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Hot-Dipped Galvanized Steel

Galvanizing HDG, or Hot-Dipped Galvanized steel is a common finish used in the realm of bicycle parking. The “hot-dipped” refers to the finish applied to the steel structure as it is completely submerged in a molten zinc bath. Before this bath occurs, the steel goes through a surface preparation consisting of three steps: degreasing/caustic cleaning in a hot alkaline solution, pickling in a hydrochloric or sulfuric acid to remove surface rust, and fluxing in a zinc ammonium chloride solution to remove oxides and prevent oxidation prior to being dipped in the molten zinc.
As the molten zinc bath begins the actual “galvanizing step” the steel is submerged in a bath of 815-850 degrees Fahrenheit. The zinc then reacts with the iron in the steel forming “a series of metallurgically bonded zinc-iron alloy layers… As the steel is withdrawn slowly from the galvanizing bath, excess zinc is removed by draining, vibrating, and/or centrifuging. The metallurgical reaction will continue after the articles are withdrawn from the bath, as long as it remains near bath temperature.” The description of the process itself helps to explain why galvanized bicycle racks appear the way that they do. Since the racks are completely submerged in the molten zinc, “breathe holes” are put into the bent steel tubing to prevent the steel from exploding under the extremely hot pressure this also allows air to escape and the molten zinc to completely fill the inside of the rack- adding interior protection to the steel to prevent corrosion from the inside out. Additionally, when the zinc is draining from the steel it has an easier escape; zinc particles can “bead” on the steel creating zinc “chunks” and some sharp particles that are sanded off at our facility. It can also create a ripple effect where some of the steel appears smooth, and other parts appear rough.

Overtime, the galvanizing will oxidize and all become a uniform ashy-grey color. The American Galvanizers Association has this to say about the appearance of galvanized steel: “The steel chemistry is the primary determinant of galvanized coating thickness and appearance. Continuously cast steel produced by the steel companies has a wide variety of chemistries, thus the different coating appearances.

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