Steel fabrications that are to be hot dip galvanized must be Designed for Galvanising. The temperature of a galvanizing bath is around 450°C meaning that such products as Aluminium and Stainless Steel can not be galvanised. There are many considerations and rules that must be followed in the galvanising process to ensure a safe and secure process.
Venting and Drainage
It is essential that the work is vented correctly for drainage. Any part of the fabrication which creates a trap requires holes of an adequate size to allow the zinc to drain freely. Sealed compartments in fabrications are an explosion hazard when dipped in molten zinc, especially if they contain any liquid that can 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.
Clean steel surfaces are an essential requirement for good hot dip galvanizing. 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 sometimes difficult to detect on the steel surface and will only show up after the galvanizing treatment. The article may then have to be regalvanized, at additional cost.
If large flat contacting surfaces can not be avoided, the the edges of the contacting areas should be continuously welded and a hole drilled through both members to eliminate the danger of an explosion in the galvanizing bath.