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Hot-dip galvanizing

Hot-dip galvanizing is the process of coating iron or steel with a thin zinc layer by passing the steel through a molten bath of zinc at a temperature of around 450C. Zinc "rusts" to form zinc oxide, a fairly strong material that stops further rusting, 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 crystalization patterning on the surface (often called a "spangle").

The bond between the zinc and steel is fairly strong, and the resulting coated steel can be used in much the same way as uncoated. One exception is that re-heating the steel will cause potentially poisonous zinc vapours to be released, so galvanized steel cannot (typically) be welded, although it is often spot welded because the point of heating is small and brief. Galvanized steel is also not safe for use in roles where fire resistance is important, so "pure" galvanized steel is not commonly found in automobile applications for instance.

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 and resistance to corrosion. Applications include: roofing and walling, consumer appliances and automotive body parts. One common use is in metal pails.

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 electrodeposition, forming a thinner and much stronger bond.



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