The short answer
If you only remember one paragraph: 304 is the default stainless for indoor wet and hygienic air; step up to 316 when chlorides are present — coastal and marine air, swimming-pool halls, de-icing salt; and choose the low-carbon L grade (304L or 316L) whenever the duct is continuously welded, which covers almost all cleanroom, pharmaceutical and marine fabrication. This page is the grade-selection companion to the broader stainless steel HVAC duct overview and the galvanised vs stainless cost comparison; start there if you are still deciding whether stainless is justified at all.
The grade selection table
Four austenitic grades cover the great majority of stainless duct. Two variables separate them: molybdenum (chloride resistance) and carbon (weldability). Everything else — the chromium, the nickel, the forming behaviour — is effectively the same.
| Grade | Molybdenum | Carbon | Chloride / pitting resistance | Best fit in duct |
|---|
| 304 | None | Standard (~0.08% max) | Good general resistance | The default — lock-seamed indoor wet & hygienic duct |
| 304L | None | Low (~0.03% max) | Good general resistance | Welded duct without heavy chloride — ISO Class 7–9 cleanroom |
| 316 | 2–3% | Standard (~0.08% max) | Much better against chlorides | Lock-seamed coastal, pool & marine duct |
| 316L | 2–3% | Low (~0.03% max) | Much better against chlorides | Welded marine, ISO Class 5–6 cleanroom & pharma sterile duct |
Compositions are nominal for standard austenitic designations. 316/316L runs roughly 25–30% more than 304/304L by weight; the molybdenum is what you are paying for. Match the grade to the environment — over-specifying 316 where 304 would last adds cost the project rarely needs.
What the molybdenum buys: chlorides and pitting
Stainless steel protects itself with a thin, self-healing chromium-oxide film. Chloride ions — from sea air, pool chlorine vapour, road-salt spray, some sterilisation gases — attack that film locally and start pitting corrosion: tiny, deep, self-accelerating pinholes that are far more dangerous than uniform rust because they perforate the duct wall while most of the surface still looks clean. The 2–3% molybdenum in 316 and 316L raises the pitting resistance substantially, which is why the 316 family is the rule for anything chloride-laden.
Practical thresholds we use when advising buyers: inland conditioned air and most indoor wet zones (kitchen exhaust, general wash-down) are comfortable in 304; a building within roughly a kilometre of surf, an indoor pool hall, or duct that sees salt spray should be in 316; and a marine vessel, a jetty plant room or pool supply sitting right above the waterline is firmly 316L. When in doubt on a coastal site, the cost of stepping up to 316 is far cheaper than re-ducting a corroded system in five years.
What the "L" buys: welded duct and sensitisation
The "L" means low carbon (around 0.03% maximum, versus about 0.08% for standard grades). It matters only because stainless duct is so often welded. When standard 304 or 316 is held in the 450–850 °C range during welding, carbon combines with chromium to form chromium carbides at the grain boundaries in the weld heat-affected zone. That locally strips chromium out of the steel — a condition called sensitisation — and the depleted zone then corrodes preferentially (intergranular corrosion), often appearing as rust lines running parallel to a weld months after handover.
The low carbon of 304L and 316L starves that reaction, so the weld zone keeps its corrosion resistance. The rule on the shop floor is simple: if the duct is continuously welded, specify the L grade. If it is only lock-seamed and never welded, plain 304 or 316 is acceptable and slightly cheaper. Specifying plain 304 for welded cleanroom duct is one of the most common and most expensive stainless mistakes — see the cleanroom duct manufacturing guide for the full ISO 14644 context.
Gauge and thickness for stainless duct
Stainless duct uses the same gauge system as galvanised. Because stainless is stronger than mild steel, designers sometimes drop one gauge versus galvanised for the same duty, but the pressure-class rules in SMACNA and AS/NZS still govern. The common bands:
| Gauge | Thickness | Typical stainless duct use |
|---|
| 26 ga | 0.55 mm | Light low-pressure supply & small hygienic branches |
| 24 ga | 0.70 mm | General commercial stainless supply / return |
| 22 ga | 0.85 mm | Medium-pressure, larger cleanroom & pool duct |
| 20 ga | 1.00 mm | High-pressure & large rectangular stainless duct |
| 18 ga | 1.31 mm | Kitchen & chemical exhaust, heavier welded duct |
| 16 ga | 1.61 mm | Heavy industrial & marine exhaust |
See the full duct sheet-metal gauge and thickness chart for the complete imperial-to-metric mapping and the SMACNA pressure-class context.
Stainless vs galvanised, side by side
For the engineer weighing the step up, the practical differences come down to this:
| Property | Galvanised steel (G90) | Stainless (304 / 316 family) |
|---|
| Corrosion mechanism | Sacrificial zinc coating; corrodes once zinc is consumed | Self-healing chromium-oxide film; no coating to lose |
| Wet / chloride air | Poor — zinc is consumed quickly | Good (304) to excellent (316) |
| Hygiene / cleanability | Adequate for general air | Smooth, wipe-clean, weld-and-passivate capable |
| Relative material cost | 1.0× baseline | ~3.0× (304) / ~3.8× (316) |
| Line speed (auto duct line) | Full speed, standard tooling | ~20% slower; harder tooling; stiffer seam |
| Cut-edge treatment | Brush zinc-rich paint on exposed edges | No edge touch-up; but welds must be passivated |
Why stainless changes the shop floor
Every austenitic stainless grade work-hardens — it gets harder the more it is deformed. That single property drives most of what changes when a duct shop runs stainless instead of galvanised:
Forming. An auto duct line runs stainless roughly 20% slower than galvanised, needs harder polished tooling so it does not transfer marks, and applies more forming force; the Pittsburgh-lock or TDF profile is stiffer to close. The machine has to be specified for stainless from the outset — retro-fitting soft galvanised tooling onto stainless work galls the tools and scores the duct.
Welding. Stainless duct is usually welded rather than only locked, for hygiene and leak-tightness, and the weld is where grade choice pays off. It wants clean, controlled heat: too much, and you get distortion, heavy heat tint and — on standard grades — sensitisation. SBKJ supplies two routes. The medium-frequency welder joins stainless from 0.4 mm up to 3.0 mm with a tidy, low-distortion seam, and the 2000 W handheld laser welder (1064 nm, water-cooled) gives the cleanest, lowest-heat seam for cleanroom and architectural feature work.
Cutting. A fibre laser cutter (2–8 mm capacity, 2000–3000 W) gives the cleanest edge on stainless fittings and feature work, where plasma can leave dross that has to be dressed off before welding.
Passivation. This is the step most often skipped. Welding burns off the passive oxide film and leaves a heat-tinted band that is markedly less corrosion-resistant than the parent metal. After welding, the joint must be cleaned to remove the heat tint and then passivated — pickling paste or an acid bath, followed by a thorough rinse — to rebuild the chromium-oxide layer. On marine and cleanroom duct this is mandatory, and on the L grades it is what protects the very welds the L grade was chosen for. Skipping it produces rust streaks at the welds within months, even on 316L.
Which SBKJ machines handle stainless
Every SBKJ forming and joining machine that lists stainless is rated for the 304 and 316 families alike — the grade does not change the machine, only the downstream weld and passivation regime. The catalog ranges:
Source: SBKJ Product Catalog 2026, manufacturer nameplate specifications. Spiral round duct conforms to EN 1506 / SMACNA round-duct construction; achievable gauge depends on material temper. For round-duct machine selection see the spiral duct machine spec comparison; for round stainless feed-stock, SBKJ also builds a dedicated stainless duct forming machine for small-diameter thin-foil work (Φ100–Φ500 mm).
Putting it together: three worked cases
Pharmaceutical sterile suite (ISO Class 5–6). Continuously welded, electropolished interior, mandatory passivation. Grade: 316L — low carbon for the welds, molybdenum for any chloride-bearing sterilant. Formed on a stainless-tooled spiral line or rectangular line, welded with the laser unit for minimal heat tint.
Coastal hotel pool hall. Salt air plus chlorine vapour, lock-seamed and partly welded. Grade: 316 for lock-seamed sections, 316L where welded. The molybdenum is non-negotiable this close to surf; 304 would pit within a few seasons.
Inland hospital wet-zone exhaust. Humid, hygienic, but no chloride load and mostly lock-seamed. Grade: 304 (or 304L if welded). Stepping up to 316 here is pure over-specification — spend the saving on a heavier gauge or better sealing instead.
Building stainless duct? Tell us your grade, gauge and weld requirement for a machine recommendation →
FAQ
304 or 316 for a coastal or marine project?
Use 316 (or 316L if welded). The 2–3% molybdenum resists the chloride pitting from salt air and pool chlorine that attacks 304 over time. 304 is fine inland; on a salt-laden site it will eventually pit. Reserve the dearer 316 family for genuine chloride exposure.
316 vs 316L — what is the difference?
Same molybdenum-bearing alloy and the same chloride resistance; the only difference is carbon. 316L is the low-carbon version, which resists sensitisation (chromium-carbide precipitation) at the weld heat-affected zone. Specify the L grade for welded duct; plain 316 is fine for lock-seamed duct that is never welded.
What grade and finish does a pharma cleanroom need?
316L for ISO Class 5–6 sterile and chloride-exposed duct, at roughly Ra 0.4–0.8 µm (2B mill finish plus electropolishing). 304L for ISO Class 7–9 at Ra 1.6–3.2 µm 2B mill finish. The L grade matters because the duct is welded; the finish matters because smooth surfaces shed fewer particles.
Do stainless duct welds need passivation?
Yes. Welding burns off the passive oxide film and leaves a heat tint that corrodes preferentially. Clean off the heat tint and passivate (pickling paste or acid bath, then rinse) to restore the chromium-oxide layer. Mandatory on marine and cleanroom duct; skipping it causes rust streaks at the welds within months.
What stainless thickness can SBKJ machines handle?
Spiral tubeformers form stainless from 0.4 mm (SBTF-1500 to 0.8 mm; SBTF-1500C/1602/2020 to 1.2 mm). The SBAL-V auto line runs 0.5–1.5 mm rectangular. The medium-frequency welder joins 0.4–3.0 mm; the 2000 W handheld laser welder suits thin cleanroom work. Figures from the SBKJ Product Catalog 2026.
Is 316L harder to fabricate than 304?
Not meaningfully for forming — all austenitic stainless work-hardens and runs ~20% slower than galvanised with harder tooling. The differences are downstream: 316L welds want tighter heat control, and the L grades plus passivation protect the welds. SBKJ machines are rated for stainless within their published gauge range.
