Hot-dip galvanized steel is coated in molten zinc to resist rust and corrosion. The process forms an alloy surface with four layers, and the zinc acts as a sacrificial anode to protect the underlying steel. While it can last over 100 years, exposure to abrasives and chemicals will eventually erode the coating. Stainless steel may be a better choice for harsh conditions.
Hot-dip galvanized steel (HDG) is steel that has been dipped in molten zinc to produce a rust-resistant coating. The term galvanizing usually refers to the hot dip method; it is also sometimes called hot-dip zinc. A distinctive crystalline pattern, called a sequin, is often left on the surface of the steel. Hot-dip galvanized steel is used in applications where the steel can be exposed to the elements, but where stainless steel is too expensive.
The steel to be galvanized is first washed and chemically treated to remove dirt and oxidation from the metal surface. It is then immersed in a kettle of molten zinc at 815-850°F (435-455°C). The hot zinc reacts with the steel, forming an alloy surface with four distinct layers. The inner layer is approximately 75% zinc and 25% steel; the outer layer is 100% zinc. The excess zinc is then drained off and the coated steel is cooled with cold air or water.
When exposed to 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. Secondly, zinc acts as a sacrificial anode. Zinc oxidizes more quickly than iron, giving up electrons in the process. These electrons then travel to the steel, where they reduce the iron’s chemical reactivity and slow down the corrosion process. The term sacrificial refers to the fact that the zinc sink available for oxidation is depleted over time, preventing the iron in the steel from suffering the same fate.
Despite this double defense, all hot-dip galvanized steel will eventually rust. The zinc coating only slows down this process. As free zinc is consumed through oxidation, its role as a sacrificial anode decreases. The coating will also physically erode from exposure to abrasives and chemicals in the environment, reducing its effectiveness as a mechanical barrier to air. Acid rain and salt water will both increase the rate at which the coated metal will break down.
The galvanizing process is more than 150 years old and hot-dip galvanized steel is used in a wide variety of applications. It is used for structural steel beams, corrugated sheets, nuts, bolts, pipes, gratings, steel railings and walkways, among other things. Industry claims that galvanized steel can last for over 100 years in typical installations. If steel is to be used in locations where it will be exposed to seawater, mechanical abrasion, acids, or other harsh conditions, stainless steel may be a better choice in the long run.
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