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the decomposition of zinc or a zinc alloy on a metal object to impart certain physical and chemical properties to the surface, mainly high corrosion resistance. Galvanizing is the most common and economical metallization method used to protect iron and its alloys from atmospheric corrosion, and approximately 40 percent of the world’s zinc output is used for this purpose. The more corrosive the environment and the longer the prospective service period, the thicker must be the zinc coating (10 to 50 microns).
Zinc is applied to steel sheets, strips, wires, fastenings, machine and instrument components, and pipes. It usually has no decorative function; however, the appearance can be made more acceptable commercially by passivating zinc-coated objects in Chromate solutions, which imparts an iridescence.
The most widely used form is zinc-coated strip, which is produced by the hot-dip method on automated lines, that is, by immersion in molten zinc. The spraying method can be used to coat objects of any size, for example, power transmission towers, but the coating has considerable porosity and much zinc is wasted. Galvanizing by electrodeposition (electrogalvanizing) is carried out by acidic or alkaline cyanide electrolytes; special additives make it possible to obtain a lustrous coating. Galvanizing by diffusion, which is carried out from the vapor or gas phase at high temperatures (375°–850°C), is used to coat pipes and other structural components that function in a humid atmosphere, in gasoline and kerosene, and in gaseous media containing sulfur. The thickness of the diffusion layer, usually between 0.1 and 1.5 mm, depends on the temperature and the galvanizing time.
REFERENCESProskurkin, E. V., and N. S. Gorbunov. Diffuzionnye tsinkovye pokrytiia. Moscow, 1972.
Lainer, V. I. Zashchitnyepokrytiia metallov. Moscow, 1974.
V. I. LAINER and G. N. DUBININ
The generic term for any of several techniques for applying thin coatings of zinc to iron or steel stock or finished products to protect the ferrous base metal from corrosion; more specifically, the hot dipping that is widely practiced with mild steel sheet and corrugated sheets. During dipping, molten zinc reacts with the steel to form a brittle zinc-iron alloy. For marine use, magnesium is added.
An electrolytic process (also called cold galvanizing or electrogalvanizing) is also used for wire, as well as for applications requiring deep drawing. An alloy layer does not form, hence the smooth electroplated coating does not flake in the drawing die. See Metal coatings