Welding galvanized steel

It is preferable to weld fabrications prior to hot dip galvanizing. Fabricators need to be aware that for structures to be hot dip galvanized electrode choice takes into account the need to have a low residual silicon level in the weld. It is sometimes necessary to weld after galvanizing. The welding of galvanized steel requires some changes to the methods employed when welding uncoated mild steel.

Fume

When welding galvanized steel white fumes of zinc oxide are produced. Inhalation of these fumes may cause physiological problems which, although short term, are easily avoided. Adequate ventilation combined with proper fume extraction completely eliminate exposure to welding fumes.

Spatter

Vapour streaming from the zinc coating causes excessive spatter to be produced when welding galvanized steel. However, this is easily removed by use of an anti-spatter agent when welding.

Weld Gap and Speeds

When welding galvanized steels, it is necessary to volatilize the molten zinc from the weld pool. As a consequence, for a given electrode, penetration rates and welding speeds are lower than with uncoated steel. In order to compensate for this, root gaps should be increased by 50% and a reduction in speed of 25% is typical as a back and forward movement is require in order to volatilize the zinc in front of the weld pool.

Weld Cracking

Fillet welds made by MIG/MAG (Metal inert gas/metal active gas) welding on 6mm and thicker galvanized steel and by MMA (Manual metal arc, shielded metal arc or stick) welding steel thicker than 10mm sometimes contains cracks through the weld throat. These are caused by the penetration of liquid zinc from closely abutting surfaces at the weld root.

This can be prevented by:

  • a). providing an edge preparation such as a single or double bevel.
  • b). removal of zinc from both adjoining surfaces by burning or grit blasting, on the application of a mask prior to hot dip galvanizing
  • c). providing a 1.5mm gap between the plates.

Arc Length

A shorter arc length is needed when welding galvanized steel in order to avoid excessive penetration or undercut.

Welding Current

With rutile (high titanium dioxide) or basic coated (high calcium) electrodes the welding current used for uncoated steel is also suitable for galvanized steels.

Choice of Electrode

In addition to the referenced electrodes in the table below, welding electrodes can be discussed with the South African Institute of Welding.

MMA electrodes for use with galvanized steel - (root runs, butt welds)

Electrode type Specification Material thickness, mm Welding position Electrode size, mm Current, A Reduction in welding speed compared to uncoated steel, % Root opening, mm
Rutile AWS E6012,
BS 1719
class E233,
EN 499

E380RC11 or E380RA13

 3.2

45

 flat 3.2

4

 126

136

 0

22

 2
Basic AWS E7016,
BS 1719
class E616H,
EN499 E423B12H5		 3.2

4

 3.2

45

 flat 109

136

 0

22

 1.6
Rutile AWS E7013,
BS 1719
class E317,
EN 499

E380R11/
RR12/RC12

 3.2

12.7

 flat 3.2

3.25

 89

110

 0

25

 2

2.5

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