Why Galvanize?
HOT-DIP GALVANIZING FOR MAJOR GREENHOUSE
Hot-dip galvanizing has been an integral part of the Metrolina Greenhouses in Charlotte, North Carolina, since the 1970s. The facility has had several expansions, the most recent of which will bring the total size of the building to 5.8 million square feet, making it the largest single-story building in the United States. Hot-dip galvanizing has been integral to all of the construction. Metrolina uses some 1 million gallons of water per day in the summer to keep plants thriving, and the owners maintain that hot-dip galvanizing is the only coating that is capable of withstanding the wet conditions necessary in a working greenhouse. Galvanizing protects all structural steel and has performed for decades without maintenance, allowing the owners to dedicate their resources to expansion and not repairs. Says Metrolina's Art VanWingerden, “Not only does galvanizing provide an exceptionally long life in the hot, moist greenhouse setting, but also corresponds with our efforts to protect the environment by eliminating the VOCs released when paint is used for corrosion protection.”
Photo courtesy of American Galvanizers Association
Metrolina Greenhouses owners maintain that hot-dip galvanizing is the only coating capable of withstanding the wet conditions necessary in a working greenhouse.
The Process
The galvanizing process consists of three basic steps: surface preparation, galvanizing, and inspection, with surface preparation representing the most important step in the application of any coating. In most instances, coating failure is due to incorrect or inadequate surface preparation. In the galvanizing process, the surface step has its own built-in quality control. Zinc will not react with unclean steel, and any failures or inadequacies in this step are obvious—when the steel is removed from the zinc bath, unclean areas remain uncoated, enabling timely action to correct the situation.
Three steps are involved in preparing the surface: degreasing, pickling, and fluxing. In the first step, dirt, grease, and oil are removed by a hot alkali solution, mild acidic bath, or biological cleaning bath; however, epoxies, vinyls, asphalt, or welding slag may require stronger methods such as grit-blasting, sand-blasting, or alternative mechanical means. In pickling, mill scale and rust are removed from the steel surface via diluted and heated sulfuric acid, and ambient hydrochloric acid. Lastly, fluxing is the final surface preparation step. A zinc ammonium chloride solution both eliminates residual oxides and deposits and creates a protective layer on the steel to prevent further formation of oxides before the steel is dipped in the molten zinc bath.
In the galvanizing phase, steel is immersed in a zinc bath heated to at least 815 degrees F. A crane hoist lowers the steel at an angle so air can escape from pockets in the steel, to be replaced by the molten zinc. In the kettle, zinc reacts with the iron in the steel to form a series of zinc-iron intermetallic alloy layers. The galvanized steel is then slowly removed and inspected primarily for coating thickness and coating appearance. Generally speaking, visual inspection is considered adequate, although basic physical and laboratory tests can establish whether thickness, uniformity, adherence, and appearance of the coating are consistent with long established and approved standards of ASTM.
GALVANIZING ART
Photo courtesy of American Galvanizers Association
As a result of hot-dip galvanizing, this 45-foot-tall artistic display will remain an aesthetic asset to the Denver Justice Center for 75 years without maintenance.
With gracefully arched petals blooming skyward, the Light Chamber sculpture blossoms at the entrance of the Denver Justice Center. Artist Dennis Oppenheim, who has incorporated galvanized steel in several outdoor artistic displays, created a 45-foot-tall free-flowing sculpture of leaves and flowers to symbolize the multitude of decisions made in the judge's chambers. All components of the main structure—more than 29 tons of steel—were galvanized. The zinc coating will protect the piece from the harsh sun and exposure to snow and rain, while the cathodic protection of the galvanized steel will protect the piece from any nicks and scratches. Time to first maintenance is projected to be 75 years or more.
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