Hot-Dip Galvanizing (HDG) is the process of coating iron, steel, or ferrous materials with a layer of zinc. This is done by passing the metal through molten zinc at a temperature of 860°F (460°C) to form zinc carbonate (ZNC03). Zinc carbonate is a strong material that protects steel and can prevent corrosion in many circumstances.
Hot-Dip Galvanizing involves three main steps:
- Preparation: The galvanizing reaction will only occur on a chemically clean surface, so the first step of the process involves removing contamination. First, the metal is degreased using a caustic solution and then dipped in hydrochloric acid to remove rust, mill scale, welding slag, paint, and grease. This is followed by a rinse and a dip in a flux solution, which is usually about 30 percent zinc ammonium chloride.
- Galvanizing: When the clean iron or steel component is dipped into the molten zinc (at 842°F (450°C)), zinc-iron alloy layers form as a result of a metallurgical reaction between the iron and zinc. When the material is pulled from the galvanizing bath, a layer of molten zinc is present on top of the alloy layer. When it cools, it has the bright, shiny appearance associated with galvanized products.
- Inspection: After galvanizing, the coated materials are inspected for coating thickness and coating appearance. A variety of simple physical and laboratory tests are performed to determine the thickness, uniformity, adherence, and appearance of the zinc coating.