Another aspect of hot-dip galvanizing is proven durability. Hot-dip galvanized steel has been specified extensively in petro-chemical, industrial, power/utility, and bridge/highway projects because of its unmatched durability in these harsh environments. Hot-dip galvanizing remains durable thanks to its abrasion resistance, uniform protection, and complete coverage.
A unique characteristic of the hot-dip galvanized coating is the development of metallurgically bonded (~3,600 psi), abrasion resistant intermetallic layers. This photomicrograph is a cross-section of a galvanized steel coating showing the three intermetallic layers (Gamma, Delta, and Zeta) and top layer of pure zinc (Eta). During the galvanizing process, these layers develop naturally during a metallurgical reaction between the iron in the steel and zinc in the kettle. As the photomicrograph also shows the hardness of each of the layers as a Diamond Pyramid Number (DPN), you can see the three intermetallic layers are harder than the base steel, while the eta layer has ductility which makes damaging the HDG coating very difficult. Hot-dip galvanizings abrasion resistance provides unmatched protection against damage caused by rough handling during transport and erection, as well as in service. Other coatings with lower bond strengths (300-600 psi) can be easily damaged during shipment and construction, weakening their effectiveness, as barrier protection is dependent upon the integrity of the coating.
Another aspect of HDGs durability is its uniform protection. During the metallurgical diffusion reaction in the galvanizing kettle, the galvanized coating grows perpendicular to all surfaces. Therefore, the coating is naturally as thick on corners and edges as flat surfaces. Since coating damage commonly occurs at edges, added protection at these junctures is important. Brush- or spray-applied coatings have a natural tendency to thin at corners and edges leaving the part prone to attack. The uniform protection of hot-dip galvanized steel leaves no weak points for accelerated corrosion.
Hot-dip galvanizing is a total immersion process, meaning the steel is fully submerged into cleaning solutions and the molten zinc coating all interior and exterior surfaces. This complete coverage ensures even the insides of hollow and tubular structures and the threads of fasteners are coated. As corrosion tends to occur at an increased rate on the inside of hollow structures where humidity and condensation occur, interior coverage is very beneficial. Hollow structures that are painted have no corrosion protection on the inside. Fully coating fasteners is equally important as they are utilized at connection points which are critical to structural integrity.
Durability Case Study
Texas Motor Speedway, Ft. Worth, TX - 1996
As they say, everything is bigger in Texas. With a seating capacity of over 150,000 in a 3,500-foot grandstand, Texas Motor Speedway is the second-largest sports facility in the United States and the third-largest in the world. Galvanized in 1996, Texas Motor Speedway is the home to hundreds of thousands of racing fans annually and remains corrosion-free today. The galvanizer collaborated closely with the fabricator to determine the most efficient and practical methods to achieve a coating that would last for decades. Their efforts produced a winning facility that was recognized as The Most Distinguished Project in the 1997 Excellence in Galvanizing Awards competition.
As an outdoor facility, Texas Motor Speedway is subject to perpetual abuse from the elements 365 days per year. HDG provided the durability and resilience that would withstand the sweltering summers and damp, brisk winters that are characteristic of North Texas. Cathodic protection and the zinc-iron alloy layers that are provided by HDG far exceeded other forms of corrosion protection, most of which offer merely barrier protection. The fact that the zinc-iron alloys become harder and more scratch-resistant than the substrate steel proved to be an additional benefit of choosing HDG over other methods when considering the millions of spectators that have come in contact with the steel structure.