How to Protect Hot Dipped Galvanised Steel Against Corrosion

Perhaps the most vital rule is that the galvanizer, fabricator, and designer of hot dipped galvanised products should work well together before products are manufactured. Such 3-way communication could eradicate most hot dipped galvanising problems. The designer could better appreciate the design requirements for hot dip galvanising if the basic steps of the whole process are understood well. Though this process may vary from one galvaniser to another, the fundamental steps in addressing the problem regarding rust are:

Grease & Soil Removal:

Often, a hot alkaline cleaner is used to get rid of soluble paint, oil, grease, and shop oil. But this will not eliminate things like epoxies, asphalt, welding slags, and vinyls. Such solids should be removed by grit blasting, or other methods of mechanical cleaning which is typically not the galvaniser's responsibility.

Pickling:

A certain acid bath is used to get rid of surface corrosion and mill scale to give a checmically clean surface. Many galvanisers choose to use hydrochloric acid since it's more eco-friendly and won't affect the steel's surface, which may be possible when sulfuric acid is used.

Prefluxing:

A piece of steel is immersed in a liquid flux pre-dip (often a zinc ammonium chloride solution), in order to get rid of oxides and stop oxidation before dipping it into a molten zinc bath.

By making use of the dry kettle process, a hot dip galvaniser could eradicate the potential of flux inclusion or the entrapment on the steel product. The wet kettle process on the other hand, where the steel undergoes a "flux blanket" on top of the molten zinc bath, could result in such particles adhering to the surface of the steel.

Galvanizing:

The steel article is immersed into a molten zinc bath at around 850°F (or 455°C). Such step will result in the formation of a zinc-iron and zinc alloy coating that is metallurgically bonded to steel.

Finishing:

After the steel is withdrawn from the molten zinc, the excess zinc is removed by vibrating, draining, or for small steel articles, centrifuging. Then the galvanised steel is quenced in water or cooled in air. The process of air quenching, which takes a little longer compared with water quenching process, will result in a better finish. This is a vital consideration in steel that's architecturally exposed.

Inspection:

Surface condition and thickness inspection are the final steps in the process of ganvanising. Steel and iron articles that are hot dip galvanised after fabrication may vary in size from small pieces of hardware like washers and bolts, to big welded steel castings or assemblies that weigh several tons. Galvanising such steel or iron articles could be improved by observing the design practices recommended in a certified manual and by consulting with a good galvaniser during the designing stage of the project.


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