Preparation of Steel for Galvanising

Preparing your steel for galvanising will help ensure that it is galvanised safely and to the best possible quality.

Here is some information regarding commonly sought advice in the preparation of certain types of steel fabrication designs. You can also learn more about this with our Venting and Draining Video or download our handbook.

Jump to
Hole Sizes
Venting
Handrails
Fully Welded Plates
Holes or Notches
End Plates
Stiffeners
Base Plates
Hanging Points
Mesh
Surface Contaminants
 

Hole Sizes

In simple terms, when it comes to placing vent holes in steel fabrications that you plan on galvanising, Larger Holes = Better Drainage = Higher Quality.

Millimetres

D

50  100  150  200 
Hole Ø Minimum 13 25  38  50 

diameter examples

Note: 2 holes required, 1 on each end. 

Venting

Sealed sections must be vented for reasons of safety and drainage. Vent holes should be placed no more than 10mm from their respective sealed ends.

tube venting

On longer members, the vent and drain holes must be positioned at diagonally opposite ends. The holes can be located in the top or the side or the end.

channel corners 

Handrails

Handrails must be vented at each closed end and at each bend. The vents can consist of drilled holes or slots. The slots must have a minimum length of 40mm.

venting handrail examples 

Fully Welded Plates

Fully welded plates having an area in excess of 100cm2 require relief holes. The holes must be sealed with silicon in order to minimise the entry of pickling liquid which can vaporise in the hot zinc bath.

welded plates

Relief holes must be sealed with silicon prior to delivery to Korvest Galvanisers.

The relief holes must be a minimum of 6mm in diameter.

Holes or Notches

With hollow section fabrications, provision must be made for venting and draining. Holes or notches must be placed within 10mm of the sealed end of the section. On vertical members, a drilled hole or ‘V’ notch should be provided at each end of the section. Each hole must be located on opposite sides of the section.

holes or notches 

End Plates

Drain holes located away from the edge of the end plates lead to some pickling liquid and molten zinc remaining in the tube. Retained zinc adds to the weight of the component. Retained pickling liquid can vaporise in the galvanising bath which can damage the structure or cause an explosion. For complex shapes contact us, and we can help.

end plate bad exampleend plate good example 1end plate good example 2 

Stiffeners

Dead pockets can lead to air locks, pooling of pickle liquid and zinc build up. The high temperature in the galvanising bath causes the pickle liquid to vaporise resulting in misses and excess ash.

Eliminating dead pockets will lead to a cleaner and more blemish free finish. Stiffeners should have the corners cropped in order to eliminate dead pockets. Large corner crops allow the molten zinc to freely drain away.

stiffeners 1

 

stiffeners 2 

Base Plates

Here are some examples of how to, and how not to prepare venting holes in base plate designs. 

base plate designs
 

Hanging Points 

Length
Up to 1.5m 1 Hanging Point on 1 End
Above 1.5m 1 Hanging Point on Both Ends

hanging points


 

Mesh

Due to the expansion and contraction of the steel during the galvanising process, there is a risk of distortion. A weld should be placed only in every alternate space or on every second wire, depending on the construction.

mesh 

Surface Contaminants & Distortion

Paint, Oil and Grease

Paint, oil and grease interferes with the galvanising process. Surface contaminants must be removed before galvanising can proceed.

Distortion

Distortion is an ever-present risk during galvanising. It results from the relieving of stresses which have built up during the steel manufacturing process and also during fabrication. Structures fabricated from sections having different thicknesses and large surface areas are particularly prone.

Preparation of Steel for Galvanising

Properly vented steel draining out after the "Quenching" stage of the hot dip process