Hot-Dip Galvanizing
Hot-dip galvanizing for steel: coating thickness, silicon reactivity and what a quote needs
Hot-dip galvanizing coats steel with a metallurgically bonded zinc layer by immersing it in a bath of molten zinc — a corrosion protection method, not a paint. This page covers what actually decides the outcome: the minimum coating thickness EN ISO 1461 requires for your section, why the steel's own silicon content can grow that coating thicker than you planned, the venting and draining a hollow part needs before it goes anywhere near the bath, and what a provider needs from you before quoting.
What drives the price
- Material thickness and mass
- Steel grade and silicon content
- Part size against the kettle's dimensions
- Coating class or thickness requirement
- Surface preparation and starting condition
- Total volume
- Special handling — masking or double-end dipping
What to send for a quote
- Grade or material number, with silicon content if known
- Part dimensions and weight
- Quantity
- Coating thickness class or standard required
- Venting and draining provisions
- Whether the part is thin, asymmetric or distortion-prone
- Whether you need a certificate — for the steel, the coating, or both
- Delivery address and required date
Minimum coating thickness — EN ISO 1461
EN ISO 1461
Indicative figures. Confirm the achievable coating and any dimensional limits with the provider for your grade, section and part size.
Steel silicon content and the Sandelin effect
| Below about 0.04% Si | 0.04% · Bright, thin coating close to the EN ISO 1461 minimum | Supplier-published | |
|---|---|---|---|
| About 0.04% to 0.14% Si | 0.04% · 0.14% · Thick, dull grey coating, growing rapidly with small changes in silicon — the reactive band galvanizers try to avoid | Supplier-published | |
| About 0.15% to 0.22% Si | 0.15% · 0.22% · More predictable, controlled growth — the range ASTM A385 practice recommends specifying | Supplier-published | |
| Above about 0.25% Si | 0.25% · Reliably thicker and greyer, but more consistent than the reactive band | Supplier-published | Source: Galvanizers Association of Australia — steel composition FAQ |
Exact percentage boundaries vary slightly between industry sources; all agree on the shape of the curve. Ask for the steel's silicon content before ordering large, decorative or dimensionally critical parts.
Venting and draining hollow sections
| Minimum vent and drain hole diameter | 13 mm | ||
|---|---|---|---|
| Minimum crop-back on stiffeners and gussets | 19 mm |
Supplier-published
Bath temperature
| Standard galvanizing | 438 – 460 °C | Supplier-published | |
|---|---|---|---|
| High-temperature (delta) galvanizing | 560 – 630 °C · Precise part fit-up and a thinner, harder coating | Supplier-published |
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What coating thickness does EN ISO 1461 require?
EN ISO 1461 sets the minimum coating thickness by the thickness of the steel itself, as local and mean values. For steel over 6 mm it's 70 µm local / 85 µm mean; from 3 mm up to 6 mm, 55 µm / 70 µm; from 1.5 mm up to 3 mm, 45 µm / 55 µm; under 1.5 mm, 35 µm / 45 µm. Castings are slightly different — 70 µm / 80 µm at 6 mm and above, 60 µm / 70 µm below it. Centrifuged and threaded articles have their own, thinner table. These are minimums the coating must meet, not a target — how far above them it lands depends on the steel's own reactivity.
Does the steel I use affect the galvanized coating?
Yes, more than most buyers expect. The steel's silicon content governs how thick and how uniform the coating grows — this is known as the Sandelin effect. Below about 0.04% Si the coating tends to be bright and close to the EN ISO 1461 minimum. From roughly 0.04% up to about 0.14% Si, coating growth accelerates sharply and produces a thick, dull grey, sometimes brittle coating — the reactive range galvanizers try to avoid. ASTM A385 practice recommends specifying steel in the 0.15% to 0.22% range instead, where growth is more predictable. Above about 0.25% Si the coating is reliably thicker and greyer again, though less erratic than in the reactive range. Phosphorus in combination with silicon can push the reaction further the same way. If the part is large, decorative, or dimensionally critical, ask for the mill's silicon content before ordering the steel.
Do I get a certificate for the galvanized coating?
Two separate documents are in play, and it's worth keeping them apart. EN 10204 covers the STEEL — the usual buyer default is a 3.1 inspection certificate for the material itself. Galvanizing does not extend that certificate; it adds its own conformity confirmation under EN ISO 1461, covering coating thickness, adhesion and appearance against that standard's requirements. Ask for both separately: the mill's EN 10204 3.1 for the steel, and the galvanizer's EN ISO 1461 test report for the coating.
Does my part need vent and drain holes?
Almost certainly, if the part is a hollow section — pipe, box section, or anything that encloses a cavity. Molten zinc and the pre-treatment solutions before it need somewhere to get in, fill, and drain back out; without that, trapped air or liquid can make the article reject the coating, or in the worst case cause a violent steam explosion in the kettle. General guidance calls for vent and drain holes at least 13 mm (1/2 inch) in diameter, placed at the high and low points as the part will hang, and stiffeners or gussets cropped back at least 19 mm so they don't need their own holes. Confirm the exact provision with your galvanizer at the design stage — adding holes after fabrication is far more expensive than designing them in.
Can galvanizing distort my part?
It can, particularly in thin, long, asymmetric, or heavily welded and cold-worked parts. The bath runs close to 450°C, and a part carrying locked-in stress from rolling, forming or welding can relieve that stress unevenly as it heats, which shows up as warping or twisting. There is no reliable numeric threshold — thickness, length and how symmetric the part is all interact, and it comes down to the part's design and the galvanizer's experience with similar work. If distortion risk is a concern, flag the part's fabrication history and ask the provider before you commit to the job.
Is there a maximum size for a part that can be galvanized?
There is no industry-wide maximum — every galvanizer's kettle is a different size, and that sets the ceiling for a single-dip part. Long or oversized parts can sometimes be double-end-dipped, but that adds cost and leaves a visible overlap line. Send your part's dimensions with the enquiry so the provider can confirm it fits their kettle before you commit to fabrication.
What temperature is the zinc bath?
The zinc bath runs at roughly 438°C to 460°C for standard hot-dip galvanizing, commonly cited around 450°C. That's the temperature range every part is exposed to during immersion, which is why thermal stress and distortion risk are part of the design conversation.
What is high-temperature (delta) galvanizing?
High-temperature, or 'delta', galvanizing runs the bath much hotter — roughly 560°C to 630°C — producing a thinner, harder coating with more precise, controlled growth. It's used where fit-up between parts matters, or where a smoother surface is wanted for painting over the galvanizing or better abrasion resistance, at the cost of being a less common process not every galvanizer offers.
What information do you need for a quote?
Send the grade or material number and its silicon content if you have it, the part's dimensions and weight, the quantity, the coating thickness class or standard you need to meet, whether the part needs venting and draining, whether it's a thin or distortion-prone shape, whether you need a certificate, and your delivery address and date. That turns an enquiry into a firm price rather than a range.
What is the lead time?
No general lead time can be stated honestly — it depends on the provider, their kettle schedule, and the size and quantity of your order. Ask for a committed date with your quote rather than assuming a standard turnaround.
Is there a minimum order quantity?
There is no industry-wide minimum order quantity for hot-dip galvanizing; it varies by provider, and small or single-piece orders are common but may carry a minimum charge. Ask as part of your enquiry rather than assuming.
How does hot-dip galvanizing compare to painting?
Hot-dip galvanizing bonds a zinc-iron alloy layer metallurgically to the steel surface, so it can't peel or flake the way a paint film can, and it protects the steel galvanically even where the coating is scratched. Paint and other applied coatings sit on top of the steel and only protect where the film stays intact. Galvanizing is usually the more durable, lower-maintenance choice for steel facing outdoor exposure or a harsh environment; painting still wins where colour, a specific finish, or a very thin coating is required — and painting over galvanizing (a duplex system) combines both.
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