Hot-Dip Galvanizing (HDG)

What is hot-dip galvanizing?

Hot-dip galvanized steel frames. Source: Galvanizeit.org

Hot-dip galvanizing is the process of immersing clean, pre-formed steel parts into a bath of molten zinc.

The main difference between HDG and continuous galvanization is that hot-dip galvanizing allows for whole, pre-fabricated parts to be immersed in the zinc bath. In contrast, continuous galvanization involves rolls of steel sheets undergoing galvanization. These are only later fabricated to their respective parts.

Ultimately, the goal is the same, to apply an unbroken protective layer of zinc to the exterior of the steel.

The HDG process begins with caustic cleaning the surface of the steel. This process is crucial because any impurities, contaminants and deposits left on the surface of the steel may prevent the protective zinc coating from properly adhering to the steel, which could compromise the corrosion resistance.

Caustic cleaning begins with the steel being degreased in an acidic bath so as to remove contaminants like dirt, paint markings, grease and oil from the metal surface. 

Next any rust or surface oxides on the metal must be removed via pickling. This involves submersing the steel parts in a solution of strong acid such as hydrochloric or sulphuric acid.

Finally, the metal is ready for its zinc coating.

Just prior to being fully immersed in the bath of molten zinc, the steel part must be coated with zinc ammonium chloride, a fluxing chemical.

Fluxing removes any remaining oxides and deposits, and can be done separately from the zinc coating bath (dry fluxing) or simultaneously with the zinc bath immersion (wet fluxing).

If dry fluxed, the steel part is put into the zinc ammonium solution, dried and then submersed in the zinc bath. If wet fluxed, the steel part is passes through the flux solution, which floats above the molten zinc bath, before passing into the zinc bath itself.

The zinc bath contains at least 98% pure zinc and is maintained at a temperature of 815 to 850° F (435 to 455° C).

While submersed in the zinc bath, the iron in the steel reacts with the molten zinc to form a very strong and tightly-bonded alloy layer of zinc and iron. The longer the submersion process, the thicker the alloy layer.

This is one of the advantages of hot-dip galvanization over continuous galvanization, a slower process allows for thicker coatings and, consequently, greater protection against corrosion. 

Once the coating is complete, the steel is withdrawn slowly from the galvanizing bath, and any excess zinc is removed by draining, vibrating or centrifuging.

The hot-dip galvanized steel now takes on the shining, spangled appearance that typifies galvanized steel.

After the galvanization process is complete, parts must be inspected with respect to the coating thickness, adhesion and surface condition. Uncoated areas can be detected through visual inspection, while technical instruments can be used to measure surface adhesion and surface condition.

Utilities, chemical, pulp and paper, automotive, mining, and transportation industries all make extensive use of galvanized steel.

While continuous galvanized sheet steel is primarily used in for automotive body panels, appliances, ducts, framing studs and roofing, HDG steel can be found in everything from bridges and light poles, to sign structures, nails and transmission towers. 

When the Brooklyn Bridge was completed in 1883, over 14,500 miles of hot-dip galvanized wire had been used in its four main cables. Over 100 years later, when the bridge underwent massive rehabilitation, the hot-dip galvanized wire was still in excellent condition; A testament to the effectiveness of galvanization as a means to protect our most important structural metal, steel.

Sources:
Evans, Charlotte. An Anecdotal History of the Galvanizing Industry. AGA (1992). URL: http://www.galvanizeit.org/uploads/publications/History_of_Galvanizing_Industry.pdf
American Galvanizers Association.

"What is Hot-Dip Galvanizing (HDG)?"
URL: http://www.galvanizeit.org/hot-dip-galvanizing/what-is-hot-dip-galvanizing-hdg
International Zinc Association. Galvanizing
URL: https://web.archive.org/web/20140802224444/http://www.zinc.org/galvanizing

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