A process by which zinc is coated over corrosive metals is known as galvanizing. It is actually a method of coating corrosive metals, such as steel and iron, with a non-corrosive metal. Zinc is melted and applied, usually via what’s known as a hot dip, to the metal, providing a coating of corrosion protection from one mil (0.001 of an inch or 0.0254 mm) to just over four mils (0.004 inches or 0.1016 mm) thick. When cured, the zinc, through reaction with the coated metal, becomes zinc carbonate.
The galvanizing process not only prevents corrosion of various “soft” metals, but adds to the strength of the original, uncoated metal. Obviously, galvanized metal is thicker than uncoated metal, so fittings and fastenings are generally measured with the additional coating thickness in mind. Various American Society for Testing and Materials (ASTM) specifications provide guidelines and continuity for the thickness of galvanized metals.
Galvanization of nails and screws is the most common method of preventing the unsightly staining seen on many types of house siding. Non-galvanized steel nails and screws, when used outdoors, will “bleed” when they corrode, causing dark stains on the siding. This staining is only eliminated by re-painting the siding. Staining on a building facade due to corroded nails, screws, or other types of fasteners, is not only ugly, but also indicates that, because they're not galvanized, the fasteners are deteriorating and must be replaced.
The hot dip procedure to galvanize metals is essentially a bath of molten zinc. The zinc is kept liquefied at a temperature of about 860°F (460°C) and the metals to be coated are dipped into or, in some cases, fed through this zinc bath. Prior to dipping, metals are cleaned and prepped for the hot dip by pickling in a light acid solution. The zinc coating is ordinarily distinguishable from shiny bare steel or iron in that it is a dull, medium gray. Magnesium is put into the hot dip solution when galvanized metals are to be used in a marine environment.
Electro-galvanizing is an electrolytic process where a thinner, tighter-bonding coat of zinc is applied to a metal via electroplating. In this process, an electric current is passed through a zinc compound, positively charging the zinc ions to adhere more securely to the conductive primary metal. It provides a somewhat stronger tensile strength to the coated metal by virtually impregnating the metal with zinc. This process is most commonly employed with iron or steel beams, angle-irons, and other items that are to be used in building construction. Hot dip and electro-galvanizing are the most common of several methods of galvanizing metal.