Hot-dip galvanisation is a form of galvanisation. It is the process of coating iron, steel or aluminium with a thin zinc layer, by passing the metal through a molten bath of zinc at a temperature of around 860 °F (460 °C). When exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a usually dull grey, fairly strong material that stops further corrosion in many circumstances, protecting the steel below from the elements. Galvanized steel is widely used in applications where rust resistance is needed, and can be identified by the crystallization patterning on the surface (often called a “spangle”).
Galvanized steel can be welded; however, one must exercise caution around the resulting zinc fumes. Galvanized steel is suitable for high-temperature applications of up to 392°F (200°C). The use of galvanized steel at temperatures above this will result in peeling of the zinc at the intermetallic layer. Electrogalvanized sheet steel is often used in automotive manufacturing to enhance the corrosion performance of exterior body panels; this is, however, a completely different process.
The process of hot-dip galvanizing results in a metallurgical bond between zinc and steel with a series of distinct iron-zinc alloys. The resulting coated steel can be used in much the same way as uncoated.
A typical hot-dip galvanizing line operates as follows: 
- Steel is cleaned using a caustic solution. This removes oil/grease, dirt, and paint.
- The caustic cleaning solution is rinsed off.
- The steel is pickled in an acidic solution to remove mill scale.
- The pickling solution is rinsed off.
- A flux, often zinc ammonium chloride is applied to the steel to inhibit oxidation of the cleaned surface upon exposure to air. The flux is allowed to dry on the steel and aids in the process of the liquid zinc wetting and adhering to the steel.
- The steel is dipped into the molten zinc bath and held there until the temperature of the steel equilibrates with that of the bath.
- The steel is cooled in a quench tank to reduce its temperature and inhibit undesirable reactions of the newly-formed coating with the atmosphere.
Lead is often added to the molten zinc bath to improve the fluidity of the bath (thus limiting excess zinc on the dipped product by improved drainage properties), helps prevent floating dross, makes dross recycling easier and protects the kettle from uneven heat distribution from the burners. Lead is either added to primary Z1 Grade Zinc or already contained in used secondary zinc. A third, declining method is to use low Z5 Grade Zinc.
Steel strip can be hot-dip galvanized in a continuous line. Hot-dip galvanized steel strip (also sometimes loosely referred to as galvanized iron) is extensively used for applications requiring the strength of steel combined with the resistance to corrosion of zinc. Applications include: roofing and walling, safety barriers, handrails, consumer appliances and automotive body parts. One common use is in metal pails. Galvanised steel is also used in most heating and cooling duct systems in buildings
Individual metal articles, such as steel girders or wrought iron gates, can be hot-dip galvanized by a process called batch galvanizing. Other modern techniques have largely replaced hot-dip for these sorts of roles. This includes electrogalvanizing, which deposits the layer of zinc from an aqueous electrolyte by electroplating, forming a thinner and much stronger bond.
In 1742, French chemist Paul Jacques Malouin described a method of coating iron by dipping it in molten zinc in a presentation to the French Royal Academy. In 1836, French chemist Stanislas Sorel obtained a patent for a method of coating iron with zinc, after first cleaning it with 9% sulfuric acid (H2SO4) and fluxing it with ammonium chloride (NH4Cl).
See also 
- Bolt manufacturing process
- Corrugated galvanised iron
- Galvannealed - galvanization and annealing
- Liquid metal embrittlement
- Metal fume fever
- Sendzimir process
- Surface finishing
- Thermal spraying
- "The Hot-Dip Galvanizing Process". V&S Hot Dip Galvanzing.
- Lohse et al., Adaptation to Scientific and Technical Progress of Annex II Directive 2000/53/EC, January 2008
Lead addition described on page 12
- European Commission, Reference Document on Best Available Techniques in the Non Ferrous Metals Industries, December 2001
Primary zinc grade table on page 15