Hot-dip galvanizing

Hot-dip galvanizing is a form of galvanization. It 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 860 °F (460 °C). When exposed to the atmosphere, pure zinc reacts with oxygen to form zinc oxide, which further reacts with carbon dioxide to form zinc carbonate, a 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").

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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. 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). Use at temperatures above this level will result in peeling of the zinc at the intermetallic layer. Galvanized sheet steel is commonly used in automotive manufacture to enhance corrosion performance of exterior body panels of some models.

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. They are 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.

New Technology

Thermo-diffusion or thermal diffusion galvanizing is a precision batch galvanizing process developed in 1993 by Distek, Ltd. under the direction of Isaac Shtikan, a Russian metallurgist. The Distek process is "green" with near zero emissions, and energy efficient. The galvanized coating creates zinc/iron alloy layers similar to the old hot-dip process, but with a precision, uniform coating on external and internal surfaces. Due to the metallurgy of the coating, it is more corrosion and wear resistant than that applied by the hot-dip method.

History

In 1742, Melouin - a French chemist - described, in a presentation to The French Royal Academy, a method of coating iron by dipping it in molten Zinc. In 1836, another French Chemist named Sorel, obtained a patent for a means of coating iron with Zinc after first cleaning it with 9% Sulfuric Acid and fluxing it with Ammonium Chloride.

See also

• Bolt manufacturing process
• Corrugated galvanised iron
• Liquid metal embrittlement
• Plating
• Sendzimir process
• Surface finishing