Hot-Dip Galvanizing (HDG)
Hot Dip Galvanizing is a process developed to prevent steel from corroding. Before the process can take place, the steel goes through a thorough chemical clean; this removes all rust, oil and mill scale from the surface. When the cleaning process has been completed and the cleaning solution has been rinsed off, the coating process can begin. The steel is dipped into a bath of molten zinc that has been heated to around 460 °C (860 °F). The steel is then removed from the bath and left to cool in a quench tank. When the cooling process is complete, the zinc coating is then metallurgically bonded to the steel.
A fairly strong material that protects the steel underneath from further corrosion in many circumstances. Galvanized steel is widely used in applications where corrosion resistance is needed without the cost of stainless steel, and is considered superior in terms of cost and life-cycle.
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 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 may be performed to determine thickness, uniformity, adherence and appearance of the zinc coating.
Advantages of Hot-Dip Galvanized Steel
1) Long life
Galvanizing provides an easy to clean surface which can give a maintenance-free life of over 50 years. (This is dependent on the environment it is being used in). When maintenance eventually becomes necessary, it is straightforward: no complex preparation treatments are necessary.
2) Competitive first cost
For many applications the cost of hot dip galvanizing is lower than that of applying alternative coatings. The reason is simple: alternatives – particularly painting – are very labour intensive compared with galvanising which is a highly mechanized, closely controlled factory process.
3) Reliability
The process is relatively simple, straightforward and closely controlled. The thicknesses (weights) of coatings formed are regular, predictable and simply specified. Hot dip galvanizing is one of the few coatings which is completely defined by a British Standard (BS EN ISO 1461).
4) Coating toughness
Galvanizing is unique: the hot dip process produces a coating which is bonded metallurgically to the steel. No other coating process has this feature, and as a result galvanised steel has by far the greatest resistance to mechanical damage during handling, storage, transport and construction – an important factor where steelwork is to be shipped around the world.
5) Complete coverage
Because it is formed by dipping steel in molten zinc, all parts of the surface of the steel are coated – inside, outside, awkward corners, and narrow gaps which would be impossible to protect in any other way.
The coating actually tends to build up at vital corners and edges – rather than thinning out as do brushed, sprayed and other dipped coatings.
See also