Hot-dip galvanizing (HDG) is the process of immersing fabricated steel or iron into a kettle (bath) of molten zinc. While in the kettle, iron in the steel metallurgically reacts with the zinc to form a tightly-bonded alloy coating. It resists corrosion by providing barrier and cathodic protection, as well as through the development of the zinc patina. These three levels of corrosion protection provide galvanized steel with maintenance-free longevity for decades.
Many specifiers are familiar with one type of corrosion protection system or another, but few realize the intrinsic value of utilizing two together, called a duplex system. A duplex system is formed by painting or powder coating over hot-dip galvanized steel. Used for decades as a means to enhance corrosion protection, the synergy between the two systems provides protection far superior to either system used independently.
Zinc, a natural, healthy, and abundant element was first used in construction in 79 AD. Zinc metal has a number of characteristics that make it a well-suited corrosion protective coating for iron and steel products. Zinc’s excellent corrosion resistance in most environments accounts for its successful use as a protective coating on a variety of products and in many exposure conditions.
Protective coatings, such as paint and powder coatings, applied to steel provide barrier protection. As barrier protection is dependent on the integrity of the coating, the selection, application, and handling of painted and powder coated materials is very important. It is imperative these coatings are handled with care during installation and if damaged are repaired to ensure they are as durable as planned.
Special steels such as stainless and weathering provide corrosion protection by taking advantage of specific chemistries and elements. Weathering and stainless steel tend to work best in narrowly defined environments or applications and each has its own limitations.