The best time to think about hot dip galvanising is before fabrication begins.
Small design changes made during planning are usually much easier (and cheaper) to make than modifications once fabrication is complete.

Designing steel for hot dip galvanising means considering the galvanising process while the fabrication is still being designed. Details such as vent holes, drainage, weld positions and enclosed sections allow molten zinc to flow freely around the steel and drain effectively during the galvanising process.

Making these decisions early helps achieve a high-quality coating while reducing the risk of delays, rework and additional fabrication costs.

Why it matters

Poor design can lead to:

• Additional fabrication work before galvanising
• Delays while modifications are made
• Poor zinc drainage, affecting coating quality
• Increased processing costs
• Components that are difficult or unsafe to galvanise

Designing for galvanising helps:

• Achieve a high-quality, corrosion-resistant coating
• Reduce the need for fabrication changes
• Improve processing efficiency
• Minimise project delays
• Maximise the service life of the steel

This guide is useful if you are:

• Structural engineers
• Steel fabricators
• Agricultural equipment manufacturers
• Designers specifying hollow sections
• Anyone designing fabricated steel for hot dip galvanising

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When designing steel for hot dip galvanising, there are four key areas to consider:

• Suitability
• Venting and drainage
• Surface contamination
• Contacting surfaces

Suitability

Hot dip galvanising is intended for suitable steel fabrications designed for galvanising. The temperature of a galvanising bath is around 450°C, meaning products such as aluminium and stainless steel cannot be galvanised.

Material type, fabrication design and overall suitability for hot dip galvanising should be considered before manufacture begins.

Venting and Drainage

It is essential that the work is vented correctly for drainage. Any enclosed section or part of the fabrication that could trap air or molten zinc requires vent and drain holes of an adequate size to allow zinc to flow and drain freely. Sealed compartments are an explosion hazard when immersed in molten zinc, particularly if they contain moisture or other liquids that can rapidly vaporise.

In square shaped fabrications the corners should be cropped to allow for an even flow of the zinc and to make sure no amounts of the molten liquid gets trapped in the corners, thus providing a cleaner, smoother finish all over.

Where hollow sections are joined together, holes must be provided connecting the sections, or each section must be individually vented to ensure complete zinc protection and to prevent any explosion hazards. Holes should be positioned to take advantage of the natural drainage characteristics of the work.

• No drain hole should be less than 10 mm
• Preferred minimum drain hole size is 25 mm
• Large hollow sections require a 100 mm diameter drain hole for each cubic metre of enclosed volume
• Hollow sections such as tube, RHS and SHS require minimum drain hole area equivalent to 25% of the section’ diagonal cross section. The preferred design option is to leave the ends of tubes, RHS and SHS open.

Surface Contamination

Clean steel surfaces are essential for achieving a high-quality galvanised coating.

Silicone based contaminations may result in black bare spots after hot dip galvanizing. Steel sections, which have been cut or drilled using suds type oil can give similar problems to burnt or anti-spatter sprays.

Contamination is often difficult to detect before galvanising and may only become visible after processing. In some cases, the article may need to be re-galvanised, resulting in additional cost and delays.

Contacting Surfaces

Where large flat contacting surfaces cannot be avoided, the edges should be continuously welded and a hole drilled through both members to eliminate the risk of an explosion in the galvanising bath.