Double Dip Galvanizing

Definition - What does Double Dip Galvanizing 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. This is done by passing the metal structure through a molten zinc bath more than once at a temperature of 860°F (460°C) to form zinc carbonate (ZnC03). Zinc carbonate is a strong material that protects steel and prevents corrosion in many circumstances.

DDG is different from the hot dip galvanizing process (HDG). HDG follows the same process but is done for smaller steel structures and the structure is only passed through the molten zinc bath one time.

Corrosionpedia explains Double Dip Galvanizing

Double dip galvanizing involves three main steps:

  1. 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 de-greased 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.
  2. Galvanizing: When the clean iron or steel component is dipped in the molten zinc, zinc-iron alloy layers form 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 present on top of the alloy layer. When it cools, it has the bright, shiny appearance associated with galvanized products. As the name suggests, in double dip galvanizing, the steel structure is dipped more than once into the molten zinc bath because of its large size or complex structure.
  3. Inspection: After galvanizing, the coated structure is inspected for coating thickness and coating appearance. A variety of simple physical and laboratory tests may be performed to determine thickness, uniformity, adherence and appearance of the zinc coating.

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