![]() These differences can occur between individual pieces and even within sections of the same piece. In fact, HDG steel is known to take on a variety of initial appearances from bright and shiny, dull, spangled, mottled, or even matte gray (Figure 2). 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 ( Read the American Galvanizers Association’s (AGA’s) Publication on Hot-dip Galvanized Coating Appearance (2016)). You May Also Like Preserving architectural treasures in New York’s historic districts ![]() This property of galvanized steel is important for applications that may not be extremely sensitive to aesthetics. 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. ![]() The zinc patina formation creates a matte, uniform, and weathered gray appearance-evening out the differences that may have existed originally (Figure 1). After a period of six months to two years, a zinc patina will develop fully. Photos courtesy AGAĪs 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. In June 2009, the structure was uniformly matte gray with little to no visible difference in appearance. Figure 1: The canopied walkway at Mark Twain Elementary in Riverside, California, was installed in October 2006, and the initial coating appearance varied from bright and shiny to matte gray on the same beam. These post-treatment options essentially provide unlimited final appearances for HDG steel. However, when projects demand a particular aesthetic, post-treatments can be applied to the galvanized surface to alter the final appearance. With proper communication between the design professional, fabricator, and galvanizer, most appearance concerns can be mitigated. Although appearance does not affect the durability or corrosion resistance of HDG steel, it can impact the intended use of the product. Design, specification, steel chemistry, and fabrication practices all have an influence on the HDG coating appearance. Producing consistent coating appearances on the varying steel articles they process each day is an impossible task. ![]() It is difficult for the batch, hot-dip galvanizer to precisely control the metallurgical reaction between iron and zinc that forms the HDG coating. This can lead to frustration on projects where a certain aesthetic is expected. Design professionals may expect all galvanized steel to consistently maintain a shiny, smooth, or spangled finish common to that of galvanized sheet metal (think HVAC ductwork or corrugated panels) and end up with a matte gray coating once the HDG steel is installed at the jobsite. 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. The appearance of batch, hot-dip galvanized (HDG) steel can be an enigma.
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