Technical Articles
Understanding Galvanising
Published 17 December 2018

Galvanising is a process of applying a zinc coating to steel components that offer corrosion protection for up to 50 years, and in some cases even longer.

 

  

Inspection

Upon arrival all steelwork is inspected to ensure the fabrication is suitable for galvanising and to check for:

  • Adequate drainage and ventilation holes
  • Surface contamination

Preparing the steelwork

Fabricated steel products are loaded onto jigs

Cleaning cycle

In common with many coating processes, the secret to achieving a good quality coating lies in the preparation of a chemically clean surface. It is essential that this is free of grease, dirt and scale before galvanising. Cleaning processes involve:      

  • De-greasing using alkaline or acidic solutions into which the component is dipped to remove light oils and rolling fluids before rinsing it in water to avoid contaminating the rest of the process
  • Dipping in hydrochloric acid at ambient temperature to remove rust and mill scale - an additive prevents the acid over pickling the parent metal of the component. The length of time the goods are immersed depends on the level of corrosion
  • The above treatments will not remove welding slag, paint or heavy grease which are removed  manually prior to galvanising
  • Rinsing in water to wash all the acid from the components and prevent contamination in the next process
  • Dipping in flux solution of zinc ammonium chloride at around 65-80°C to remove the last traces of oxide to promote bonding of the zinc to the steel surface

Galvanising

When clean iron or steel is dipped in molten zinc, commonly at 450°C, zinc/iron layers are formed by a metallurgical reaction. The main thickness of coating is formed during an initial rapid rate of reaction which slows down parabolically with time. Coating thickness is not increased even if the component is in the bath for longer periods. A typical immersion is about four or five minutes but can be longer for heavy articles with high thermal inertia or where the zinc has to penetrate internal spaces. Upon withdrawal from the bath, a layer of zinc will be taken out to the alloy layer which often cools to exhibit the shiny finish associated with galvanised products.

Final treatment             

After galvanising, the final treatment process involves:

  • Quenching into water or air cooling to cool the component to ambient temperature - Additives prevent the formation of “white rust”
  • A visual inspection – all items are fettled to remove any spikes/snags
  • Batches can be tested to check the thickness of the coating 
  • No post-treatment of galvanised components is necessary however paint or powder coating can be applied for aesthetics or for additional protection in demanding environments
  • Once they conform to BS EN ISO 1461 components are despatched

 

 

 

 

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