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We use uncoated galvanized steel as a sustainable finish in many of our projects. It is sustainable because galvanizing produces a durable, no maintenance finish with a sixty-year plus lifespan. Recently we have noticed that its appearance can be less uniform than we have seen on previous projects.
This material is sustainable because galvanizing produces a durable, no maintenance finish with a sixty-year plus lifespan.
The galvanized coating on steel is both protective and decorative. ASTM A123 Zinc (Hot Dip Galvanized) Coatings on Iron and Steel is the industry standard for specifying typical galvanized steel coatings. This specification focuses on the protection from corrosion (cathodic protection) afforded by galvanized steel. Many factors affect the APPEARANCE of galvanized steel, but do not reduce protection from corrosion.
The support structure for the rooftop photovoltaic array at our 7th and H project provides an example of adequate cathodic protection, yet a problematic appearance if the structure was located in a highly visible location.
During the pre roofing conference held on January 24, , the PV support structure was observed to have a very un-uniform galvanized finish appearance. Portions were dark dull gray while other portions of the same I beam were shiny.
The PV array support structure is acceptable, but Mogavero is concerned that the appearance of the galvanized finish on the second floor terrace gazebos will not be as intended unless this appearance is not addressed by the galvanizer. The galvanizer should be made specifically aware that finish is an important part of the design and that steps need to be taken to ensure a more uniform finish with a natural reflectivity range of 70% or greater.
My research indicates that there are several potential causes and remedial steps to ensure a more uniform finish.
The following comments are based upon research conducted at the American Galvanizers Association. Galvanized coating is created in several layers, the surface (or &#;zeta: layer) grows in columns perpendicularly outward from the base steel. It appears that the surface layer of the zinc coating (zeta layer) is overgrown (too thick) in the dull areas and thinner in the shiny areas. My research indicates that there are several potential causes and remedial steps to ensure a more uniform finish.
Steel with chemistry outside the recommended limits and ranges for galvanizing is known as &#;reactive steel.&#; This steel may have higher levels of phosphorous and silicon than is appropriate for an exposed galvanized surface. Steel Mil Certificates indicating phosphorous and silicon content may not be a reliable indicator as the certificates are generally an average of sample pieces and do not necessarily reflect the actual chemistry of each part. The galvanizer should specify steel phosphorous and silicone limits within optimum galvanizing tolerances. The galvanized coating is created through a metallurgical reaction between the steel and the zinc in the galvanizing kettle. For non-reactive steel, the metallurgical reaction is complete within six or seven minutes. After that time, no additional zinc is deposited on the steel. For reactive steel, the coating experiences linear growth with respect to the time it is exposed in the galvanizing tank: the longer reactive steel is left in the galvanizing tank, the thicker the coating.
Nickel can be added to the galvanizing bath to help control reactive steels by reducing intermetallic formation. This process is effective as long as the steel has 0.20% silicone content or less. Tin and aluminum can also be added to the galvanizing bath to produce a better overall appearance.
During hot-dipped galvanizing, the steel is heated to approximately 830 degrees F. Reactive steel continues reacting as long as there is free zinc available and the temperature is 500 degrees F or higher &#; even when outside the galvanizing bath. This happens when reactive steel on thick, heavy parts is cooled slowly. If the reactive steel consumes the entire eta layer, the zeta intermetallic layer will grow to the surface and will transform the part from shiny to dull as the part cools. When steel is dipped into a quench tank immediately after galvanizing, the temperature of the steel drops several hundred degrees quickly; thus stopping the metallurgic reaction. Quench options include water, chromate and phosphate.
Withdrawal speed can affect the thickness of the galvanized coating. The faster steel is removed from the galvanizing kettle, the less zinc is able to drain off the steel before solidifying. This means faster withdrawal rates can create thicker coatings.
Blasting is a process whereby abrasive particles are propelled at the steel surface using compressed air. Blasting interferes with the growth of the zeta layer, rather than growing in long columns when the surface is flat. Blasting creates many peaks and valleys so the growing crystals collide and interfere with each other to the point that they cannot continue to grow. The American Galvanizers Association comments that &#;Blasting is not a replacement for sourcing steel with the recommended chemistry; however, in those times when you must galvanize reactive steel, blasting can help decrease the coating thickness.&#;
The specifications should make the intent clear to the Galvanizer. For example:
It is critical that the Architect inform the galvanizer to the importance of the visual surface appearance at the second floor terrace gazebos and develop an action plan to assure the final result is as intended. Action plan may include all or some of the following:
One additional item: the American Galvanizer&#;s Association web site notes concern that adequate drain holes and vent holes be provided to allow galvanizing the interior of capped tube steel members. Gusset plates may need to be cropped (would require the approval of the Structural Engineer) to ensure unimpeded flow of the galvanizing solution. These features were not included in the gazebo shop drawings. The galvanizer should review the shop drawings and provide input prior to fabrication.
Specifying Hot Dip Galvanized Steel
For the vast majority of applications, hot dip galvanizing can be simply specified using the following reference: Hot dip galvanized to BS EN ISO : by a member of Galvanizer Association (Galco, Sperrin). This standard contains coating thickness requirements as shown in table below which will typically be sufficient to achieve acceptably long coating life.
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Minimum coating thicknesses achieved by hot dip galvanizing to EN ISO (not centrifuged)
Microstructure of a typical hot dip galvanized coating
Thicker Coatings by Surface Roughening
For instances where an increased coating thickness is required to achieve a longer coating life, or where work will be exposed in a severe service environment, steelwork may be grit blasted prior to galvanizing. This produces a coarse profile so opening up more steel surface to react with molten zinc resulting in a thicker galvanized coating. Typically the following reference may be used: Grit blast to Sa2½ with G24 chilled angular iron grit before hot dip galvanizing to BS EN ISO : to achieve a nominal coating thickness by a member of Galvanizing Association (Galco, Sperrin). It should be noted that the term `nominal` implies a target to be aimed for but not guaranteed, the value of which can vary with the steel section thickness, with upper limits being indicated in table below. For steel section thickness well in excess of 6 mm, a coating thickness in excess of 140 μm might be achieved.
Nominal coating thicknesses on samples that are grit blasted prior to hot dip galvanizing
Microstructure of a thick coating obtained by grit blasting steel prior to galvanising
Use of a Reactive Steel
The silicon (and to a lesser extent the phosphorous) content of a steel can affect its reactivity so causing the galvanized coating to grow more rapidly during immersion in the galvanizing bath. Typically steel with a silicon content in excess of 0.25% may be reactive and in the very rare cases where prior grit blasting will not achieve a sufficiently thick coating the specification of a steel`s chemistry can enable a still thicker galvanized coating to be specified using the following reference: Use a high silicon steel hot dip galvanized to BS EN ISO : to achieve a nominal coating thickness of 200 μm, by a member of Galvanizers Association (Galco, Sperrin). The 200 μm requirement would only apply for heavier steel section thicknesses. For light gauge steelwork a more realistic figure might be circa 120 μm. Where such a specification is used the coating may be comprised fully of zinc-iron alloy such that it will be dull grey in appearance and might be more susceptible to mechanical or handling damage. As a result, increased care should be taken when handling such product. You should contact your steel stockholder to discuss material availability.
Microstructure of thick coating obtained on a reactive steel
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You can find more information about Hot Dip Galvanizing on Galvanizing Association website
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