STAINLESS STEEL DUCT FOR CORROSIVE AND WASHDOWN AIR
Galvanized duct is the default for most commercial work, and it does the job in dry, clean, conditioned air. The moment the airstream turns salty, wet, hot, or chemically aggressive, that zinc coating starts to lose. Stainless steel is the answer when the duct has to survive the environment it lives in.
This is a practical guide for mechanical contractors and process engineers deciding when galvanized is not enough, which stainless grade to specify, and what it costs.
Published July 13, 2026 · JMC Fabrication
The short answer: spec stainless when the airstream or the surrounding air will corrode galvanized faster than the system can be allowed to fail. That covers coastal and high-humidity air, food and pharma washdown, chemical and industrial exhaust, and wet or high-temperature streams.
Galvanized protects steel with a sacrificial zinc coating. Once that coating is consumed by salt, moisture, acids, or heat, the base steel rusts and the duct is on a clock. Stainless resists corrosion through the metal itself, so it holds up where galvanized would need early replacement.
Four environments push a job from galvanized to stainless:
- Coastal and high-humidity air — Salt-laden Gulf Coast air attacks zinc coatings and standing condensation accelerates it. Exposed or rooftop duct near the water is a common trigger.
- Food and pharma washdown — Surfaces that get sprayed, sanitized, and scrubbed daily need a material that tolerates water, cleaning chemicals, and repeated contact without shedding or corroding.
- Chemical and industrial exhaust — Fume hoods, process exhaust, and corrosive vapors chew through galvanized quickly. Stainless handles the chemistry and the temperature that often comes with it.
- Wet or high-temperature streams — Moisture-heavy or hot airstreams that would condense, sweat, or exceed the limits of a zinc coating call for stainless from the start.
The first grade decision is almost always 304 versus 316. Type 304 is the workhorse stainless: it covers general washdown, food processing, and most indoor corrosive service well. Type 316 adds molybdenum, which improves resistance to chlorides and pitting.
The rule of thumb for the Gulf Coast: where the air carries salt or the process involves chlorides, bleach-based sanitizers, or coastal exposure, 316 buys real margin. In cleaner indoor washdown without heavy chloride load, 304 is often the cost-effective choice. Match the grade to the actual exposure, not to a blanket spec, and confirm it against the mechanical or process requirements for the run.
In washdown and hygienic service, the finish matters as much as the grade:
- Smoother finishes shed water and soil, which makes duct easier to clean and slower to harbor bacteria
- Continuous, ground-smooth welds avoid the crevices where corrosion and contamination start
- Interior surfaces sized and detailed so the duct can actually be sprayed down and drained
- Fewer seams and cleaner joints reduce the number of places a washdown environment can attack
How the duct is joined is a corrosion decision, not just a leakage decision. In corrosive and washdown service, welded seams are usually preferred because they leave no gap for moisture, chemicals, or product to collect and no sealant to break down over time.
Sealed and gasketed joints are faster and lower cost, and they suit lower-pressure, less-aggressive runs. When the airstream is wet, chemically loaded, or subject to daily washdown, continuous welds are the more durable answer. The tradeoff is more shop labor for a joint that lasts.
Stainless costs more than galvanized on material and on labor, since welding and finishing take more time than sealed galvanized construction. On the sticker, galvanized wins.
The comparison that matters is life-cycle, not first cost. In an environment that corrodes galvanized, you are paying for early replacement, downtime, and the risk of a duct failure in a process or hygienic space. Stainless is the lower total cost where the environment is genuinely aggressive, and galvanized is still the right call where it is not.
JMC Fabrication is a commercial and industrial metal fabricator based in Pascagoula, Mississippi, serving Mobile, New Orleans, and the Mississippi Gulf Coast. The salt air and humidity that define this region are exactly the conditions that push duct from galvanized to stainless.
We fabricate SMACNA duct in stainless, carbon, and galvanized steel and build to your mechanical or process spec. Send us the drawings and the service conditions and we will help you land on the right grade, finish, and joint detail.
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