Acid dew-point corrosion: the silent ID-fan killer below the dew point

An induced-draft fan handling boiler flue gas can run for years on hot, dusty, mildly aggressive gas and show almost no distress — then thin, pit and perforate on the coldest parts of the impeller and casing, seemingly out of nowhere. The mechanism is rarely erosion and rarely a metallurgy defect. It is acid dew-point corrosion: liquid sulphuric acid condensing on metal surfaces every time they fall below the acid dew point of the gas. The failure is quiet because it happens where you cannot see it — the back of the backplate, the shroud-to-blade fillets, the casing floor — and it accelerates precisely when the fan is doing the least: at start-up, at shutdown, and at low load.

Where the acid comes from

When you burn a sulphur-bearing fuel — coal, lignite, heavy oil, pet-coke, some biomass and process residues — most of the sulphur leaves as SO2. A small fraction oxidises further to SO3. That SO3 combines with water vapour in the flue gas to form gaseous sulphuric acid (H2SO4). As long as every wetted surface stays hot, the acid stays as vapour and does no harm. The moment a metal surface drops below the temperature at which that acid vapour condenses — the acid dew point — a film of concentrated sulphuric acid forms on it and attacks the steel. It takes only a very small SO3 concentration to raise the acid dew point far above the plain water dew point, which is why a gas that feels "dry" on paper can still wet cold steel with acid.

The acid dew point is not one number

There is no single dew-point temperature you can design to. The acid dew point rises with the SO3 concentration and with the moisture content of the gas, both of which follow your fuel and your combustion and cleaning process — the fuel sulphur, the excess air, the degree of SO2-to-SO3 conversion, and the flue-gas humidity. It is commonly cited in the region of a few tens of degrees above the water dew point, but the value moves with the gas and must be established from the flue-gas condition you specify, not assumed. For a boiler ID fan, the design question is therefore never "what is the dew point" in the abstract — it is "what is the acid dew point of your stated flue gas, and where on the fan does metal fall below it." Treat any single degree figure as a starting point to be confirmed against your own gas analysis.

Why start-up, shutdown and low load are the danger zone

At full load with the gas path hot, the fan metal usually sits above the acid dew point and stays dry. The exposure comes at the transitions:

• Start-up — cold metal meets acid-bearing gas before the fan and ducting have warmed through; condensation is guaranteed until surfaces climb past the dew point.
• Shutdown and standby — the fan soaks, the casing cools first, and residual gas condenses onto it; a fan left dirty and wet over a weekend outage can lose more metal than a week of running.
• Low load / turndown — gas temperature drops, the margin above the dew point shrinks, and the coldest structural members cross the line first.
• Cold spots at any load — an un-lagged casing panel, a bare drain boss, a shaft-seal pocket or a stiffener acts as a heat sink and condenses acid while the bulk gas is still safe.

This is the same "the coldest part governs" logic that drives thermal design on hot-start machines; see designing furnace and hot-gas fans for hot starts for the expansion and cold-metal side of the same coin.

Four defences, chosen at the enquiry stage

Acid corrosion is designed out before the fan is built, not patched after the first perforation. Four levers, usually in combination:

Defence	What it does	Where it applies
Keep metal above the dew point	Removes the condensing surface — lagging/insulation on the casing, minimum-gas-temperature operation, avoiding cold structural sinks, warm-through on start	Casing, inlet box, bearings pedestal shielding
Material selection	Tolerates the acid film when condensation cannot be fully avoided — SS304, SS316L, 2205 duplex, and for aggressive gas higher alloys such as C-276, or FRP linings	Impeller, shaft-wetted parts, casing liners
Protective coatings / linings	Barrier between acid and base steel where alloy upgrade is not warranted	Casing internals, backplate, low-velocity zones
Drainage & no dead pockets	Lets condensate leave instead of pooling and concentrating	Casing low points, drain plugs, self-draining geometry

On materials specifically, the step from SS304 to SS316L (and beyond) is not cosmetic — the molybdenum in 316L changes how the steel behaves against a chloride- and acid-bearing film. We cover the trade-off in SS304 vs SS316 fan construction. Note that acid corrosion and fly-ash erosion are different attackers needing different answers: erosion wants hard-faced or radial-tip geometry, corrosion wants the right alloy or coating. A wheel specified only for wear can still dissolve below the dew point.

Getting the specification right

Because the acid dew point is a property of your gas, the corrosion outcome is owned by the flue-gas condition you state — the fuel sulphur, the SO3 and moisture, the temperature at fan inlet, and the load profile including how often the machine starts, stops and idles. The more completely those are stated, the more precisely the build can be matched to them. A duty point given only as flow and pressure hides exactly the information that governs corrosion life; see why the duty point needs the full gas condition. It is also worth separating corrosion from the vibration story — deposits and imbalance are a distinct failure path, addressed in ID-fan resonance vs imbalance, and both belong in the same enquiry.

How we handle it

We treat acid dew-point corrosion as a gas-condition problem, resolved on the datasheet before any steel is cut. Working to your stated flue-gas analysis — fuel sulphur, SO3/moisture, inlet temperature and start-stop-turndown profile — we select the wetted-part material (SS304 and SS316L, 2205 duplex, C-276 for aggressive gas, or FRP linings), specify casing insulation and warm-through to keep metal above your acid dew point where practical, add protective coatings or linings where an alloy upgrade is not warranted, and detail casing drainage with low-point drains and self-draining geometry so condensate leaves instead of concentrating. The build is matched to your gas; the corrosion margin is stated against the condition you give us, not offered as a blanket guarantee. This is standard practice across our induced-draft fan work. Tell us the flue-gas condition and we will match the fan to it — get a quote.

Talk to us about an ID-fan corrosion duty →

Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.