Hot-Dip Galvanizing

Hot-dip galvanizing is a process that coats steel products with zinc to prevent corrosion.  Industry has used this process for the last 250 years to protect steel.  Hot-dip galvanizing became prominent in American industry in the 1890s. The process has continued as a mainstay since.

Galvanizing protects many common steel products.  For example, a zinc coating protects highway guardrails.  Galvanized steel is a prominent feature of historic landmarks like the Brooklyn Bridge. It is also a feature of more recent projects.  NASAs launch pad sound suppression system uses galvanized steel.

The Basic Process
Hot-dip galvanizing takes the steel product through the following steps

1. This step removes soil, grease, and other surface contaminants.
2. Pickle the surface with a dilute acid solution.  This step removes surface rust and mill scale.  It provides a clean surface for bonding.
3. Dip the steel in a liquid flux bath.   This step removes oxides.  It also prevents oxidation as the structure moves through the remaining steps in the process.  Zinc ammonium chloride solution is the most common fluxing bath.
4. Dip the steel in a molten zinc bath at 815-850 F (435-455 C).
5. Drain, vibrate, or centrifuge the structure to remove excess zinc.  The structure can be air dried and cooled.  Or it can be quenched in a liquid.
6. Inspect the structure for the desired coating thickness and surface conditions.

Galvanizing causes the zinc to form a metallurgical bond with the steel.  A series of zinc-iron alloy layers form.  These layers are extremely resistant to abrasion.  Atop the zinc-iron alloy layers, a layer of impact-resistant pure zinc forms.

Dry and Wet Processes
Hot-dip galvanizing can follow a dry or wet process.  In the dry process, the steel structure dries after it is immersed in the flux bath and before it is immersed in the molten zinc.  In the wet process, the flux bath floats atop the molten zinc.  The steel passes through the flux bath immediately before it enters the molten zinc.

Service Life
The American Galvanizers Association has estimated the service life of zinc coated steel structures.  The estimates assess corrosion rates in rural, temperate marine, tropical marine, suburban, and industrial climates.  These values and other facts about the service life of zinc coated components appear on the AGA web site (http://www.galvanizeit.org).  Even in the most hostile settings, service lives of 150 years are likely with zinc coatings of 200 mils in thickness.  Service life is the time required for a steel product to rust over 5% of its surface.