Unless a welded structure is too large for the galvanizing kettle, best practice is to weld prior to galvanizing to ensure the entire structure is coated with corrosion-inhibiting zinc. Throughout North America, variously sized hot-dip galvanizing kettles are available, allowing a broad scope of structural fabrications to be galvanized. However, as with any fabrication to be galvanized, it is important for the steel’s surface to be entirely clean for zinc to react with the steel.
Two contaminants welding can introduce on the steel surface are weld flux and weld slag. It is important these residues be removed from the surface before sending to the gavlanizer as the chemical cleaning solutions of the process cannot remove them. Best practice for welding articles to be galvanized is to use an uncoated electrode to prevent flux deposits. If using a flux-coated electrode is unavoidable, mechanical cleaning will be necessary to prepare the steel for galvanizing.
Anti-spatter sprays should also be avoided when possible, as they also leave behind contaminants on the surface not removed by chemical cleaning. In order to minimize costs and turnaround times, removal of any surface contaminants from welding processes that chemical cleaning will not eliminate should be performed at the fabrication facility.
When welded items are galvanized, two factors affect the galvanized coating’s quality and appearance – cleanliness of the weld area and metallic composition of the weld itself. Silicon is a catalyst for coating growth in the galvanized process, and high-silicon areas tend to develop thick coatings with a matte gray appearance. Therefore, the composition of the weld electrode is important, and should be as similar as possible to the steel chemistry to minimize differences in appearance. The composition compatibility will yield a more uniform zinc surface appearance and thickness. As with galvanizing high silicon steel, welding rods high in silicon may cause excessively thick and/or darkened galvanized coatings to form over the weld.
|Welding Process||Welding Electrode||Silicon Content|
|SMAW||Jetweld 2 (E6027)
Fleetwood 35 LS (E6011)
Fleetwood 7 (E6012)
|FCAW-S||NR-203 NiC+ (E71T8-K2)
The specifics of welding techniques can best be obtained from the American Welding Society (800-443-9353) or your welding equipment supplier. Several welding processes and techniques have been found to be successful for items to be galvanized.
Preferred Welding Processes
Welding processes such as gas metal arc welding (GMAW) or gas tungsten arc welding (GTAW) are recommended since they essentially produce no slag.
If the gap between two surfaces to be welded is less than 3/32” (2.5mm), all edges must be completely seal-welded
Stitch-welding vs. Continuous Welding
It is often beneficial to use stitch-welding techniques as opposed to continuous welding. If air is trapped in a seal-welded space it will expand rapidly upon immersion into the zinc bath. This may cause a sudden fracture of the steel structure. A stitch weld in conjunction with a gap of 3/32” (2.5mm) or more between pieces is suggested to avoid this.
Welding Residue Stud Insulators or ferrels may also cause problems when galvanizing. Residue or impurities may remain around the base, leaving imperfections somewhat like welding slag. These residues must be mechanically cleaned prior to galvanizing.
Induced Stresses, Differences in Steel Thickness: Welding two parts together induces areas of increased stress around the weld joint. Warping and distortion may occur upon immersion into the molten zinc. Additionally, if the parts of the assembly are unequal in thickness, temporary bracing may be necessary to limit warping and distortion.