Welding after hot-dip galvanising
Because it is not possible in some cases to hot-dip galvanise a complex structure as a whole, for example because the structure is too large for the zinc bath or for transportation, structures of this kind will have to be assembled after galvanisation. In these cases, bolted connections are by far the preferred approach given technical corrosion considerations. See Contact Corrosion. When bolted connections are not possible, the separate galvanised parts have to be welded together.
Welding hot-dip galvanised steel
As a rule, welding hot-dip galvanised steel is no different from welding non-galvanised steel. Research has demonstrated that the mechanical properties of steel, whether galvanised or non-galvanised, welded or not welded, are identical.
It is necessary to look at the area around the weld. The zinc coating must be removed on either side of the weld to such an extent that the blank steel can be welded without burning the zinc coating.
If this precaution is not taken, the high-temperature generated during welding will burn away the zinc coating on either side of the weld, releasing zinc oxide fumes that are detrimental to health. Of course, a weld fume extractor is indispensable during welding work. However, so much smoke is produced during the welding of galvanised material that it is very difficult to maintain a clear view of the welding work, and quality can therefore be affected. Furthermore, a thick zinc coating will, logically, generate more smoke during welding. In addition, the splattering of the burnt zinc during the welding can make the welding process very irregular, leading to pores in the welds.
Retouching / Reconditioning
After welding, the weld surface and the non-galvanised areas around it must be retouched, as described in Retouching/Reconditioning. The maximum permitted size of the surface to be reconditioned stated in this information sheet (0.5% of the total surface area of an item) is not applicable here because it is agreed in advance with the client that the individual galvanised parts have to be welded together. In practice, the most widely used approach to retouching /reconditioning is zinc-rich paint.