What Happens When You Weld Galvanized Pipe
Galvanized steel pipe carries a zinc coating that protects the base steel by sacrificial action. During welding, the zinc around the joint vaporizes at about 907°C, far below arc temperature, so the coating in the weld zone is destroyed and zinc vapour can contaminate the weld pool. The results are porosity, lack of fusion and zinc-oxide fumes that are hazardous to breathe. Understanding these effects is the first step to producing sound, code-acceptable welds on galvanized pipe.
Effect of Zinc on Weld Properties
Porosity:
zinc vapour trapped in the solidifying weld metal forms gas pockets that weaken the joint.
Lack of fusion:
zinc residues on the joint edges reduce wetting of the base metal.
Reduced strength:
porosity and inclusions lower the weld's load capacity below that of the base pipe.
Arc instability:
zinc fumes disrupt the arc, making consistent penetration difficult to maintain.
Preparation Before Welding
Remove the zinc:
grind the coating back at least 25-50 mm from the joint on both pipe and fitting surfaces.
Clean the surface:
remove residual zinc, oil and dirt down to bright metal before fit-up.
Ventilation:
set up local exhaust ventilation or weld outdoors to control fume exposure.
PPE:
use respiratory protection rated for metal fumes; never weld galvanized steel in confined spaces without extraction.
Welding Process Selection
| Process | Behaviour on Galvanized Steel | Best Practice |
|---|---|---|
| SMAW (stick) | Tolerates zinc residues better | Cellulosic or E6010 root pass for penetration |
| GMAW (MIG) | Porosity risk from fumes | Remove zinc thoroughly; keep shielding gas flow correct |
| GTAW (TIG) | Cleanest welds when zinc removed | Preferred for thin-wall and high-quality joints |
Restoring Corrosion Protection
Welding destroys the zinc in the heat-affected zone, leaving bare steel that will corrode rapidly in service. After welding and inspection:
Clean the weld area by wire brushing or light abrasive blasting.
Apply a zinc-rich repair paint or, where possible, re-galvanize the repaired area.
For critical service, apply a compatible two-component coating system per the project specification.
Verify the repair film thickness with a coating gauge where specified.
The repaired coating should provide protection equivalent to the original galvanizing so the joint does not become a premature corrosion site.
Frequently Asked Questions
Is it safe to weld galvanized steel pipe?
Yes, with proper preparation and ventilation. The main hazard is zinc-oxide fume, which causes metal fume fever, so extraction and respiratory protection are essential.
Why do galvanized welds get porous?
Zinc vapour from the coating boils into the weld pool and becomes trapped as gas pockets when the weld solidifies.
Must the zinc be removed before welding?
For structural and pressure welds, yes. Grinding the coating back 25-50 mm prevents porosity and gives full fusion.
Does welding weaken the galvanized pipe?
The weld itself, if properly made, matches or exceeds pipe strength; the real risk is local corrosion where zinc was burned away, so coating repair is essential.
How is the zinc coating repaired after welding?
By cleaning the affected area and applying zinc-rich paint or a full repair coating system to restore sacrificial protection.
Can galvanized pipe be welded for high-pressure service?
Yes, but the weld must be made to the project code after zinc removal, inspected by radiography or hydrostatic test, and the coating repaired; threaded or mechanical joints are often preferred for small bores.
