Hot dip galvanized steel (HDG) is steel that has been dipped in molten zinc in order to produce a rust-resistant coating. The term galvanizing usually refers to the hot dip method; it is also sometimes called zinc hot dipping. A characteristic crystalline pattern, called spangling, is often left on the surface of the steel. Hot dip galvanized steel is used in applications where steel may be exposed to weather, but where stainless steel is too expensive.
Steel to be galvanized is first washed and chemically treated to remove dirt and oxidation from the surface of the metal. It is then dipped in a kettle of molten zinc at 815-850°F (435-455°C). The hot zinc reacts with the steel, forming an alloyed surface with four distinct layers. The inner layer is about 75% zinc and 25% steel; the outer layer is 100% zinc. Excess zinc is then drained off, and the coated steel is cooled with cold air or water.
Hot-dip galvanized coatings are used on a multitude of materials ranging in size from small parts such as nuts, bolts, and nails to very large structural shapes. The size of available zinc baths and material handling restricts the size of steel that can be galvanized. Molten zinc baths 60 feet long and eight feet deep are common in North America. However, the maximum size that can be accommodated in the zinc bath is increased substantially, to near double bath length or depth by progressive dipping (immersing one portion of the product and then the other).