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How Does Hot Dip Galvanizing Work?

Jan. 09, 2023

Once you’ve invested in quality iron or steel equipment, it’s important to maintain that equipment well. In addition to cleaning that equipment properly, it’s essential to protect it, and one way to do this is through galvanization. But what is galvanizing, and how does it work? This article will answer those questions.

What Is Galvanizing?

Galvanizing is the process of protecting a metal from corrosion using zinc. In the galvanization process, a zinc coating is added to the surface of the metal to prevent rusting. By reducing the corrosion of the metal on which the process is performed, galvanization increases the lifespan of the metal. There are several methods of galvanization; the most popular is hot-dip galvanization, but other methods are also common, including shepardizing, zinc-rich painting, and electroplating.

Hot Dip Galvanized Iron Wire

Hot Dip Galvanized Iron Wire

How Does Hot Dip Galvanizing Work?

Firstly, the steel itself must be thoroughly inspected to ensure that drainage and ventilation requirements are met. A product that is free from defects to begin with will not do.

Cleaning the steel before hot dip galvanising

Before the galvanising process begins, the steel must be cleaned as thoroughly as possible. This is not just for cosmetic reasons. If foreign matter is present, the zinc coating simply will not bond to the steel surface. There are several steps to cleaning.

1. Immerse the steel in a lye bath at 180° F to remove any paint residue. The steel is then rinsed in a fresh water tank at ambient temperature to remove corrosive substances.

2. The steel is pickled by immersing it in a heated sulphuric acid tank at 140°-145° F to remove any oxidation skins. Afterwards, it is again rinsed in a fresh water tank.

3. Finally, the steel is washed at 140°-170°F in a flux bath containing a mixture of zinc chloride and ammonium chloride to provide antioxidant protection prior to galvanising.

4. Once these steps are complete, the steel is ready to receive its new protective coating.

The Hot-Dip Galvanizing Kettle

The freshly cleaned steel is immersed in a pot of molten zinc at 830°-850°F. The zinc then combines with the steel in a diffusion reaction to form a completely new metallurgically bonded zinc alloy layer. Before removing the newly coated steel from the bath, the surface of the molten zinc must be swept to remove any residue to ensure that the steel drains through the pure zinc.

Cooling down

Once removed from the galvanising bath, the steel is quenched at ambient temperature in an aqueous solution of sodium dichromate to help cool it down before the final cleaning process. This also helps the galvanised steel to retain its shiny finish for a longer period of time.

Final cleaning

After galvanising, the coated surface of the steel is cleaned of any dirt, drip marks or excess zinc to ensure the final product is ready for testing.

Testing the results of hot dip galvanising

The steel is then moved outside and placed on a rod for millage tests. These tests verify that the process is successful and that the zinc coating is the right thickness. Different applications of the steel may require different coating thicknesses.