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September 20, 2008

Zinc Coating by Hot dipping (Galvanizing)

The zinc coating of steel by hot dipping is confusingly known as galvanizing, although no use is made of electricity in the process. It provides a cheap and effective method of producing a good rust-resisting coating on steel sheet, steel wire and on miscellaneous fabricated steel and cast iron articles, such as tanks, vessels, hardware, pipes and fittings and structural components, most of which will be exposed to the weather, or to damp air or water. Galvanizing is carried out on a very large scale and, with perhaps the exception of paint, is the most widely used protective coating system. Thus in 1965 in the UK, out of the total consumption of 357,000 tons of zinc, a total of 92,052 tons (25%) was used for galvanizing, comprising sheet and strip 23,836 tons, wire 19,596 tons, tube 13,4333 tons, and general 35,187 tons.

Galvanizing consists on essence of dipping the cleaned and fluxed steel into a skimmed bath of molten zinc, and then withdrawing and cooling it as soon as possible. A certain rapidity of processing is necessary because the molten zinc not only wets the steel, but actively reacts with it to form an intermetallic compound which grows rapidly in thickness at temperatures above the melting point of zinc. This alloy layer is intensely brittle, therefore it is essential to keep it as thin as possible; and galvanized articles should not be deformed excessively for fear of fracturing this layer.

Hot dip galvanizing of steel sheet (corrugated or otherwise) and of steel wire can be mechanized; galvanizing of fabricated parts is mostly done by hand. The parts must first be free from scale, rust and oxide; casting must be free from sand. This is mainly accomplished by pickling in acid, but large castings are sometimes shot-blasted or sang-blasted, followed by a lighter pickling process. It may be necessary to brush off detritus and residues adhering to the surface after pickling. The parts are then thoroughly rinsed and immersed in a fluxing solution, usually an aqueous solution of zinc ammonium chloride. They can held in this solution without danger of further oxidation or attack until required for galvanizing.

The galvanizing bath is a heavy vessel of welded steel plate, fired from below and from the sides. Although this is often unlined, it may be lined with refractory brick. Such a vessel may hold many tons of molten zinc (spelter)-usually between 30 and 75 tons. During galvanizing the temperature is about 450oC. The surface of the molten metal must be kept clean by constant skimming when galvanization is in progress, or a thin film of ammonium chloride may be present, although this volatilizes to irritating fumes.
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