The batch hot-dip galvanizing process, also known as general galvanizing, produces a zinc coating on iron and steel products by immersion of the material in a bath of liquid zinc. Before the coating is applied, the steel is cleaned to remove all oils, greases, soils, mill scale, and rust. The cleaning cycle usually consists of a degreasing step, followed by acid pickling to remove scale and rust, and fluxing, which inhibits oxidation of the steel before dipping in the molten zinc.

There are two different fluxing methods, dry and wet.  The dry process is accomplished by pre-fluxing in a zinc ammonium chloride solution. The wet process uses a molten flux blanket on the zinc bath surface.

Hot-dip galvanized coatings are used on a multitude of materials ranging in size from small parts such as nuts, bolts, and nails to very large structural shapes.  The size of available zinc baths and material handling restricts the size of steel that can be galvanized.  Molten zinc baths 60 feet long and eight feet deep are common in North America. However, the maximum size that can be accommodated in the zinc bath is increased substantially, to near double bath length or depth by progressive dipping (immersing one portion of the product and then the other).

Because the material is immersed in molten zinc, the zinc flows into recesses and other areas difficult to access, coating all areas of complex shapes thoroughly for corrosion protection.

See Also:
Zinc Coatings Publication
Hot-Dip Galvanizing Process Animation

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Comments

Dan Barlow

Hi Chris,

The chemistry of the coating depends on the chemistry of the substrate steel as the coating is dependent on a reaction between the steel and the molten zinc. Therefore, changes in the steel chemistry (albeit small ones), can change the chemistry of the galvanized coating.

From one end of the I-beam to the other, there can be small changes in the steel chemistry. These small changes will cause local changes in the coating chemistry from area to area on the steel’s surface. This should not affect the corrosion protection performance of the galvanized coating.

Chris

We are a transportation research group at the University of Kentucky. In conjunction with the Kentucky Transportation Cabinet we are building a hot dipped Galvanized Bridge which is going to be a rapid replace project. The rapid replace will consist of Hot dipped Galvanized I beams. We are in the construction phase of work and are inspecting the beams for their zinc coatings before blast and paint. We are using a XRF to look for the zinc and aluminum in the coatings. The problem is we are getting very large differences in zinc and aluminum concentrations on the same beam displayed as percentages by our XRF. Do you have an explanation for the large differences? Differences of 20% to 30% zinc and aluminum.

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