Why Welding Galvanized Rectangular Pipe Is Different
The zinc coating on a galvanized rectangular pipe melts at about 419 C and boils at about 907 C, both far below the temperature of the welding arc. As the arc approaches, the coating vaporises and zinc vapour is trapped in the solidifying weld pool, producing porosity and, in severe cases, zinc embrittlement of the weld metal. Zinc oxide fume is also a health hazard, so fume extraction and respiratory protection are part of the procedure rather than an optional extra. Every welding process used on galvanized pipe therefore attacks the same three problems: removing or displacing the coating locally, limiting heat input, and restoring corrosion protection after welding.
Process Selection at a Glance
| Process | Best suited to | Main advantage | Main limitation |
|---|---|---|---|
| Shielded metal arc welding | Thick wall, load-bearing joints, site work | Flexible, tolerant of poor fit-up | Slower, heavy fume |
| Gas metal arc welding with mixed gas | 2 to 4 mm wall, batch production | Fast, low spatter with argon plus CO2 | Needs clean, dry gas supply |
| Tungsten inert gas welding | Thin wall, high precision, visible joints | Clean, controllable bead | Sensitive to residual zinc and oil |
| Resistance spot or seam welding | Thin wall lap joints, furniture, frames | Almost no zinc fume | Lap joints only, electrode wear |
Shielded Metal Arc Welding of Galvanized Pipe
Manual arc welding suits thick-walled, load-bearing rectangular pipe. Choose anti-porosity, low-hydrogen electrodes: E4303 for general non-load-bearing joints because it strikes and runs easily, and E4315 for load-bearing structures or low-temperature service because its basic coating keeps hydrogen out of the weld. Basic electrodes must be dried in the holding oven before use, since moisture reintroduces the hydrogen that causes cold cracking.
Zinc removal: grind the joint area with an angle grinder, 10 to 15 mm on each side of the seam, until silvery steel is exposed on thick hot-dip coatings.
Parameters: low current and short arc. For a 3 mm wall, 90 to 120 A with an arc length not exceeding half the electrode diameter.
Technique: broken-arc welding, pausing to cool after every 10 to 15 mm of bead to stop zinc vapour from accumulating, with the electrode held at 45 to 60 degrees so it does not run over residual zinc.
Gas Metal Arc Welding with Mixed Shielding Gas
Gas metal arc welding is the practical choice for batch welding of medium and thin wall rectangular tube. Use a silicon and manganese deoxidised wire such as ER50-6: 0.8 mm diameter for 1.5 to 2 mm wall and 1.0 mm for 2 to 4 mm wall, because the deoxidisers suppress porosity caused by zinc vapour.
A mixture of 80 % argon and 20 % CO2 is preferred over pure CO2. Compared with pure CO2 the mixed gas cuts spatter by more than half and gives a more stable arc and shielding pattern; pure CO2 is reserved for outdoor work in low wind. Typical settings for a 2 mm wall are 80 to 100 A, 18 to 20 V and a travel speed of 30 to 40 cm per minute, which keeps heat input low enough to avoid distortion and zinc embrittlement. Hold the torch at 15 to 20 degrees to the pipe, keep the wire extension at 10 to 15 mm, and leave a 0.5 to 1 mm root gap on butt joints to guarantee penetration.
Tungsten Inert Gas Welding
TIG gives the cleanest bead on thin-walled, high-precision galvanized tube, but it is the least forgiving process. Use a cerium tungsten electrode such as WC20 in 2.0 to 2.4 mm diameter with the tip ground to a 30 to 45 degree cone to steady the arc, and argon of at least 99.99 % purity at 8 to 12 L per minute. Clean the joint with abrasive paper or solvent until all oil and zinc is gone: even a trace of zinc residue causes porosity under argon shielding.
Weld thin wall up to 1.5 mm autogenously, and add filler manually for 1.5 to 2 mm wall, feeding the rod from the leading edge of the pool without touching the tungsten. Keeping the heat-affected zone within about 5 mm of the seam limits distortion on long rectangular sections.
Resistance Welding of Thin-Walled Tube
Resistance spot and seam welding is the lowest-fume option and is widely used for lightweight frames, shelving and furniture where lap joints are acceptable. Use copper alloy electrodes with smooth faces so that molten zinc cannot adhere and build up on the tip. Typical parameters are an electrode force of 0.3 to 0.5 MPa, a current-on time of 0.5 to 1.0 s and a current of about 5 to 8 kA for 1 mm wall. The spot diameter must be at least three times the wall thickness, so at least 3 mm for a 1 mm wall, and the overlap must be at least five times the wall thickness to prevent the joint from peeling. Electrogalvanized or light coatings can simply be sanded before welding, because resistance heat melts the thin zinc layer without trapping it.
Post-Weld Repair and Safety Controls
Zinc residues and discoloured coating remain along every weld. Grind the bead smooth, then rebuild the coating with a cold galvanizing zinc-rich paint applied to the specified dry film thickness, checking the result with a magnetic coating gauge. Distortion on long rectangular frames is controlled by balanced welding sequences and by clamping the assembly until it has cooled. Because zinc oxide fume causes metal fume fever, enclosures require local exhaust ventilation, and welders should wear suitable respiratory protection; cleaning the coating before welding reduces both fume generation and the risk of porosity.
Frequently Asked Questions
Q: Can galvanized rectangular pipe be welded without removing the zinc coating?
Thin electrogalvanized coatings can often be welded directly with resistance welding, but arc processes need the coating ground back 10 to 15 mm from the seam to prevent porosity.
Q: Which welding process is best for galvanized rectangular pipe?
There is no single best process. Thick load-bearing joints favour manual arc welding, medium walls favour gas metal arc welding, thin precision parts favour TIG, and lap-jointed frames favour resistance welding.
Q: Why do pores appear in welds on galvanized steel?
Pores come from zinc vapour generated by the arc and trapped as the pool freezes. Grinding the coating, using deoxidised filler wire and reducing heat input all suppress them.
Q: Which electrode should be used for load-bearing galvanized pipe joints?
Use a basic low-hydrogen electrode such as E4315, dried before use, and reserve rutile electrodes such as E4303 for light, non-critical joints.
Q: How is corrosion protection restored after welding?
Grind the weld and heat-affected zone, then apply zinc-rich cold galvanizing paint at the specified film thickness and verify the coating with a magnetic gauge.
Q: Is welding galvanized steel dangerous?
Zinc oxide fume can cause metal fume fever, so local exhaust ventilation, respiratory protection and good general ventilation are required in enclosed spaces.