Design right for galvanizing
When using steel in construction, hot dip galvanizing is an essential part of the project to protect the longevity of the metal. You carefully design your steel to fit the overall build of the construction project, but sufficient care and attention are also required for designing steel for galvanizing. If you design right, you’ll prevent any processing problems at the galvanizing plant and will also ensure you get the best steel coating possible.
When sending steel to a galvanizing plant, you will have several goals in mind:
- Fast turnaround
- Premium quality coating
- Excellent service
- Good value for money.
Ensuring steelwork is suitably designed for hot dip galvanizing helps achieve these goals and ensures the steel is safe to be galvanized. Poor design can result in poor coating quality, additional costs, low production rates, and failure to meet delivery dates. Poor design can also pose serious health and safety risks for the people carrying out the galvanizing.
What does good design look like?
The importance of venting and drainage
To maintain the safety of our employees and achieve the best quality zinc coating, it is important to ensure that during immersion in the galvanizing bath, superheated air can escape all internal spaces and the molten zinc can flow freely over all surfaces, external and internal. So, where there are sealed hollow sections or cavities, vent and drain holes are essential, otherwise it could result in serious injuries to our personnel.
When steel is immersed in the galvanizing bath, add vent and drain holes to sealed hollow sections or cavities to ensure superheated air can escape all internal spaces and the molten zinc can flow freely over all surfaces, external and internal. Failure to adequately vent would result in trapped air causing an explosion and endangering galvanizing workers. It would also cause zinc build-up where the excess zinc cannot drain properly.
The sizing and location of vent holes are just as important as the provision of these holes. As a guide, make the venting holes as large as the steelwork can manage (but talk to your galvanizer for the best advice for your fabrications). The hole locations will be determined by how the steelwork is hung on the jigs at the galvanizing plant.
When adequately sized and correctly located, vent and drainage holes will:
- Significantly reduce the risk of injury to galvanizers.
- Allow faster immersion and withdrawal speeds which will help to minimise excessive coating build-up and reduce the risk of distortion by minimising temperature differentials.
- Virtually eliminate the risk of damage to product, plant and equipment caused by the explosive vaporisation of entrapped moisture or air.
- Improve the characteristics of the galvanized coating by restricting the effects of zinc solidification during drainage and reducing the mass of zinc required to achieve a good finish.
Overall, a correctly vented product will ensure the safety of our operatives, total corrosion protection, produce an optimum quality finish and reduce costs.
Due to fabrication design and/or processing orientations, the venting requirements for certain fabrications may deviate from the advice provided here. Where required, this will be communicated by your Joseph Ash representative.
Size, weight, and shape matter
A fabrication design should ideally incorporate easily handled components or fabrications which can be single dipped or subsequently assembled by bolting with galvanized fastenings. Double dipping is a possible alternative; seek advice from your galvanizer before proceeding.
Suspension holes or lifting lugs may be needed if there are no suitable points for locating hooks or wires. Once again, lifting points should be positioned to maximise venting and drainage.
Minimising distortion
Fabrications should ideally be symmetrical, suitable for single dipping, and incorporate sections of as near equal thickness as possible at the joints. Components should also fit perfectly to avoid force or restraint during joining. Use continuously welded joints with balanced welding techniques to reduce uneven thermal stresses and the largest possible radii on all curved members.
Venting requirements
Suitably-sized vent holes
The photo shows how large structural hollow section members should have suitably sized vent holes, as well as holes on the opposite side.
Internal venting
Before welding on the SHS leg, create internal flow by venting the main RHS member in appropriate places, as per the photo.
End plate venting
This is how a hollow section end plate should be cropped at all corners prior to attaching to prevent zinc and air traps.
Overlapping surfaces
Leave minimum gap of 2mm between overlapping surfaces. Where seal welding is not practicable stitch welding should be used.
Air lock prevention
The photo shows how stopping short the end plate prevents an air lock.
Zinc free flow
Stopping short cross beams eliminates air locks and allows free flow of zinc through the fabrication.
External venting in hollow section frames
The photo shows a RHS fabrication. The hole size is adequate.
External venting in hollow frames
Note how the holes drilled close to the point of insertion will prevent air locks and zinc traps.
Hole stamping
Holes stamped out prior to weld assembly
Here to help
Always look for a galvanizing company happy to be on hand to provide technical advice on all aspects of pre-galvanized steel preparation.
Our Technical Support Team is available to answer any queries about the galvanizing process from design to end of life. The Team is also happy to consult with design staff, carry out site inspections and provide technical advice to prevent instances of galvanizing zinc explosions.
This information and more can also be found on our Safe Design for Hot Dip Galvanizing poster.