batch galvanizing vs. continuous sheet galvanizing

Zinc has been used for its sacrificial protection of steel for centuries and galvanizing for over 150 years. A variety of galvanized coatings have been developed to take advantage of galvanic protection.

hot-dip or after-fabrication
Galvanizing at its most basic level is dipping steel products 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 and A153 / A153 M-03, Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware detail how the process is done today.

sheet steel or continuous
Another series of hot dip galvanized steel products also exists. They are sheet steel products and while the coating is still formed by dipping steel into molten zinc, the process is fully mechanized and done at very high speeds. Coils of steel sheet metal are fed as ribbon through a molten metal bath where it reacts to leave a protective surface coating. The operation grew out of traditional after-fabrication hot-dip galvanizing into a very sophisticated process that can be used to applythin and specific coating grades. These coating grades are in the form of a letter G, Z, and A followed by a coating weight in mass per area.

This process is also called continuous galvanizing and is specified in ASTM A 653/ A653 M-02, Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process. Common coating weights specified for sheet products are: G60, G90 and G185. These also exist as metric counterparts with G90 being equivalent to a Z275 coating.

other coating alloys
Along with galvanized sheet, other coating alloys have been developed. These include: galvanneal, galvalume, galvfan, aluminized and terne.Sheet products are typically used for interior applications because they provide only short-term corrosion protection if used outside. Additionally, many of these coatings are used extensively for exterior use, but in those instances products are typically coated with another protective system on top of the zinc, such as paint.

Since the two galvanizing processes are very different, there are also two different associations that support these industries. The American Galvanizers Association (AGA), formerly the American Hot-Dip Galvanizers Association (AHDGA), assists the batch/general galvanizers and the GalvInfo Center represents the continuous steel sheet products.

A more detailed description of the differences in the two processes is available here in GalvInfo Note #9.

Questions concerning common sheet products like G60, G90, and G185 and the other sheet products listed above can be answered by the GalvInfo Center.