Zinc has been used for its sacrificial protection of steel for centuries and galvanizing for over 150 years. A variety of zinc coatings have been developed to take advantage of galvanic protection. For more information, download Zinc Coatings.
Hot-dip galvanized after-fabrication (batch or general)
Galvanizing at its most basic level is dipping steel products (structural beams, weldments, grating, handrail, fasteners, etc.)into a molten zinc bath. A metallic reaction ensues and a thin coating of zinc is formed, evenly covering all areas of exposed base metal. This simple approach is still used today; called after-fabrication hot dip galvanizing, this process is also commonly referred to as batch or general galvanizing. Products, assemblies, and fabrications coated with zinc by this process are most widely used in exterior applications but are also used in interior designs.
ASTM specification A123/A123 M-02, Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, A153/A153 M-03, Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, and A 767 Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement detail how the process is done today.
Hot-dip galvanized – continuous
In the hot-dip continuous process hot-rolled steel in coil form is uncoiled, passed through a series of cleaning steps and molten zinc at very high speed (500-600 fpm), dried, and recoiled. (The immersion elements of the process are virtually identical to hot-dip galvanizing after fabrication.) The galvanized steel in coil form is then either slit into narrower coils for use in specific high speed fabrication machines or cut to length for general use in bending, rolling or stamping operations.
Because the coating on hot-dip continuous sheet steel is relatively thin compared to after-fabrication hot-dip galvanized steel, it is mostly used for interior applications such as ductwork or as the substrate for appliances and automotive bodies to be painted or powder coated.
ASTM A653 / A653M – 07 Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process governs the coating of steel via the continuous process.
Paints containing zinc dust, commonly referred to by the misnomer “cold galvanizing,” are used to provide some level of corrosion protection when hot-dip galvanizing after fabrication is not possible, such as when the part/weldment/structural beam is too large to fit in the galvanizing kettle containing the molten zinc. The paint may be applied either by spray or brush. Inherent in the application process is uncertainty in applying a consistent coating thickness on flat surfaces and a thinner coating on edges and corners, where corrosion usually begins. Zinc-rich paints from different manufacturers contain zinc dust in a variety of concentrations, thus providing uncertain cathodic protection.
Metallizing (zinc spray)
Zinc in wire or powder form may also be applied to clean steel by feeding it into a heated gun, where it is melted and sprayed on using combustion gases and/or auxiliary compressed air to provide the necessary velocity. Metallizing may be used on any size steel, although applying a consistent coating and reaching recesses, hollows and cavities is difficult. Although there is little or no zinc-iron alloy layer formation as in the hot-dip galvanizing process, metallizing does apply pure zinc and thus there is some level of cathodic protection.