Answer:
There are two big problems with welding galvanised steel.
The first is that zinc has a fairly low boiling point and so the heat from welding will put zinc vapour into the air. Zinc is toxic and cumulative a large dose in one go can cause flu-like symptoms, smaller doses may cause no ill effect at the time but can lead to long term health problems and make you more sensitive to further exposure. Very large doses, especially after repeated exposure can be fatal.
The cumulative effect can also make people complacent
The hazards of zinc fumes can be mitigated by properly specified ventilation and respiratory protection.
The other issue is with weld quality, zinc can cause porosity and embrittlement in a weld in steel. Braze-welding ie using a non ferrous filler such as silicon bronze can be effective in mitigating this.
In some cases it is possible to design processes which can successfully weld through galvanising but in most cases it is better to completely remove the zinc form the heat affected zone either by grinding or stripping with acid.
A third problem is that if you weld galvanised steel you will boil off or oxidise the zinc from around the weld and thus lose the corrosion protection.
