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.
When exposed to the air, zinc reacts with oxygen, water, and carbon dioxide to form a layer of tough zinc carbonate (ZnCO3). This protects the underlying steel in two ways: first, it provides a mechanical barrier between the steel and the atmosphere, minimizing exposure to oxygen. Second, the zinc acts as a sacrificial anode. Zinc oxidizes more rapidly than iron, giving up electrons in the process. These electrons than travel to the steel, where they reduce the chemical reactivity of the iron and slow the corrosion process. The term sacrificial refers to the fact that the well of zinc available for oxidation is used up over time, preventing the iron in the steel from suffering the same fate.
Despite this two-pronged defense, all hot dip galvanized steel will rust eventually. The zinc coating only slows this process. As free zinc is used up through oxidation, its role as a sacrificial anode becomes diminished. The coating will also physically erode as a result of exposure to abrasives and chemicals in the environment, reducing its effectiveness as a mechanical barrier to the air. Acid rain and salt water will both increase the speed at which the coated metal will break down.
The galvanizing process is over 150 years old, and hot dip galvanized steel is employed in a wide variety of applications. It is used for structural steel beams, corrugated steel sheets, nuts, bolts, pipes, grating, railing, and steel walkways, among other things. The industry claims that galvanized steel can last for over 100 years in typical installations. If the steel is to be used in places where it will be exposed to seawater, mechanical abrasion, acids, or other harsh conditions, stainless steel may be a better choice for the long term.