fabrication/design practices of concern cannot be avoided, the susceptible articles can be metallized while the remainder of the steelwork is hot-dip galvanized.
The Castleton Bridge in Indiana, which has temperatures similar to southern Alberta, is a great example of a bridge currently using both hot-dip galvanized and metallized steel to achieve long-term savings and corrosion protection. Originally half-painted and half-hot-dip galvanized to test performance in 1970, the bridge is subject to a harsh suburban atmosphere and rough winters, complete with road de-icers.
After more than 40 years of service, the original hot-dip galvanized coating on the northbound side still exceeds ASTM 123 requirements for newly galvanized steel—while the southbound side was repainted after 14 years, and then considered a failure and metallized for protection in 2002. According to officials affiliated with the construction and maintenance of the bridge, it cost more to maintain the painted southbound side than it originally cost to build the entire bridge.
In 1970, the only experience the owners had with hot-dip galvanizing was with guiderail. Today, engineers need look no further than Castleton to reaffirm the reliability of hot-dip galvanized steel for bridge applications. In accordance with ASTM A896A, Standard Practice for Conducting Case Studies on Galvanized Structures methodology, inspection, and testing was completed in September 2011.
Based on the results of the testing performed onsite, the owners can expect an additional 60 years of life to first maintenance (five per cent rust) from the galvanized side of this bridge, lending credibility to the dream of the 100-year bridge design (Figure 1).
With the paint system on the southbound side of the bridge replaced by metallizing, the bridge will benefit from the complementary metallized and galvanized coatings. The two coatings can be used together without the fear of corrosion from dissimilar metals, and will age similarly, giving the adjacent structures a cohesive look. There is no need for a coating maintenance program, as the two zinc coatings will remain virtually maintenance-free over the remaining life of the project. Metallizing was an excellent way to support the corrosion protection of this bridge project together with hot-dip galvanized steel.
For large or complex projects, hot-dip galvanizing and zinc spray metallizing can be used in tandem to overcome the limitations of using either coating application method alone. In such applications, structures benefit from the advantages of both coating systems, without sacrificing corrosion protection. The zinc coatings are compatible, have the same electrical potential, and can be used freely together without developing corrosion cells. Additionally, the two coatings will weather similarly, giving the structure a cohesive look as time progresses. The bond strengths of HDG and metallizing to steel will also provide excellent abrasion resistance and are much greater than paint coatings to steel.
The next time a project requires the consideration of another coating system due to limitations in hot-dip galvanizing, one should consider a joint method of galvanizing whatever parts possible, and metallizing the rest. By utilizing hot-dip galvanizing and zinc spray metallizing together, one can ensure outstanding durability and corrosion protection of hot-dip galvanized steel without continual maintenance, while also receiving a reliable back-up on parts that cannot be easily or successfully galvanized.
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. Hochstein can be reached via email at email@example.com.
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