In the United States most galvanized products, such as telegraph wire, garbage cans, and roofing and siding for buildings, are galvanized by the hot dipping method. In this process, iron or steel sheets are dipped into molten zinc. The zinc combines with the iron or steel to produce a protective layer of zinc-iron alloy overlaid by a layer of zinc. The entire coating is usually from 0.0002 inch to 0.004 inch thick.
Electrogalvanizing, a less costly process, produces a thinner, less durable zinc coating. Here the iron or steel is placed in a solution of zinc or zinc salts. A strong electric current is passed through the solution to deposit a layer of 99.99 percent pure zinc on the surface of the steel or iron.
Sherardizing is a process used to galvanize small items, such as nails or bolts. These are placed in a rotating drum containing fine zinc dust. The drum is heated to a temperature from 350 °C to 370 °C. (660 °F to 700 °F.), well below the melting point of zinc. As in hot dipping, a layer of zinc-iron alloy forms on the surface of the iron or steel.
Large objects are often zinc-coated by being sprayed with molten zinc. This does not result in true galvanizing, since an alloy is not formed with the zinc.
Galvanizing is named for Luigi Galvani, an 18th- century Italian physician who performed early experiments in electricity.
