How do iron-zinc alloy layers form during hot-dip galvanizing?

During the hot-dip galvanizing process, iron-zinc alloy layers form on the surface of the iron or steel when the metal is submerged in a bath of molten zinc at approximately 450 °C (840 °F).12

As the cleaned and flux-treated steel enters the molten zinc, a metallurgical reaction occurs at the interface between the iron and the zinc, resulting in the growth of intermetallic iron-zinc compounds. These alloy layers form directly on the surface of the base metal and are typically topped with an outer layer of pure zinc.12

In some specialized applications, such as continuous coating, small amounts of aluminum are added to the zinc bath to suppress the formation of these brittle iron-zinc compounds, which results in a more flexible coating. Conversely, other methods—such as heating articles in a drum of zinc powder—can also induce the formation of these alloy layers at temperatures below the melting point of zinc.13