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Galvanizing is a process of coating iron or steel with zinc in order to provide greater protection against corrosion for the iron or steel base. The process of galvanizing sheet iron was developed simultaneously in France and England in 1837. Both of these methods employed a "hot dipping" process to coat sheet iron with zinc. Like tinplate, early galvanized metals were hand dipped. Today almost all galvanized iron and steel is electroplated.

The following are the most common methods for applying protective coatings of zinc to iron and steel:

Hot-dip Galvanizing

The immersion of iron or steel in molten zinc, after the surface of the base metal has been properly cleaned.

This process gives a relatively thick coating of zinc that freezes into a crystalline surface pattern known as spangles.

During the process, a multiple layered structure of iron- or steel-zinc alloys is formed between the inner surface of the zinc coating and the iron or steel. These middle layers tend to be hard and brittle and may peel or flake if the iron or steel element is bent.

Electro galvanizing

The immersion of iron or steel in an electrolyte, a solution of zinc sulfate or cyanide. Electrolytic action deposits a coating of pure zinc on the surface of the iron or steel.

Advantages:

路The thickness of the coating can be accurately controlled using this process.

Limitations:

路The thick coatings provided by the hot-dip galvanizing process are not usually possible with this method.

Sherardizing

The placing of a thoroughly cleaned iron or steel element in an air-free enclosure where it is surrounded by metallic zinc dust. The architectural element is then heated and a thin, zinc alloy coating is produced.

Advantages:

路The coating will conform to the configurations of the element.

Limitations:

路This process is usually limited to relatively small objects.

Metallic Spraying

The application of a fine spray of molten zinc to a clean iron or steel element. The coating can then be heated and fused with the surface of the iron or steel to produce an alloy.

Advantages:

路Coating is less brittle than those produced by some of the other processes.

路Coating will not peel or flake on bending.

Limitations:

路The coating is more porous and becomes impermeable with time as products of corrosion fill in the pores.

Painting

Paint containing zinc dust pigments may be applied as a protective Advantages

Advantages

路The paint may be applied in situ.

Limitations:

路This is a less effective method of zinc coating than the others listed above. Paint does not adhere well to pure zinc, nor to galvanized iron or steel.

路When paint peels from galvanized iron and steel, it usually comes off completely along with the primer, exposing a clean metal surface.

路If sheet metal features are well-painted, it is difficult to identify whether they are zinc or galvanized iron or steel.

路If the metal is galvanized, it will have a spangled appearance and may show some rust or rust stains from the iron or steel base metal. Both galvanized iron and steel are magnetic

路If the metal is cast or pressed zinc, it will have a grayish-white appearance. Pure zinc is not magnetic so a magnet will not stick.

路A magnet test will also reveal whether a painted sheet metal feature is zinc or galvanized iron or steel. Both galvanized iron and steel are magnetic, pure zinc is not.