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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 . 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 , 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.
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.ftp://ftp.jrc.es/pub/eippcb/doc/nfm_bref_1201.pdf
Primary zinc grade table on page 15
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, Handrail, consumer appliances and automotive body parts. One common use is in metal bucket. 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.
In 1772, Luigi Galvani, for whom galvanizing was named, discovered the electrochemical process that takes place between metals during an experiment with frog legs.
In 1801, Alessandro Volta furthered the research on galvanizing when he discovered the electro-potential between two metals, creating a corrosion cell.
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 (sulfuric acid) and fluxing it with ammonium chloride (NH4Cl).
Further performance and design information for galvanizing can be found in BS EN ISO 14713-1 and BS EN ISO 14713-2. The durability performance of a galvanized coating depends solely on the corrosion rate of the environment in which it is placed. Corrosion rates for different environments can be found in BS EN ISO 14713-1, where typical corrosion rates are given, along with a description of the environment in which the steel would be used.
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