Hot-dip galvanization

Hot-dip galvanization is a form of galvanization (the process of coating iron and steel with zinc) in which the iron or steel is immersed in a bath of molten zinc at a temperature of around 450 °C (842 °F). In such process, zinc alloys with the surface of the base metal. 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 protects the steel underneath from further corrosion in many circumstances.

Galvanized fumes are released when the galvanized metal reaches a certain temperature. This temperature varies by the galvanization process used. In long-term, continuous exposure, the recommended maximum temperature for hot-dip galvanized steel is 200 °C (392 °F), according to the American Galvanizers Association. The use of galvanized steel at temperatures above this will result in peeling of the zinc at the inter-metallic layer.

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:

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), help prevent floating dross, make dross recycling easier and protect the kettle from uneven heat distribution from the burners. Environmental regulations in the United States disapprove of lead in the kettle bath. 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, such as 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 some cases, it may be desirable to have designated parts of the metal as non-galvanized. This is often desired when metal will be welded after galvanization. To accomplish this, a galvanizer will typically use a masking compound to coat the areas that will not be galvanized during the hot dip process.