Stainless steel spiral duct (grades 304, 304L, 316, 316L) carries a chromium-nickel passive layer that protects the base metal throughout its thickness, while galvanized spiral duct relies on a zinc coating (commonly G60 or G90 per ASTM A527) that sacrifices itself first [S2][S1]. The difference between "coating" and "alloyed protection" is the single biggest decision driver when the airstream carries moisture, chlorides, acids, or sanitize-in-place wash-down.
For indoor commercial HVAC, warehouses, and most dust-collection runs, G60/G90 galvanized spiral remains the workhorse because of lower cost and adequate durability in moderate humidity [S1]. For chemical fume extraction, food/pharma, coastal rooftops, and wastewater aeration rooms, specifiers move to 304 or 316 stainless because the zinc layer cannot survive sustained acidic or chloride exposure [S2][S3].
Material spec ranges and what the numbers actually mean
Galvanized spiral duct per ASTM A527 is supplied in 10ga to 22ga (roughly 3.4 mm down to 0.85 mm) with a hot-dip zinc coating designated by coating weight: G60 carries about 0.60 oz/ft² total both sides, G90 about 0.90 oz/ft², and the heavier G90 is normally specified for humid or mildly corrosive runs [S1][S2]. Stainless spiral duct is supplied in 10ga to 18ga across diameters 2" to 84" in 304/304L/316/316L, and 316 adds 2 to 3% molybdenum over 304 for better chloride pitting resistance [S2]. The chemistry gap matters: a scratch on galvanized sheet that penetrates to bare steel creates a corrosion initiation site, while the same scratch on 304 or 316 re-forms its passive oxide layer in oxygen and remains protected [S2].
Galvalume, sometimes offered as a third option, uses a 55% aluminum / 43.4% zinc / 1.6% silicon coating and is rated for stronger atmospheric corrosion resistance than G90, but galvanized outperforms galvalume at cut edges and scratches because the zinc in galvalume is depleted and exposed steel is not sacrificially protected the same way [S4]. The trade is therefore environment-specific, not absolute.
Decision matrix: galvanized vs stainless spiral duct
Four criteria drive nearly every spec call between the two materials. (1) Upfront cost: galvanized is the lower-cost option, stainless is a premium that can be 2 to 4 times the galvanized material price on equivalent gauge, though lifecycle cost often reverses the comparison in corrosive service [S1][S6]. (2) Corrosion mechanism: galvanized relies on sacrificial zinc action, so coating damage is a service-life liability; stainless relies on a self-repairing passive film, so surface damage is not a service-life liability [S2]. (3) Pressure and structural rating: galvanized spiral is well matched to low and medium-pressure HVAC (typical supply, return, exhaust), while stainless is preferred for high-pressure industrial and fume-extraction duty because of higher base-metal strength [S3]. (4) Cleanability and hygiene: stainless is smoother, easier to wash down, and is the default in food, pharma, and sanitary process ducts; galvanized is acceptable for general industrial dust collection but not for sanitary wash-down [S1][S2].
Stainless spiral duct also has an edge on continuous high-temperature service, holding mechanical integrity and corrosion resistance at temperatures that would accelerate zinc coating breakdown on galvanized [S7]. Galvanized ducting reference material consistently cites "moderate humidity" and "normal temperature swings" as the operating envelope where zinc performs as designed, not as a high-temperature option [S1][S5].
Where each material is the right pick, and where each fails
Galvanized G60/G90 spiral duct is the right pick for general office HVAC, retail, warehouses, light manufacturing, and indoor dust or fume collection where the airstream is dry to moderately humid and free of acids, chlorides, and sanitize chemicals [S1][S3]. It fails in coastal outdoor service, chemical fume exhaust, lab acid exhaust, pool natatoriums, and food/pharma wash-down rooms, where the zinc layer is consumed faster than the design service life expects and the underlying carbon steel then red-rusts through the coating [S2].
Stainless 304/316 spiral duct is the right pick for chemical plants, food processing, pharmaceutical cleanrooms, laboratories, coastal and high-humidity plants, and any duct carrying acidic or chloride-laden airstreams [S2][S3][S8]. 316 in particular is specified where chloride exposure is sustained, such as marine intake plenums and bleach or HCl vapor service, because the molybdenum content raises pitting resistance above 304 [S2]. A common failure mode for stainless in this comparison is the wrong grade in the wrong place: 304 in heavy chloride service still stains and pits, so grade selection must match exposure, not just "use stainless" [S2][S7].
Installation, joining, and maintenance differences
Galvanized spiral duct is lighter than equivalent-gauge stainless, easier to rig in overhead trapeze hangers, and pairs with the same rolled-lip, flange, or clamp-together fittings used on stainless systems [S2][S3]. On the maintenance side, galvanized duct in corrosive zones should be inspected for surface rust at flanges, seams, and cut edges, with touch-up zinc-rich primer applied where the coating has been disturbed [S1].
Stainless spiral duct is heavier per linear foot, often needs stronger supports and careful rigging, and on sanitary runs typically uses TriClamp or flanged connections that allow full disassembly for clean-in-place [S2]. For general-purpose stainless runs the same rolled-lip and clamp connections used on galvanized are available, which means the installer's tooling is largely shared between the two materials and the decision truly comes down to environment and lifecycle cost, not install method [S2]. For background on duct geometry and how the spiral lockseam is formed from coil stock, the spiral duct reference page covers the standard diameters, gauges, and SMACNA-based pressure classes used in both selections.
Spec, sourcing, and standards checklist
Two material standards dominate. Galvanized sheet is typically called out as ASTM A527 with G60 or G90 coating weight, sheet thickness picked per SMACNA HVAC duct construction standards for the static pressure class [S1][S2]. Stainless sheet is called out by ASTM A240 grade (304, 304L, 316, 316L), with 316/316L preferred for chloride service and low-carbon "L" variants used where welded seams will see corrosive condensate [S2]. For ductwork that doubles as a process or hygienic line, surface finish matters: 2B mill finish is the default for industrial stainless duct, while higher-polish sanitary finishes are reserved for pharma and semiconductor exhaust.
For a broader view of the stainless family used across duct, pipe, and sheet, the stainless steel grade reference covers the 300-series chemistry and the difference between 304 and 316. Stainless pipe in process piping adjacent to the duct run is covered separately on the stainless pipe spec page and often shares the same 304/316 callout for material consistency across the system.
Final call: pick G60/G90 galvanized spiral duct for indoor, dry, moderate-humidity HVAC and dust collection where the zinc sacrificial layer is sufficient and the budget favors lower first cost. Pick 304 or 316 stainless spiral duct for any service that is wet, coastal, acidic, chloride-bearing, sanitary, or high-temperature, and accept the higher upfront price as the lifecycle premium for a passive oxide layer that does not get consumed like a coating [S1][S2][S3][S8]. Track two signals going forward: the grade (304 vs 316) actually called out on submittals for chloride service, and whether G90 is being substituted for G60 in humid regions as a cost-effective middle ground before a full stainless upgrade.
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