254 SMO vs Super Duplex 2507 in FGD Scrubbers: A Grade Selection Guide
Flue Gas Desulphurisation (FGD) scrubbers present one of the most aggressive corrosion environments in power generation — hot dilute sulphuric acid, chlorides, and fluorides in a pH 3–5 slurry at 50–70°C. Both 254 SMO (EN 1.4547, 6% Mo austenitic) and Super Duplex 2507 (EN 1.4410) are widely specified for FGD internals and piping. The choice between them is not simply about PRE — it involves welding constraints, stress corrosion cracking risk, and the specific zone of the scrubber.
FGD Scrubber Service Conditions
A wet limestone FGD absorber handles: inlet flue gas at 120–160°C (cooled to 55–65°C inside); circulating slurry at pH 4–6 with 10,000–20,000 ppm chloride (concentrated from coal-burn); sulphite and sulphate salts; traces of HF and HCl from flue gas absorption. The combination of moderate temperature, high chloride, and acid pH creates conditions that defeat 316L within months (historical failures in Gulf-region FGD plants are well-documented) and make duplex grades and 6Mo austenitic grades the minimum specification.
PRE Comparison
| Grade | Cr% | Mo% | N% | PRE | CPT (6% FeCl₃) |
|---|---|---|---|---|---|
| 316L (1.4404) | 17 | 2.1 | 0.05 | ~24 | ~0°C |
| Duplex 2205 (1.4462) | 22 | 3.1 | 0.17 | ~35 | ~20°C |
| 254 SMO (1.4547) | 20 | 6.1 | 0.20 | ~43 | ~50°C |
| Super Duplex 2507 (1.4410) | 25 | 3.8 | 0.27 | ≥42 | ~45°C |
PRE values for 254 SMO and Super Duplex 2507 are similar (≥42–43). CPT is also close. In FGD slurry service at 55–65°C, both grades are operating at or above their CPT — meaning grade selection alone is not sufficient; crevice-free design and weld quality (G48-tested) are equally important.
Key Differences That Drive Selection
Chloride SCC: 254 SMO, as a fully austenitic grade, is susceptible to chloride SCC at elevated temperature despite its high PRE. FGD chloride concentrations can reach 15,000–20,000 ppm — at 60°C, this is above the practical SCC threshold for austenitic grades. Super Duplex 2507 (mixed microstructure) is effectively immune to chloride SCC. For FGD piping subject to mechanical stress (pump nozzles, vessel connections), 2507 has a meaningful advantage.
Welding: 254 SMO is welded with ERNiCrMo-3 (Inconel 625 filler) — a proven, widely available filler. Super Duplex 2507 requires ER2594 filler with strict interpass temperature control (≤100°C) and ferrite measurement. 254 SMO is significantly easier to weld in the field and in fabrication shops that do not have qualified 2507 WPS.
Acid resistance: In the sulphuric acid component of FGD liquor (H₂SO₄ at low concentration), 254 SMO's higher Mo content (6.1%) provides marginally better reducing-acid resistance than 2507 (3.8% Mo). For zones with pH <4 or H₂SO₄ peaks, 254 SMO or Alloy 825 / C-276 are preferred.
Zone-by-Zone Grade Recommendation for FGD
| Zone | Conditions | Recommended Grade |
|---|---|---|
| Absorber vessel wall | pH 4–6, 55–65°C, slurry | 254 SMO or 2507 — both acceptable |
| Recirculation pump piping | High velocity, mechanical stress | Super Duplex 2507 (SCC immunity) |
| Spray nozzle headers | High velocity, erosion-corrosion | Super Duplex 2507 (high yield strength) |
| Acid pH sump (pH <4) | High H₂SO₄, high chloride | Alloy 825 or Hastelloy C-276 |
| Gas inlet duct | Hot flue gas, acid condensate | 254 SMO or C-276 (high Mo for HCl) |