Electrogalvanizing

The development of continuous electrogalvanizing lines added another dimension to zinc-coated steel, that is, very thin, formable coatings ideally suited to deep drawing or painting. Zinc is electrodeposited on a variety of mill products by the steel industry: sheet, wire, and, in some cases, pipe. Electrogalvanizing at the mill produces a thin, uniform coat of pure zinc with excellent adherence. The coating is smooth, readily prepared for painting by phosphatizing, and free of the spangle that is characteristic of some other zinc coatings.

Electrogalvanized steel is produced by electrodepositing an adhering zinc film on the surface of sheet steel or wire. These coatings are not as thick as those produced by hot dip galvanizing and are mainly used as a base for paint. Processing details applicable to electrogalvanizing can be found in the article "Continuous Electrodeposited Coatings" in this Volume.

The coating produced on strip coils or sheets has a coating weight in the range of less than about 18 to 60 g/m2 (0.06 to 0.2 oz/ft2), or 1.3 to 4.3 pm (0.05 to 0.17 mil) thick on each side. A small amount carries considerably less--approximately 7.6 g/m2 (0.025 oz/ft2), or 0.5 pm (0.21 mil) on each side.

Zinc is usually electrodeposited on steel wire in the range of 90 to 915 g/m 2 (0.3 to 3 oz/ft2). The diameter of plated wire (including wire that is cold drawn after plating) usually ranges from 0.23 to 4.9 mm (0.009 to 0.192 in.). Steel carbon contents range from 0.08 to 0.85%. Tensile strengths range from 345 to 2070 MPa (50 to 300 ksi). Heat-treated and coated wire can be cold drawn to approximately 95% reduction in area, depending on chemical composition, heat treatment, and diameter.

Nature of the Electrogalvanized Coating. The pure zinc coating deposited is highly ductile. Because of its excellent adhesion, electrogalvanized steel strip and wire have good working properties, and the coating remains intact after severe deformation.

Electrodeposited zinc coatings are simpler in structure than hot dip galvanized coatings. They are composed of pure zinc and have a homogeneous structure. Surfaces have a smooth texture whose appearance can be varied by additives and special treatments in the plating bath. They can be used where a fine finish is needed.

Electrogalvanizing provides adequate protection for many types of mild exposures. These coatings are frequently treated with chromate conversion solutions to improve appearance, reduce staining, and retard the formation of white corrosion products under high-humidity conditions.

Advantages and Limitations. In electrogalvanizing, steel strip or wire is continuously fed through suitable entry equipment, a series of washes and rinses, and a plating bath. Either an acid sulfate zinc or cyanide zinc bath is used as the plating bath. Both produce even, adhering zinc deposits. Although brighteners are not used for electrogalvanizing, grain refiners are usually added to help produce a fine, tightknit zinc surface on the steel.

Zinc electrodeposits are considered to have the best adhesion of any metallic coating. Good adhesion depends on very close physical conformity of the coating with the base metal. Therefore, particular care must be taken during initial cleaning. Electrodeposition affords a continuous process for applying zinc coatings to parts that cannot be hot dipped. They are especially useful where a high processing temperature could damage a part.

Applications. Electrogalvanized sheets are produced in various tempers suitable for simple bending or forming, for curving, and for rolling into cylinders without fluting. Spot welding is easily accomplished if care is taken.

Electrogalvanized steel is easily prepared to receive decorative finishes. Much of it is produced with a phosphate treatment or an organic coating. The phosphate treatment provides an adequate surface for a good bond with organic finishing materials. Organic coating applied over electrozinc thus treated maintains good adhesion in adverse conditions, such as sudden changes in temperature and high humidity. Phosphated electrogalvanized steel is used for parts subject to atmospheric corrosion or salt spray and for parts that will be lacquered or painted. Phosphate treatment increases corrosion resistance markedly, particularly in atmospheres with a high sulfur content.

Electrogalvanized sheet is used for manufacturing water cooler housings, exterior panels of ranges, freezers, dryers, washers, air conditioners, and other major appliances. It is used for deep-drawn parts for kitchen cabinets, refrigerators, and allied products instead of plain cold rolled sheet because zinc holds better in the dies and reduces breakage significantly. Bakery goods and other merchandizing display cases, stud systems for steel building construction, acoustical ceiling members, and television antennas are also made of electrogalvanized steel.

Sheets for automotive applications can be electrogalvanized on one side only. The galvanized side protects against corrosion, and the bare side can take the baked enamel finish required by the outer automobile surface.

Electrogalvanized wire is especially useful in applications in which the wire must be bent, twisted, or wrapped around its own diameter. When formed, the coating does not crack, peel, or flake. Many chain link fences are made from zinc-electrocoated wire because it is not rough and therefore is safe to handle. The wire is used for conveyor belts, twisted wire brushes, chains, baskets, kitchen utensils, staples, cages, bobby pins, clotheslines, and telephone and transmission wire.

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