Double Dip Galvanizing (DDG)
Definition - What does Double Dip Galvanizing (DDG) mean?
Double dip galvanizing (DDG) is the process of coating large iron, steel or ferrous materials with a layer of zinc by dipping them multiple times in a molten zinc bath at a temperature of 460°C (860°F) to form zinc carbonate (ZnCO3).
Zinc carbonate has corrosion resistance properties that in many instances protect steel from corrosion.
There is a difference between double dip galvanizing (DDG) and hot dip galvanizing (HDG) where smaller steel structures are passed only once through a molten zinc bath.
Corrosionpedia explains Double Dip Galvanizing (DDG)
Double dip galvanizing involves three main steps:
- Preparation: The galvanizing reaction occurs on a chemically clean surface, so the first step of the process involves removing impurities. First, the metal is de-greased using a caustic solution and then dipped in hydrochloric acid to remove rust, welding slag, paint, mill scale and grease. It is then rinsed and dipped in a flux solution. This flux solution is usually about 30 percent zinc ammonium chloride.
- Galvanizing: When the clean iron or steel component is dipped in the molten zinc, a zinc-iron alloy layer is formed as a result of a metallurgical reaction between the iron and the zinc. When the material is pulled from the galvanizing bath, a layer of molten zinc is formed on top of the alloy layer. After cooling it has the bright, shiny appearance associated with galvanized products. As the name suggests, the steel structure is dipped more than once in the molten zinc bath due to its large size and complex structure.
- Inspection: After galvanizing, the structure is inspected for coating thickness and appearance. A variety of simple physical and laboratory tests may be performed to determine thickness, uniformity, adherence and appearance of the zinc coating.