Galvanized and coated steels are the hardest duty cycle a seam welding wheel will ever see — not because the steel is difficult to weld, but because the coating actively attacks the electrode. Lines that run bare steel happily on chromium-copper wheels often watch the same wheels fail in days once zinc-coated strip goes through. This guide explains the pickup mechanism, why tungsten copper handles it, and how to specify wheels for coated work.
What zinc actually does to a wheel
Zinc melts at roughly 419 °C and boils near 907 °C — temperatures the electrode face reaches every welding cycle. At the wheel-to-strip contact, three things happen in sequence:
- Alloying. Molten zinc diffuses into the copper-based face and forms a thin brass layer. Brass is softer and has higher resistivity than the parent alloy, so the contact heats faster on every subsequent cycle.
- Glazing. The alloyed layer oxidizes into a hard, insulating glaze. Current now passes through a resistive skin, contact resistance climbs, and the heat balance of your schedule quietly shifts.
- Sticking. The contaminated face starts to grab the strip. Coating material tears out and transfers, welds show pickup marks, and shunt conditions wander along the seam.
The visible symptoms in the plant: dressing intervals collapse from hours to minutes, seam width drifts, and operators start compensating with current — which accelerates the whole loop.
Why tungsten copper breaks the loop
The tungsten skeleton in a W/Cu wheel is what resists pickup mechanically. Zinc cannot alloy into tungsten the way it does into copper, so a tungsten-copper face stays metallic far longer than a Cr-Cu or Be-Cu face under the same coated-strip schedule. Less pickup means less glaze, less sticking, and dressing intervals that stretch back toward bare-steel behavior.
The copper phase still does its usual jobs — carrying the welding current and pumping heat out to the cooled shank — so the face runs cooler, and cooler faces pick up less coating to begin with. This is, in our experience, the single most common reason lines migrate from copper-alloy wheels to tungsten copper.
Choosing the grade for coated schedules
Coated strip usually demands more current than bare steel of the same gauge, because the coating adds contact resistance. That pushes the alloy decision:
- W/Cu 70/30 (≈ 42% IACS) — the choice when the schedule is current-limited: thick coated stock, high line speed, or a transformer already near its ceiling. Higher conductivity means less resistive heat in the wheel itself, a cooler face, and less sticking.
- W/Cu 75/25 (≈ 195 HB) — the choice when wheel life limits the campaign: high electrode force, long runs, and maintenance windows that favor fewer wheel changes. The harder face holds its profile longer under coated work.
Both grades ship in the same geometries and the same tolerance package, so switching a station between them is a purchasing decision, not a process change.
Profile and dressing practice on coated strip
- Single or double bevel faces are the standard on can-body, drum-lid and other lap seams in coated stock — the bevel concentrates current on the sheet side and helps the nugget form through the resistive coating layer.
- Dress on interval, not on symptom. Once glaze forms, the face degrades non-linearly. A fixed light dress keeps the wheel ahead of the contamination curve.
- Keep the cooling honest. A W/Cu face only delivers its anti-pickup advantage when conducted heat actually leaves the wheel. Check water flow whenever sticking seems to appear without a schedule change.
What to confirm before ordering
Coated lines usually cannot tolerate a trial-and-error wheel fit: three dimensions decide whether a replacement drops straight onto your shaft. Bore — we cut H7 as standard, keyed bores to drawing. Runout — 0.02 mm, the value that stops the wheel hunting on the seam. OD — 0/+0.5 mm to match your arm geometry. Send the drawing, or a worn wheel if the drawing is gone, and we match all three.
Common questions
Do we need to change the weld schedule when moving from Cr-Cu to W/Cu? Typically the schedule comes down slightly, because contact resistance is lower and stays stable. Retune once at changeover, then expect the schedule to hold through the wheel’s life instead of drifting with glaze buildup.
Is 70/30 or 75/25 better for galvanized work specifically? Neither universally. If your problem is heat and sticking, move conductivity up to 70/30; if your problem is wheel life and profile drift, move hardness up to 75/25.
Get a quote
If zinc pickup is eating your dressing intervals, send us the wheel drawing and your current alloy — request a quote and we will confirm whether a W/Cu 75/25 or 70/30 wheel suits the schedule, with pricing valid 7 days and standard lead time of 10–14 days, no minimum order.