Insights · Reference
Stainless steel duct costs around three times galvanized, so it earns its place only where the air would corrode ordinary steel. This reference covers when stainless is the right call, how to choose between 304 and 316, the gauges used, and what changes on the shop floor when you form and weld it.
Galvanized steel is the default for conditioned air. Stainless steel is the answer when the environment would eat through zinc-coated steel: wet or high-humidity air (kitchen exhaust, pool halls, wash-down areas), chloride and salt exposure (coastal sites, swimming pools, marine), and hygienic settings where the duct must be cleanable and corrosion-free (food, pharmaceutical, hospital, lab exhaust). Outside those cases, stainless is usually over-specification — see galvanized vs stainless for the cost trade-off.
Almost all stainless duct is one of two austenitic grades. The difference is one alloying element — molybdenum — and it decides chloride resistance.
| Grade | Alloy | Corrosion resistance | Use it for |
|---|---|---|---|
| 304 | ~18% chromium, 8% nickel, no molybdenum | Good general corrosion resistance | The default — most stainless duct, indoor wet/hygienic air |
| 316 | 304 + 2–3% molybdenum | Much better against chlorides & pitting | Coastal, pool, marine, chloride-bearing exhaust |
316 costs roughly 25–30% more than 304. Specify 304 by default and step up to 316 only where chloride exposure genuinely warrants it — the molybdenum is what you are paying for.
Stainless duct uses the same gauge system as galvanized — typically 26 to 22 gauge (0.55–0.85 mm) for low-pressure work, heavier as duct size and pressure rise. Because stainless is stronger, designers sometimes drop a gauge versus galvanized for the same duty. SBKJ spiral tubeformers and auto lines form stainless from 0.4 mm up to 1.2 mm; for the per-machine ranges see the spiral and auto line comparisons.
Stainless behaves differently from galvanized through the fabrication process:
Forming — stainless work-hardens, so an auto duct line runs about 20% slower, needs harder tooling and more forming force, and the Pittsburgh-lock seam is stiffer to close. The machine has to be rated for it.
Welding — stainless duct is usually welded rather than only locked, for hygiene and leak-tightness. It wants clean, controlled heat: a medium-frequency or laser welder gives a tidier weld with less discolouration than a mains-frequency machine, and the medium-frequency welder handles stainless up to 3.0 mm.
Cutting — a fiber laser cutter gives the cleanest edge on stainless feature work, where plasma can leave more dross to dress.
Stainless is roughly 3x galvanized by weight, and 316 adds another 25–30% on top of 304. That is why the rule is to match the grade to the environment: galvanized where it lasts, 304 where moisture or hygiene demand it, and 316 only where chlorides are genuinely present. Over-specifying stainless adds cost the project rarely needs.
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Where galvanized would corrode: wet air (kitchen exhaust, pools), chloride/salt (coastal, marine), and hygienic settings (food, pharma, hospital). Otherwise galvanized is more cost-effective.
304 (18Cr/8Ni, no moly) is the default. 316 adds 2–3% molybdenum for chloride/pitting resistance — use it for coastal, pool and marine duct. 316 costs ~25–30% more.
Same gauge system as galvanized — usually 26–22 ga (0.55–0.85 mm) low-pressure. SBKJ forms stainless 0.4–1.2 mm; the medium-frequency welder joins up to 3.0 mm.
Slightly — it work-hardens, so it runs ~20% slower with harder tooling, and welding needs cleaner heat (medium-frequency or laser). SBKJ machines are rated for stainless in their gauge range.
Building stainless duct and need machines rated for it? Tell us your grade, gauge and weld requirement and an SBKJ engineer replies within 12 hours — not a salesperson.
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