Welding galvanised steel: zinc fume safety and restoring the coating

Galvanised steel can be welded, but the zinc coating must be handled with care: it gives off zinc oxide fume that causes metal fume fever, so the coating is removed from the weld zone, the work is ventilated, and the burnt area is re-coated with zinc-rich paint afterwards. Done properly, a galvanised part welds much like the bare steel underneath. This educational guide explains the health risk, the safe method step by step, and how to restore the corrosion protection once the joint is made.

Why welding galvanised steel needs care

The steel beneath a galvanised coating is ordinary structural steel and welds normally. The complication is the zinc on top. Zinc melts and boils well below the temperature of a welding arc, so heat vaporises it at the joint, and that vapour does two things: it releases fume that is a health hazard, and it can disturb the weld itself with porosity and spatter if it is not dealt with. This is why galvanised steel is not simply welded as-is, and why the coating around the joint is removed first. ULAMEX supplies galvanised material for bolted and welded structural connections, and this guide covers the welding done by the fabricator.

Zinc fume and metal fume fever

Heating zinc produces zinc oxide fume, and breathing it causes metal fume fever, an acute flu-like illness. The symptoms, chills, fever, headache, nausea and body aches, typically begin 3 to 12 hours after exposure, often the same evening after a shift, and usually pass within 24 to 48 hours once the exposure stops. It is often mistaken for a cold. While a single mild episode resolves, repeated exposure over time is linked to longer-term respiratory harm, so it is a hazard to control, not to shrug off. The whole safe-welding method is built around keeping this fume out of the welder’s breathing zone. Good milk or extra fluids do not prevent it, contrary to a common workshop belief; only keeping the fume away from the lungs works, which is why ventilation and position matter more than any remedy.

Controlling the fume: ventilation and protection

The first line of defence is to keep your head out of the plume and to move the fume away from the breathing zone. Work in a well-ventilated space, and use local exhaust ventilation or a fume extractor positioned at the arc to capture the fume at source, which is far more effective than general room air flow. Where ventilation cannot be assured, an approved respirator suited to metal fume is worn. These controls matter most in enclosed or poorly ventilated spaces, where zinc oxide can build up quickly, and they apply to anyone nearby, not only the welder.

Removing the coating from the weld zone

The key preparation step is to remove the zinc from the weld area before striking an arc. The coating is ground or wire-brushed back from the joint, typically 10 to 25 millimetres each side of where the weld will run, down to bright bare steel. This does two jobs at once: it cuts the amount of zinc that vaporises, reducing both the fume and the risk to weld quality, and it gives a clean steel surface for a sound weld. Grinding the zinc off produces dust rather than fume, so the same care over dust and ventilation applies during preparation.

Welding the joint

With the zinc removed, the exposed steel is welded using the process and filler suited to its grade, the same choices as for the equivalent uncoated structural steel. Any zinc that remains at the edges of the cleared zone can still cause a little porosity, so a slightly slower travel speed and good technique let residual zinc vapour escape from the weld pool rather than being trapped. The aim is a full, sound weld in clean steel, after which attention turns to restoring the protection that grinding removed. The principles of heat and fume are shared with other metals, as our guide to welding stainless steel discusses for that material.

Welding processes and filler for galvanised steel

Once the zinc is stripped back to bare steel, the choice of process and filler follows the base grade, not the coating, because you are welding clean structural steel. MIG and MAG (gas metal arc) welding is the common workshop choice, giving good control and productivity, while manual metal arc (stick) welding suits site work and is more tolerant of any residual zinc. Flux-cored wire is used for heavier or outdoor work. Whatever the process, a filler matched to the parent steel, such as a standard carbon-steel wire or electrode for S235 and S355 grades, gives a sound weld. The technique adapts more than the consumable: a slightly slower travel speed and, on fillet welds, a gentle weave help any trace of zinc vapour bubble out of the pool rather than being trapped as porosity.

Restoring the coating after welding

Welding leaves a strip of bare, heat-affected steel where the zinc was removed and burnt back, and this must be re-coated or it becomes the one spot on the part that rusts. The standard repair is a zinc-rich paint, sold as cold galvanising compound, which carries a high proportion of zinc in the dried film and restores the sacrificial protection by corroding before the steel does. The weld and surrounding area are cleaned of slag and spatter, wire-brushed to bare metal, and given enough coats to build a zinc thickness comparable to the original coating. This closes the gap in the corrosion protection.

A safe method in summary

Bolting as an alternative to welding

Because welding destroys the coating locally and calls for repair, bolted connections are often preferred for galvanised steel where the design allows, since they leave the zinc intact and need no fume control or re-coating on site. Galvanised profiles, angles and sections are readily bolted, and galvanised fasteners keep the whole connection protected. Where a weld is required for strength or continuity, the method above applies; where a bolt will do, it keeps the corrosion protection unbroken and the work simpler. The choice is part of designing with galvanised steel, alongside its corrosion protection.

Galvanised steel from ULAMEX

ULAMEX has supplied the steel trade since 1988 and stocks galvanised steel intended for bolted and welded structural connections, including profiles, sections, sheet, fence systems and gratings in structural grades such as S235JR and S355J2. As an EN 1090-1 EXC2 and ISO 3834-2 certified supplier, ULAMEX provides galvanising documentation and mill test certificates to EN 10204 type 3.1 on request, with delivery across continental Europe.

Planning a galvanised fabrication? Tell ULAMEX the product, grade and quantity and the desk will confirm availability and return a non-binding quotation. See our guides to galvanised steel profiles and galvanised steel and DX51D, and our supply capabilities, or contact the export desk at [email protected] or +48 504 424 761.

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