PP Duct vs Galvanized Duct: The Zinc Coating, Condensation and When Each Material Wins

Key takeaways

1. The comparison is usually argued on price per metre. The material that actually decides the outcome is a layer of zinc about 19 µm thick, and that layer is consumed in service.
2. G90 and Z275 are the same coating: 0.90 oz/ft² or 275 g/m² over both faces, which is roughly 137 g/m² or 19 µm of zinc on each face. That is the whole corrosion allowance.
3. One metre of φ315 mm duct carries about 0.54 kg of zinc coating. At a zinc loss of 2 µm/year that coating is spent in under 10 years; at 5 µm/year it is gone in under 4.
4. Galvanized duct rarely fails uniformly. It perforates where liquid collects — the cold end of a run, a roof penetration, the first metres after the hood, cut edges and unfinished welds.
5. Condensation, not concentration, is the switch. A dry exhaust above its dew point can run in galvanized steel for decades; the same gas arriving wet will take the coating in a few years.
6. The temperature limit usually quoted for galvanized duct belongs to the steel, not to the coating. Zinc is not a high-temperature material, and the corrosion resistance is the zinc.
7. Galvanized steel is genuinely the right material in dry hot air and in the downstream half of a correctly analysed hybrid run — and at φ500 mm PP duct is actually heavier than thin-gauge galvanized, so “PP is always lighter” is wrong too.

PP Duct vs Galvanized Duct — the Decision Is 19 µm of Zinc

Nearly every discussion of PP duct vs galvanized duct settles on the same two claims: galvanized is cheaper, and PP resists chemicals better. Both claims are true and neither one is a specification. The first ignores that the galvanized duct has to be replaced at some interval, and the second ignores that a large share of industrial exhaust is neither clearly corrosive nor clearly benign.

The material that actually decides the answer is not the steel and not the polymer. It is a zinc coating roughly 19 micrometres thick on each face. Everything galvanized steel does to resist corrosion, it does by spending that zinc. Once the zinc at a given point is gone, what remains is bare steel in a wet, acidic environment, and the perforation that follows is a matter of months rather than years.

That reframes the comparison into three questions that can each be answered with a number: how much zinc is there, how fast is it being consumed in this duty, and is there anywhere in the run where liquid water can form. The first is fixed by the coating specification, the second follows from the chemistry and the temperature, and the third is a layout question that is normally ignored until the duct fails.

This page works through all three, with the coating expressed as a thickness and a mass per metre rather than as a mill thickness, with the temperature limit separated into the coating’s limit and the steel’s, and with the honest cases where galvanized steel is the better material named explicitly. Where the comparison turns on how a PP duct is installed rather than on the material itself, we link to the installation data instead of repeating it here.

The two materials are described from a manufacturer’s position: we fabricate polypropylene duct and fittings, and we also work alongside galvanized steel ductwork on hybrid exhaust systems, so the comparison below is the one we give to customers rather than a case for one material over the other. If the answer at the end of it is galvanized steel, that is a valid outcome and it is better reached now than after the second replacement. A useful starting point for the polymer side is our guide to what a PP duct is.

What Each Material Actually Is

Before any numbers, the two ducts need to be understood as different kinds of object, because the shape of a pp duct vs galvanized duct comparison follows from it. One is a single material through its whole wall thickness, and the other is a composite in which the structural material and the corrosion-resistant material are different substances with different limits and different failure modes.

Polypropylene Duct — Homogeneous Wall, No Coating to Lose

A polypropylene duct wall is polypropylene from the bore to the outer surface. There is no coating, no lining and no interface between materials. If the bore is scratched, the surface exposed by the scratch is the same polymer that was already there, and it resists the same chemicals at the same rate. The material picture, the grades used for ductwork and how they differ are covered in our polypropylene duct guide.

That homogeneity is the whole engineering advantage. There is no point in the wall that is materially weaker than any other point, so there is no preferred location for failure. A PP duct usually degrades uniformly, which makes its remaining life predictable from a temperature and a concentration. It cannot be inspected for hidden thin spots the way a coated steel duct can, because it does not develop them.

Galvanized Steel Duct — Steel Carries the Load, Zinc Carries the Corrosion

Galvanized duct is a steel shell with a thin zinc coating applied by hot-dip or continuous galvanizing. The steel determines the stiffness, the span and the temperature rating of the structure. The zinc determines whether the duct survives the chemical environment, and the zinc is a small fraction of the thickness of the steel beneath it. A 1.0 mm steel wall under a 19 µm zinc coating is a coating that is 1.9 percent of the wall thickness.

The two materials are also selected by different people at different times. The steel gauge is chosen by the duct fabricator from a pressure and span table, and the coating mass is chosen by whoever writes the material specification, often as a single line. In practice the second decision matters far more to service life in a chemical exhaust, and it gets a fraction of the attention.

Why the Comparison Is Not Symmetrical

The asymmetry matters because it changes what “better” means. A PP duct is replaced when the whole wall has thinned or embrittled to some fraction of its original thickness, which is a whole-wall condition. A galvanized duct is replaced when one point has perforated, which is a local condition reached long before the rest of the duct is significantly degraded.

This is why the two materials cannot be compared on average wall-loss rates. A galvanized duct can be at 95 percent of its steel thickness everywhere except one square decimetre at a roof penetration, and that one point is what takes it out of service. If you are weighing PP duct against PVC duct next, that comparison runs on different axes again and is set out in our article on PP duct versus PVC duct. Against galvanized steel, the axis is condensation and coating thickness, and that is where the rest of this page goes.

How Zinc Protects Steel, and When That Stops Working

Zinc protects steel in two ways at once, and the distinction between them decides how a galvanized duct behaves over its life. One of them is a barrier and the other is a sacrifice, and they fail differently and at different times. The general mechanism is described under galvanization.

Sacrificial Protection — the Zinc Spends Itself

The primary mechanism is galvanic. Zinc is less noble than iron, so when the two are in contact in the presence of an electrolyte, the zinc corrodes preferentially and the steel is left intact. This is why a galvanized duct can have a visible scratch and still not rust at that scratch: the surrounding zinc becomes the anode and protects the exposed steel nearby.

The consequence is that the zinc is not protecting the duct by staying there. It is protecting the duct by being consumed. Every hour of protection is an hour of zinc loss, and the coating has a finite amount to lose. A galvanized duct’s service life in a corrosive duty is therefore a corrosion-allowance calculation, not a barrier-integrity question, and the allowance is about 19 µm per face.

White Rust and Red Rust — the Two Failure Messages

Zinc corrosion produces a white or grey powdery deposit, usually zinc oxide and zinc hydroxide, which is commonly called white rust. It looks alarming and it is not a failure; it is the coating doing its job and being used up. The condition to act on is what comes after. When the local zinc has been consumed, the first iron corrosion products appear as red rust, and at that point the steel is no longer protected at that location.

In an industrial exhaust this sequence can run in the opposite order to intuition. A duct that looks clean and grey on the outside can be several years into white rust on the inside, and the first external sign is a rust stain at a joint or a low point where condensate has been sitting. By then the internal loss is well advanced.

The Throw Distance at a Scratch or Cut Edge

Galvanic protection does not travel far. The distance over which intact zinc can protect adjacent bare steel is limited by the conductivity of the electrolyte on the surface, and in practice it is on the order of a few millimetres in a wet film and effectively zero in dry air. A scratch a few millimetres wide is protected. A cut edge in a large fabricated panel, or a length of uncoated weld, is not protected across its width.

This is why fabricated galvanized duct is more vulnerable than the base sheet suggests. The sheet arrives fully coated, and then every cut, every welded seam and every drilled hole creates a length of bare steel that relies on a throw distance of a few millimetres at each end. On a long longitudinal weld, the unprotected strip in the middle has nothing protecting it at all.

Barrier and Sacrifice Are Not the Same Claim

The barrier contribution of zinc is small because the coating is thin. Once you accept that, several common statements about galvanized duct stop making sense. “Galvanized does not rust” is a statement about a coating that is consumed, so it holds only until the coating is consumed. “The zinc will protect the scratch” holds only within the throw distance. “The duct is coated so it is protected” ignores that fabrication removed the coating at exactly the points where corrosion starts.

For a PP duct, none of these conditions exist, because there is nothing to consume and no edge to protect. That is a genuine advantage and it is also the reason the PP-versus-galvanized question reduces to whether the duty has liquid in it — a dry duty will not consume the zinc, and a wet one will.

The Zinc Coating Is a Consumable, Not a Barrier

If the coating is a corrosion allowance, then it should be specified the way an allowance normally is: in micrometres, with a stated consumption rate, so the interval before it is exhausted can be calculated. Galvanized sheet is sold by coating mass, which is a weight per area, and the conversion to thickness is simple arithmetic that is almost never done in a duct specification.

What G90 and Z275 Mean in Microns

Coating designations describe the total zinc on both faces of the sheet. G90, from the ASTM A653 system, is 0.90 oz/ft², and Z275, from the ISO and EN system, is 275 g/m². These are the same coating: 0.90 oz/ft² converts to 274.6 g/m², which is the 275 of Z275. A specification quoting one and a supplier delivering the other is not a downgrade.

Converted per face and then to thickness, the coating is smaller than most people expect. Half of 275 g/m² is 137 g/m² on each face, and at a zinc density of 7,140 kg/m³ that is a layer just over 19 micrometres thick. A human hair is several times thicker. That 19 µm is the entire corrosion protection on each face of the duct, and there is no more underneath it.

How Much Zinc One Metre of Duct Carries

Expressed per metre of duct and accounting for both faces, the coating mass is small in absolute terms. The figures below are calculated from the 275 g/m² designation and the perimeter of each diameter, so they are the zinc actually present in a metre of duct rather than a nominal figure.

For comparison, a metre of the same φ315 mm duct in 1.0 mm galvanized sheet is 7.77 kg of steel. The corrosion protection on it is 0.54 kg of zinc, about seven percent of the metal present. The duct is bought by the tonne and protected by the fraction.

Turning Zinc Loss Into a Service Interval

Zinc loss rate depends on the environment. In dry, clean indoor air it can be well under a micrometre per year. In an urban or industrial atmosphere it is typically a few micrometres per year. In a wet acidic exhaust, where the coating is in contact with a liquid film carrying chlorides or mineral acid, the rate can be an order of magnitude higher again, and it becomes strongly localised.

Divided into the 19 µm allowance, those rates give the interval before the coating at a point is exhausted — which is the point at which steel corrosion begins, not the point at which the duct fails, but close enough to be the governing number.

The bottom two rows are where galvanized duct gets its reputation for failing in two to five years, and the top two rows are where it earns its reputation for lasting decades. Both reputations come from the same product. The difference is entirely whether liquid forms on the coating, which is the subject of the next section. Where the coating is genuinely adequate, the duct itself is a standard fabricated item, and the range we supply is listed under industrial ductwork.

What Voids the Allowance — Cut Edges, Welds, Field Damage

The calculation above assumes the coating is continuous. Fabrication breaks that assumption at predictable places, and these are the points that govern the life of the duct regardless of the allowance figure. Cut edges have bare steel across the full sheet thickness. Welded seams burn off the coating in the heat-affected zone and leave a bare strip whose width depends on the process. Drilled and punched holes expose bare steel at the bore. Bolted flanges concentrate both a bare edge and a crevice at the same point.

The practical consequence is that a duct built from fully coated sheet can still have unprotected steel at every seam. A coating specification that does not say how the fabricator repairs those areas — by a zinc-rich primer, by a hot-dip after fabrication, or by accepting them — is incomplete, because on a wet duty those areas decide the replacement date. This is also where a PP duct avoids the whole class of problem, since a welded PP seam is the same material as the wall around it.

Condensation, Not Concentration, Decides the Material

Ask whether galvanized duct is suitable for a chemical exhaust and the usual answer is framed around concentration: how much acid is in the gas. In a pp duct vs galvanized duct comparison that is the wrong controlling variable, and it is usually the one asked last. Zinc corrodes orders of magnitude faster in a liquid film than in dry air, so the question that decides the material is whether the gas ever condenses on the duct wall. A strongly acidic gas that stays dry may leave a galvanized duct alone for years; a weakly acidic gas that condenses every shift will take it out in a fraction of that time.

The Acid Dew Point

Exhaust gases condense on a surface when that surface falls below the dew point of the gas. For exhausts carrying water vapour the dew point is often well above ambient, and for gases carrying sulfur oxides, hydrogen chloride, hydrogen fluoride or nitrogen oxides, the condensate is acidic and the effective dew point is higher than the water dew point alone because the acids concentrate in the first liquid film. The underlying behaviour of condensation as a function of temperature and moisture is described under dew point.

The practical figure for most industrial exhausts is that any duct surface below roughly 60 °C to 120 °C should be assumed wet, and the higher end of that range applies to exhausts with sulfur or halide content and high moisture. Since a duct wall is only a few degrees above the air temperature on the cold side, an uninsulated run exposed to winter air or running through an air-conditioned space will be below the dew point for much of the year even when the gas leaving the process is comfortably hot.

Why Dilute Exhaust Still Fails Galvanized

Dilution is the standard defence of galvanized ductwork in laboratory and process exhaust, and it is a reasonable one. By diluting the exhaust with enough room air to bring the concentration of the aggressive species below a small fraction of its occupational limit, the assumption is that the gas reaching the duct is effectively benign. Where the duct stays dry, that assumption holds and galvanized steel performs well.

The failure is that dilution controls concentration and does nothing about moisture. Diluting a hot wet stream with cool room air lowers the temperature of the mixture and raises its relative humidity at the same time, which moves the mixture closer to its dew point rather than further from it. Add any source of water — a scrubber, a wet process tank, a steam line, an open drain — and the diluted exhaust condenses in the duct even though the chemistry has been diluted to nothing. This is the single most common route by which a galvanized exhaust duct that was correctly sized for concentration still fails.

The Three Zones in a Typical Run

A real exhaust run does not have one condition. It has a hot end near the source, a middle section, and a cold end at the discharge, and the three behave differently in galvanized steel.

• Hot zone, first few metres from the source. Gas temperature is high, the wall is above the dew point, and the surface is dry. Zinc loss is slow and galvanized steel is at its best.
• Transitional zone. The gas has cooled toward the dew point, often at a change of direction, a fan, a damper or a long uninsulated length. This is where the first liquid appears, and it appears intermittently, which produces alternating wet and dry cycles that are harder on the coating than continuous wetting.
• Cold zone, discharge end and any cold surface. Gas is below the dew point, the wall is wet continuously, and any acidic species present concentrates in the film. Zinc loss here is at its fastest and is the point that will perforate first.

Laid over this are the local cold spots — a roof penetration in winter, a fire damper with a large unlined frame, a fan casing with a thin wall, a run passing through a refrigerated area. Each is a localised version of the cold zone and each is a candidate for the first perforation, which is why failures cluster at those points rather than in the middle of a straight run.

The table is a screening test, not a design calculation. Its purpose is to establish whether the corrosion-allowance arithmetic from the previous section applies at a dry-air rate or at a wet-acid rate, because those two rates differ by more than an order of magnitude and they lead to opposite material decisions.

Temperature — Separate the Steel From the Coating

The temperature figures quoted for galvanized duct are usually the steel’s, and that is the most common error in a pp duct vs galvanized duct comparison. Two materials are present in a galvanized duct and they have very different thermal limits, so a single temperature rating for the product is not a meaningful number. The coating is the part providing corrosion resistance, and it is the part with the lower limit.

What the Zinc Can Take

Zinc itself is a low-melting metal, and long before it melts the coating begins to change. Continuous service in the region of about 200 °C is generally accepted as the practical limit for a hot-dip galvanized coating in a corrosive environment, with intermittent excursions somewhat above that. Well below the melting point, zinc oxidises faster, the coating can interdiffuse with the steel and form brittle intermetallic layers, and the alloy layer thickens at the expense of the pure zinc that does the sacrificial work.

Above roughly 250 °C the coating is no longer behaving as a galvanized coating. The steel underneath still has its full structural strength and is entirely capable of carrying the duct at that temperature — but the corrosion protection has been spent by heat rather than by chemistry, and in a wet acidic exhaust the duct is then unprotected steel. Quoting a galvanized duct as suitable for 400 °C is quoting the steel, and it says nothing about whether the duct will survive the gas.

What the Polymer Can Take

Polypropylene duct is rated by our own factory data at 90 °C to 100 °C continuous service, with short-term excursions handled as a separate case. That band is not a marketing figure; it is the temperature above which the material’s creep behaviour, stiffness and resistance to chemical attack all begin to move together, and it is why the fusion and support parameters in our installation data are quoted against an operating temperature rather than a material name.

Figures of 80 °C and of 100 °C or more appear in various published comparisons of PP duct. The lower figure is conservative for continuous service and the higher is realistic only for limited durations or for a lower-stress configuration. Neither is wrong in its own context, but a specification should state which one it means and for how long, because the support spacing and the expansion allowance depend on it.

Where the Two Limits Cross

Set side by side, the two materials have complementary temperature behaviour and the crossover is wider than a single number. Galvanized steel holds a clear advantage in dry hot air, where its structural limit well exceeds anything a polymer can take and there is no moisture to consume the zinc. Polypropylene holds the advantage anywhere the gas is wet or acidic, because it has no coating whose chemistry changes with temperature — its rating is a mechanical and chemical limit on a homogeneous material.

The consequence is a band between roughly 90 °C and 200 °C where the choice turns entirely on moisture and chemistry rather than on temperature. In that band, dry and non-corrosive goes to galvanized steel, wet or acidic goes to PP, and a hot wet stream at 150 °C is the case where neither material’s temperature figure helps and the condensation analysis decides. The PP side of that decision is covered by our range of polypropylene duct, which is fabricated to the same wall and pressure data used throughout this page.

Weight, Support and Handling at Equal Diameter

Weight is the second comparison that is routinely made without a gauge, and the missing gauge changes the answer by a factor of more than two. Galvanized duct is bought in a range of steel thicknesses, and the same nominal diameter can be specified in 0.6 mm, 1.0 mm or 1.5 mm sheet depending on the pressure class and the span. Comparing a PP duct to “galvanized duct” without saying which one is not a comparison.

Weight by Diameter and Steel Gauge

The figures below come from the same wall-thickness model used for our own fabrication data: mass per metre is the wall area of the tube multiplied by the material density, with polypropylene at 910 kg/m³ and steel at 7,850 kg/m³. The PP column is our production wall for that diameter; the galvanized columns are the same tube at three common sheet thicknesses, before the weight of flanges, hangers or stiffeners.

Why “PP Is Eight Times Lighter” Is Not a Specification

The density ratio between the two materials is genuinely large: steel is 8.63 times as dense as polypropylene, so the same volume of material is 88 percent lighter in PP. Duct walls are not solid, however, and the comparison that matters is between two shells whose wall thicknesses were chosen independently. A PP duct compensates for its lower stiffness with a thicker wall, which recovers much of the density advantage.

The result is that across 1.0 mm galvanized sheet, PP is between 36 percent and 65 percent lighter depending on diameter — real and useful, and considerably less than eight times. Against 0.6 mm sheet the margin narrows from about 42 percent at φ110 mm to about 13 percent at φ400 mm, and at φ500 mm the PP duct at 7.86 kg/m is actually heavier than the 0.6 mm galvanized duct at 7.40 kg/m. Anyone quoting a single weight advantage for PP duct has either picked a diameter, a gauge, or neither.

What It Does to Support Spacing

Installed weight is what the supports carry, and that includes the flanges, the stiffening rings, the hangers themselves and any insulation. On a large-diameter run the added steel for flanges and stiffeners can be a significant fraction of the duct weight, and it is added to both materials, which narrows the percentage gap further.

Weight translates into support spacing through deflection, and the two materials get there by different routes. Galvanized steel is stiff, so its spans are limited by the sheet’s local buckling and by the load on the hangers. PP is far more flexible and its spans are limited by deflection at operating temperature, which is why PP support spacing has to be quoted against a temperature rather than a diameter alone. The by-diameter wall and weight data behind the table above is set out in our guide to what a PP duct is, and the support spacing and hanger loads that follow from it are given by diameter in our PP duct installation data. Where a specific diameter is being specified, the corresponding fabricated item is listed as a product, for example the φ315 mm polypropylene duct.

Internal Roughness and Fan Energy

Weight and corrosion get the attention in a pp duct vs galvanized duct comparison, and the internal surface is treated as identical because both ducts look smooth from the outside. They are not the same bore. The inside of a spiral galvanized duct carries the helical seam and, over time, the pitting and rust scale of a coating that is being consumed, and both raise the resistance to flow in a way that is paid for on every hour the fan runs.

Roughness of the Two Bores

Expressed as an absolute roughness for use in friction calculations, a polypropylene duct bore is in the region of 0.007 mm. A welded galvanized duct with dressed seams is around 0.045 mm, and a spiral galvanized duct is around 0.15 mm because of the standing seam geometry. Those look like small differences in isolation, and at large diameters they are. At the diameters used in most exhaust work they are not.

The relevant quantity in the friction calculation is not the roughness but the roughness divided by the diameter, so a fixed roughness matters more in a small duct than in a large one. That is why the airflow penalty is worst in the branch ducts near the source, which are typically the smallest sections of the system and are often where the total pressure demand is set.

Friction Factor at a Working Point

Taking a φ315 mm duct at a velocity of 10 m/s in air, the Reynolds number is about 209,000, which is fully turbulent and in the range where the Colebrook formulation applies. The background to the pressure-loss calculation and the friction factor is set out under the Darcy–Weisbach equation.

The spiral galvanized bore carries about 19 percent more friction than the PP bore at the same diameter and flow. A welded and dressed galvanized duct carries about 6 percent more. In either case the difference is not large enough to change a material decision on its own, but it accumulates in the right direction.

What It Costs Over a Year of Running

Fan power scales with the friction factor at a fixed flow, so a 19 percent higher friction factor is roughly a 19 percent higher fan energy for that section. On a long run or a system with several small-diameter branches, the compounding across the network can put the whole-system penalty in the same range as the headline figure. Over a year of continuous operation that is a recurring cost, and it belongs on the same side of the ledger as the coating’s replacement interval.

The gap also widens with age, which is the part that is usually left out. A PP bore stays at its manufactured roughness because the material does not corrode and does not build scale. A galvanized bore starts at its seam roughness and then gains the roughness of the corrosion products as the zinc is consumed and the steel begins to pit. By the time the duct is approaching the end of its coating life, the friction factor has moved away from the value in the table above, and the fan is working against a rougher duct than the one that was designed. Where the bore matters to the running cost, the corresponding fabricated diameters are listed individually, for example the φ355 mm polypropylene duct.

Cost — What to Compare and What Not To

Cost is where pp duct vs galvanized duct comparisons are most often published and least often useful. Galvanized steel is cheaper per metre and that is the end of the part that is uncontested. The rest depends on a replacement interval that is specific to the duty and on an access cost that is specific to the building, and a single total-cost figure that does not state both is not a number anyone can design against.

First Cost Structure

On a like-for-like diameter and a stated steel gauge, galvanized steel duct is the lower first cost. The material is inexpensive, the fabrication is fast, and it is made by more shops in more places. Polypropylene is a more expensive raw material, has to be welded rather than folded and riveted, and takes longer to fabricate per metre. Across the industrial exhaust market the customary first-cost index puts PP duct at roughly 1.5 to 3 times the cost of galvanized steel duct, and the position within that range depends on the diameter, the wall class and the number of fittings.

Two structural differences sit inside that index and are worth extracting. Galvanized duct cost rises steeply with the steel gauge, so specifying a heavier sheet for corrosion margin is a direct addition to first cost. Polypropylene duct cost rises with the wall thickness needed for stiffness, which is set by the diameter and the support arrangement rather than by a corrosion margin, because there is no corrosion margin to buy. The second point is the one that matters: a PP duct’s first cost already includes the full corrosion resistance, and a galvanized duct’s first cost includes only the 19 µm of it.

The Replacement Interval Is the Real Cost Line

The correct way to frame the cost of a galvanized duct is as a first cost divided by the interval before the coating at its weakest point is exhausted. The arithmetic from the coating section applies directly: at a wet acidic zinc loss rate of 5 µm per year the 19 µm coating is spent in under four years, and at 10 µm per year in under two. A duct that is replaced every four years has a very different cost profile from one that lasts thirty, even though the two ducts are bought at the same price per metre.

The second half of that line is the replacement cost, and it is almost never the duct. On an exhaust system the duct is usually the cheapest part of a replacement. The cost sits in access — scaffolding, a crane, a roof penetration to open and reseal, or the dismantling of a ceiling or a plant item to reach the run — in production downtime while the system is down, and in re-commissioning and re-testing afterwards. On an elevated or concealed run the access cost can exceed the material cost several times over, and it is incurred again at every replacement. A material whose protection is consumed on a short cycle turns that access cost into a recurring charge.

Why We Do Not Publish a Single TCO Number

Total cost of ownership comparisons for these two materials circulate widely, usually as a ten-year figure with an implicit assumption inside it. That assumption is almost always the zinc loss rate, and it is the one input that varies most between installations — a duty that keeps the coating dry and a duty that wets it every shift differ by more than an order of magnitude in the same calculation. A ten-year figure that does not state its zinc loss rate, its replacement access cost and its assumed production downtime is a number produced by its own assumptions and not by the duct.

What can be given instead is the method and the inputs. The first cost is quoted per metre by diameter and gauge. The consumption rate comes from whether liquid forms and what is in it, which the condensation screening above establishes. The replacement cost comes from the building, not the duct. Put those three together and the comparison becomes specific and defensible; leave any of them out and it becomes a preference. In most of the systems we see, the honest answer is that galvanized steel wins on cost wherever it stays dry, and PP wins wherever it does not — the cost comparison follows the condensation analysis rather than replacing it.

Inspecting a Galvanized Duct — What to Measure

Once a pp duct vs galvanized duct decision has gone in favour of galvanized steel, the condition of the coating becomes an ongoing responsibility. A galvanized duct that is already installed can be inspected, and the inspection is different from the inspection of a PP duct. There is no coating to lose on a PP duct and no local thinning to find, so its condition is judged from temperature history and chemical exposure. On a galvanized duct there is a real quantity to measure, and where it is measured matters more than how precisely.

Thickness Gauging at the Right Points

The useful measurement on a galvanized duct is remaining steel thickness, and the useful places to take it are the places where the coating is under attack. Measuring at the middle of a straight, clean, dry run will confirm that the duct is fine and will tell you nothing about the point that is about to perforate. The measurement points that matter are the condensate collection points, the lowest point of each run, the first few metres after the source, the discharge end, every roof or wall penetration, the upstream and downstream faces of dampers and fans, all cut edges and welded seams, and any location where a cold surface is exposed.

Ultrasonic gauging reads the steel beneath the coating, so it gives remaining wall regardless of what the zinc is doing. Because the wall is thick compared with the coating, ultrasonic is the right instrument for the steel and a poor one for the coating. Visual inspection, and often a camera survey on long runs, is what establishes whether the coating itself is intact, and it is the coating’s condition that predicts where the steel measurement will be lost next. On a duct that has been in wet service, the interior is where the answer is and it is also where nobody has looked.

The Eighty Percent Rule and When to Replace

The usual engineering criterion for a corroded duct wall is replacement when the measured thickness falls below about 80 percent of nominal. That rule was written for structures carrying pressure or load, and on an exhaust duct it is a reasonable but not complete trigger, because a duct can perforate at a single point while the surrounding wall is still above the threshold. A localised reading below the threshold at one of the points listed above is a stronger signal than a slightly low average across a whole run.

For a galvanized duct there is an important qualification. The 19 µm coating is far below the resolution that matters for a steel thickness measurement, so a duct whose steel is still at nominal thickness can already have lost its protection entirely at one point. Gauging tells you how much steel is left; it does not tell you how much protection is left. Both have to be assessed, and the coating assessment is the earlier warning. A duct that is showing red rust at a seam or a low point has already passed the stage where a thickness reading would alarm.

Internal Condition — the One You Cannot See

Exhaust ducts corrode from the inside far more often than from the outside, because that is where the process gas and the condensate are. The external surfaces are usually dry and well ventilated, which is the condition under which zinc performs at its best. This produces the characteristic failure pattern of a galvanized exhaust duct: an exterior that looks serviceable, with the perforation appearing from within at a joint or a low point and showing first as a stain.

The practical response is to inspect internally at commissioning and then on a schedule set by the duty rather than by the calendar year. Where the duct cannot be entered, a camera survey through access doors at the known condensation points is enough to establish whether the wall is dry or wet and whether white rust or red rust is present, and that single observation is the most useful piece of condition data available on a galvanized exhaust system.

The Hybrid Run — Where Galvanized Belongs in a PP System

The most useful answer to pp duct vs galvanized duct is often that the correct system contains both. A single exhaust run does not have a single condition, and a material chosen for the worst point in the run is over-specified for most of it while a material chosen for the average is under-specified at the point that fails. Splitting the run by condition is standard practice in industrial and laboratory exhaust, and it is the design that usually gives the lowest installed cost for a given service life.

The First Metres From the Source

The section immediately downstream of the source carries the highest concentration, the highest temperature and the greatest likelihood of moisture, and it is also the section where a failure is most disruptive. It is short, it is usually accessible, and it is the point at which the whole run should be built from the corrosion-resistant material. In laboratory and process exhaust the customary arrangement is a corrosion-resistant section covering the first few metres from the hood or the process connection, sized on the worst credible duty rather than the average.

Downstream of that section the gas has been diluted, cooled and mixed, and if the analysis shows it stays above its dew point it can be carried in galvanized steel for the remainder of the run at a lower cost per metre. The saving is real because the corrosion-resistant section is short and the galvanized section is long, which is the opposite of the arrangement that puts a single expensive material through the whole system.

Transition Details Between the Two Materials

The transition is the weak point of a hybrid run, and most hybrid failures happen at it rather than in either material. Three things have to be handled at the joint. The two materials expand at very different rates under the same temperature change, so a rigid connection between a long PP section and a long galvanized section develops a large differential movement. The joint has to allow that movement with a flexible element designed for it, and the movement the run has to absorb is calculated the same way whether or not the run is hybrid.

The second detail is the transition itself. A bolted connection between a polymer flange and a steel flange concentrates stress and creates a crevice in exactly the wet location the hybrid was designed to manage. A properly made transition uses a compatible flange arrangement with a gasket selected for the medium, or a purpose-made transition fitting, rather than a pair of drilled flanges pulled together by bolts. The fittings that make these connections are the reason a hybrid system should be designed as one system, and the relevant items are grouped with the rest of the PP duct fittings.

The third detail is that the transition point is a fixed point in the layout, because it is where one material’s movement stops and the other’s begins. Its position therefore sets the expansion-joint arrangement for the PP section, which is why the transition is normally located and fixed early in the design rather than adjusted on site. The support spacing, the movement allowance and the position of the fixed points that follow from it are given in our PP duct installation data.

When the Run Should Be One Material Only

A hybrid run is only correct when the downstream condition is genuinely dry. If the analysis shows the gas is still below its dew point at the point of transition, or if it will fall below it later in a different season or a different operating mode, then the galvanized section will corrode at its own wet rate and the hybrid has simply moved the failure downstream and added a joint. The same applies when the downstream section passes through a cold space, or terminates at an outside discharge where the last metres are cold in winter.

The screening is straightforward. A hybrid run is appropriate when the gas is above its dew point from the transition to the discharge in every operating mode and every season, when there is no chloride or strong acid load that could concentrate in a transient film, and when the downstream section is accessible enough that a future failure is not a major event. If any of those fails, the whole run should be one material. Choosing the material for the run, rather than for its average condition, is covered in the installation data referred to above and in the decision sequence that follows.

Choosing Between Them — a Decision Sequence

When the duty rules out both materials in this comparison, the third candidate is usually the composite one, and it carries its own reversal: FRP’s stiffness advantage over PP is real, but hydrofluoric acid attacks the glass fibre that carries its load. That pair is compared in our article on PP duct vs FRP duct.

The material decision for a pp duct vs galvanized duct question can be reduced to four questions asked in order. Each has a defensible answer from information that is normally available before the duct is priced, and each one can end the comparison on its own. Working through them in sequence avoids the usual outcome, which is a decision made on price per metre and revisited when the first duct is replaced.

Step 1 — Can Liquid Water Ever Form?

This is the controlling question and the one that most often decides the material. It is not about the gas composition; it is about whether the wall temperature can fall to the dew point of the gas anywhere in the run, in any operating mode, at any time of year. Include cold starts, standby periods, off-shift operation, seasonal ambient conditions, roof and wall penetrations, and any upstream item that introduces water such as a scrubber, a wet tank or a steam line.

If liquid can form anywhere, the galvanized coating at that point is being consumed at a wet rate, which is several times to an order of magnitude faster than a dry rate. That does not automatically rule out galvanized steel, but it moves the comparison onto the coating arithmetic and requires the corrosion allowance to be sized and the inspection arranged accordingly. If liquid cannot form anywhere, galvanized steel is a strong candidate and the remaining questions are about temperature and consequence.

Step 2 — What Is the Temperature at the Coldest Point?

The temperature that matters is not the process temperature; it is the lowest wall temperature anywhere in the run. A hot process stream can be entirely dry at the source and entirely wet at a discharge 40 metres away in winter. Compare that coldest wall temperature with the dew point of the gas to close Step 1, and separately compare the process temperature with the two material limits — the polymer’s continuous rating at the hot end and the coating’s limit at the hot end.

A dry gas above the dew point at every point and below about 200 °C at the coating is the case where galvanized steel is straightforwardly the right material. A gas above the polymer’s continuous rating, in the region of 90 °C to 100 °C for our PP duct, with moisture present, is the case where neither material is comfortable and the answer usually involves a different material class rather than either of these two.

Step 3 — What Is the Consequence of a Perforation?

A perforated duct on a general ventilation system is an inconvenience. A perforated duct on an exhaust carrying a hazardous substance is a containment failure with a regulatory and safety dimension, and a perforation that leaks liquid onto a surface below is a different event again. Where the consequence is high, the material decision should be made against the worst credible condition rather than the average, because the cost of the material is small compared with the cost of the event.

This step also interacts with the inspection regime. A galvanized duct can be managed by inspection where its condition can be observed and its perforation would be detected before it becomes significant. Where the duct is concealed, above a ceiling, inside a shaft or otherwise uninspectable, a material whose failure mode is a local perforation at an unobservable point is a poor choice regardless of its corrosion arithmetic, because the inspection intended to protect it cannot be performed.

Step 4 — What Is the Access Cost to Replace?

The final question converts the previous three into a cost. Where a replacement means scaffolding, a crane, opening a roof and a production stop, the recurring cost of a short-interval material dominates everything else in the comparison and argues for the material with the longest interval. Where the duct is at floor level, flanged, accessible and replaceable in an afternoon, the first-cost advantage of galvanized steel is real and can be taken.

The sequence in short form

1. Liquid can never form anywhere — galvanized steel is a strong candidate, continue to Step 2.
2. Liquid forms anywhere — size the coating allowance, and unless the interval is acceptable and the duct is inspectable and accessible, that point decides for the corrosion-resistant material.
3. Coating temperature above about 200 °C with moisture present — neither galvanized nor standard PP is the answer; reconsider the material class.
4. Perforation consequence high, or the duct uninspectable — choose against the worst credible condition, not the average.
5. Replacement access expensive — the shortest-interval material carries a recurring access cost that usually outweighs its lower first cost.

In practice most real systems resolve at Step 1 or at Step 3, and the middle of the comparison — where both materials are defensible and the difference is a few years of life against a lower first cost — is narrower than the published comparisons suggest. Where the answer is a hybrid run rather than a single material, the transition details in the previous section determine whether the hybrid performs as designed.

Where Galvanized Duct Is Genuinely the Right Answer

A pp duct vs galvanized duct comparison written by a polypropylene manufacturer should still say plainly where galvanized steel wins, and it wins in more situations than a material-supplier comparison usually admits. Galvanized steel is stiff, non-combustible, dimensionally stable, cheap, widely available and easy to modify on site. None of those advantages disappear because the coating is thin. They apply wherever the coating is not being consumed quickly, which is a larger set of duties than the corrosion-focused parts of this page might suggest.

Dry, Above-Dew-Point, Non-Corrosive Duty

General ventilation, warm air exhaust, dust extraction with no moisture, and any process exhaust that is dry at every point in the run and at the coldest ambient condition are the natural territory of galvanized steel duct. Zinc loss in dry air is measured in fractions of a micrometre per year, so the 19 µm coating is not the governing limit and the duct life is set by the steel and the fixings rather than by corrosion.

This is the majority of ductwork installed in the world, and it is the correct answer for it. The error to avoid is assuming that a duct is in this category because the process is dry, when the exhaust is actually wet at a cold end, at a roof penetration, at a scrubber, or during a winter night. The category is defined by the whole run and all conditions, not by the process description.

High-Temperature Dry Exhaust

Where the exhaust is dry and hot, galvanized steel has no competitor in this comparison. Its structural strength is retained to temperatures far above anything a polymer can reach, it does not soften or creep, and with no moisture present the coating is not being consumed at a corrosive rate. Some caution is needed above the coating’s own thermal limit, as discussed earlier, but for a dry exhaust in the range where the coating remains intact galvanized steel is the straightforward choice.

This is the one area of the comparison where the temperature advantage of steel is fully available, because the condition that would otherwise undermine it — moisture — is absent. The same gas with water in it reverses the conclusion entirely, which is why the moisture question is asked first in the decision sequence.

Non-Critical, Accessible, Short-Life Service

Where a perforation is an inconvenience rather than an event, the duct is at floor level and flanged, and the system has a limited service life or may be rearranged, galvanized steel is the rational choice even in a mildly corrosive duty. The low first cost, the availability and the ability to cut and re-join the duct on site without welding equipment all favour it, and a shorter replacement interval is acceptable when the replacement is genuinely easy.

The trade is only sound when the access assumption holds. A duct that was accessible at installation often is not accessible two years later, after a plant item, a cable tray or a mezzanine has been installed around it. The access condition should be assessed for the whole intended service life, not for the day the duct is installed.

The Downstream Half of a Correctly Analysed Hybrid

Where a run has been analysed and the downstream section is genuinely dry, that section can be galvanized steel and is usually the most economical way to build it. The corrosion-resistant material is used where it is needed, in the short section near the source, and the long remainder is built in the cheaper material. This is a deliberate design outcome rather than a compromise, and it is common practice in industrial exhaust.

What makes it correct rather than optimistic is the analysis behind it: the transition point chosen where the gas is provably above its dew point, the transition detail designed to accommodate the differential movement, and the downstream section inspected so that a change in the process that wets it can be detected before the coating is lost. Done on that basis, the hybrid run uses each material where it is strongest.

Six Mistakes We See in This Comparison

These are the six errors that most often decide a pp duct vs galvanized duct selection incorrectly. Each one replaces a number with an assumption, and each one is easy to check once it is named.

1. Comparing Price Per Metre Without a Gauge

A galvanized duct price is meaningless without the sheet thickness, because the same nominal diameter is sold in a range from around 0.6 mm to 1.5 mm and the cost and the weight both move with it. Comparing a PP duct to a galvanized duct quoted in the thinnest available sheet flatters the galvanized option on both cost and weight, and the duct that arrives may be notably different from the one that was compared. Always state the gauge in the comparison, and compare at the gauge the duty actually requires.

2. Quoting a Steel Temperature Limit as a Duct Rating

Galvanized duct is frequently rated at 400 °C or higher in published comparison tables. That figure describes the steel, which will indeed carry the duct at that temperature. It does not describe the coating, which is no longer providing corrosion protection well before then, and it says nothing about the gas. A galvanized duct in a wet acidic exhaust is unprotected steel at any temperature above the coating’s limit, and quoting the steel’s figure as the product’s rating conceals exactly the information needed to make the decision.

3. Treating Galvanized as Rust-Proof in Chemical Exhaust

The phrase “galvanized does not rust” is true only while there is zinc left at the point in question. In a chemical exhaust, zinc is a consumable and the passivation of the steel ends when the local coating is gone. A specification that treats the coating as a permanent barrier will not size an allowance, will not set an inspection regime and will not plan for replacement, and the duct will be discovered to be at the end of its life by the perforation rather than by the inspection.

4. Reading the Zinc Figure Without the Thickness

Coating designations such as G90 or Z275 state a mass per area over both faces, and converting them to a thickness is what makes the number usable. Without that step the figure has the appearance of a substantial protective layer when it is in fact about 19 µm per face — thin enough that fabrication removes it at every cut and weld. Any comparison that cites the coating mass without the thickness, and without the effect of fabrication on it, has stopped one step short of the number that decides the service interval.

5. Assuming a Smooth Bore Because It Looks Smooth

Both ducts look smooth on the outside and neither is smooth on the inside at the resolution that matters to a friction calculation. A spiral galvanized duct has a standing seam that raises its absolute roughness by more than an order of magnitude compared with a PP bore, and that difference appears as a recurring fan-energy cost for the life of the system. It is not large enough to decide a material on its own, but it belongs in the comparison and it is routinely left out.

6. Costing the Replacement on the Duct, Not the Access

The recurring cost of a short-interval material is dominated by what it takes to reach and replace the duct, not by the duct itself. A comparison that multiplies the duct price by the number of replacements misses the scaffolding, the crane, the roof work, the downtime and the recommissioning, all of which are incurred again at each cycle. On an elevated or concealed run those items can exceed the material cost several times over, and including them is usually what reverses an apparently obvious first-cost decision.

Frequently Asked Questions

Is galvanized duct suitable for chemical exhaust?

It depends almost entirely on whether the exhaust condenses on the duct wall. Where the gas stays above its dew point at every point in the run and at the coldest ambient condition, galvanized duct can give a long service life even with some acid content. Where liquid forms, the zinc coating is consumed at a wet rate and the duct can perforate in a few years, and at that point a corrosion-resistant material is the correct choice. The concentration of the gas is a secondary factor; condensation is the controlling one.

How long does galvanized duct last?

The coating life is the 19 µm zinc allowance divided by the zinc loss rate at the point in question. In dry clean air that is measured in decades, around 38 years at 0.5 µm per year. With occasional condensation and mild chemistry it falls to around ten years, and with frequent acidic condensation to about four, or about two in continuously wet acidic service. The duct itself may last longer than the coating, but it is no longer protected once the coating at a point is gone.

What temperature can galvanized duct handle?

The steel will carry the duct well above 400 °C. The coating’s practical limit is much lower, in the region of about 200 °C for continuous service in a corrosive environment, and above roughly 250 °C it is no longer behaving as a galvanized coating. For a dry exhaust the steel’s limit governs and galvanized steel is a good choice. For a wet or acidic exhaust the coating’s limit governs, because the coating is providing the corrosion resistance.

Does PP duct rust?

No. Polypropylene contains no iron and cannot form iron corrosion products. It degrades by chemical attack, by creep and by embrittlement over time, and those processes depend on temperature and on the medium rather than on moisture in the way steel corrosion does. This is the main structural difference between the two materials: a PP duct has no coating to consume and therefore no corrosion allowance, and its life is set by the whole wall rather than by a local point.

Can I connect PP duct to galvanized duct?

Yes, and hybrid runs are common, but the transition has to be designed rather than improvised. The two materials expand at very different rates, so the joint must accommodate the differential movement with a flexible element sized for it. A bolted flange pair pulled together by bolts creates a crevice in the wettest location in the system, which is the usual cause of failure at a transition. The transition also establishes a fixed point in the layout, so its position affects the expansion arrangement for the whole PP section.

Is PP duct more expensive than galvanized?

Yes, on first cost, typically by a factor of about 1.5 to 3 depending on diameter, wall class and fittings. The comparison does not end there, because the galvanized duct’s corrosion protection is a consumable allowance whose replacement interval depends on the duty, and the replacement cost is usually dominated by access and downtime rather than by the duct. Where the duct stays dry, galvanized steel is the lower-cost option over its life. Where it does not, the recurring replacement usually outweighs the first-cost difference.

Which is better for a fume hood exhaust?

The customary arrangement is a corrosion-resistant section covering the first few metres from the hood, where the concentration, the temperature and the moisture are highest, and galvanized steel downstream where the gas is diluted and provably dry. Fume hood exhaust often carries moisture from wet chemistry and from the room air itself, so the assumption that dilution alone keeps the duct benign should be checked against the dew point rather than assumed. Where the whole run is wet, the whole run should be one corrosion-resistant material.

Does galvanized duct corrode from the inside or outside?

Usually the inside. The external surfaces of an exhaust duct are normally dry and ventilated, which is the condition in which zinc performs at its best, while the interior carries the gas and any condensate. This produces the characteristic failure pattern of an externally serviceable duct perforating from within at a joint, a low point or a cold section. Internal inspection at the known condensation points is therefore the most useful condition check on a galvanized exhaust system.

Getting the Material Right for Your Duty

A pp duct vs galvanized duct decision comes down to a coating thickness, a zinc loss rate and whether liquid forms anywhere in the run. Once those three are established the answer is usually clear: galvanized steel where the duct stays dry and the access is easy, polypropylene where it does not, and a correctly analysed hybrid where the run changes condition along its length. What makes the decision go wrong is not the material data but the missing input — a dew point never checked, a gauge never stated, or a replacement interval costed on the duct instead of on the access.

Xicheng manufactures polypropylene duct, fittings and the components a run needs, and we work with the by-diameter wall, weight, support and expansion data used throughout this page as our own fabrication and site data. If you are choosing between the two materials, replacing a galvanized duct that has perforated, or checking whether an existing run can be kept in galvanized steel, send us the medium, the operating temperature, the moisture content, the airflow and the layout, and we will work through the condensation question with you and come back with a material recommendation and, where it is PP, the wall and support figures for your system.

Useful starting points: the polypropylene duct guide for the material picture, PP duct versus PVC duct if the polymer choice is still open, and what a PP duct is for the by-diameter wall and weight data behind the comparison on this page.

Contact our engineering team with the medium, the temperature and the layout, and we will give you the material recommendation and the fabrication data for the run you are planning.