Dew Point Corrosion in Boiler Flue Gas Systems: Material Selection for Acid Condensate Service
When flue gas from sulphur-bearing fuels cools below the sulphuric acid dew point — typically 120–160°C depending on SO₃ concentration — concentrated H₂SO₄ condenses on pipe and fitting surfaces. This acid condensate is more aggressive than bulk dilute H₂SO₄ because it forms at high concentration and deposits on surfaces that are simultaneously experiencing oxidising conditions. Carbon steel corrodes rapidly; most stainless steel grades are also attacked. Understanding the correct material boundary for each flue gas zone prevents premature failure in economisers, air preheaters, stack connections, and FGD inlet ducting.
The H₂SO₄ Dew Point Mechanism
Sulphur in fuel burns to SO₂, with a small fraction (typically 1–5%) further oxidised to SO₃ in the presence of excess O₂ and catalytic surfaces (particularly vanadium pentoxide deposits on boiler tubes). SO₃ combines with moisture in the flue gas to form H₂SO₄ vapour. The acid dew point is the temperature at which H₂SO₄ vapour starts condensing — typically 120–160°C for oil-fired plant, 100–130°C for gas-fired, and up to 180°C for high-sulphur heavy fuel oil. Below the dew point, condensed acid is approximately 75–85% H₂SO₄ — this is the intermediate concentration range where carbon steel corrodes fastest and where even grades like 316L are only marginally acceptable.
Material Selection by Zone
| Zone | Temperature | Condition | Fitting Grade |
|---|---|---|---|
| High-temp flue gas duct | >300°C | Dry, above dew point | Carbon steel WPB |
| Economiser / air heater | 150–300°C | Approaching dew point | P11 or 304L depending on SO₃ level |
| Dew point zone | 100–160°C | Concentrated H₂SO₄ condensate | Alloy 20, 904L, or Hastelloy C-276 |
| FGD quench / absorber inlet | 50–90°C | Dilute H₂SO₄ + Cl⁻ | Hastelloy C-276 or C-22 |
| Stack below dew point | 40–80°C | Dilute H₂SO₄ + HCl condensate | C-276, duplex 2205 minimum |
Why 316L Is Marginal in This Service
316L has a PRE of ~24 and is generally acceptable in dilute H₂SO₄ at ambient temperature — but in the dew point zone it faces a combination of concentrated acid, elevated temperature, and residual chloride from HCl also present in flue gas. This triple combination — acid concentration, temperature, and chloride — takes 316L well outside its reliable service envelope. Field experience on European coal-fired plant has shown 316L fittings in the 100–150°C flue gas duct surviving 2–5 years before showing wall thinning at extrados surfaces where condensate pools. The correct grade for the dew point zone is Alloy 20 as a minimum; Hastelloy C-276 for plant handling high-sulphur or waste-derived fuel.
Specifying for Dew Point Service
On the PO: state the maximum SO₃ concentration in ppm, the expected dew point temperature, whether chloride is present in the flue gas, and the minimum operating temperature. These parameters determine whether Alloy 20 (adequate for clean coal gas) or C-276/C-22 (required for high-sulphur, waste, or mixed-fuel combustion with HCl present) is the right choice. For fittings in the FGD inlet zone where both H₂SO₄ and HCl are present, C-22 (higher Cr than C-276) often outperforms C-276 in long-term corrosion testing.