What Is Flue Gas Desulfurization (FGD)?
Flue gas desulfurization (FGD) is the dominant technology for removing sulfur dioxide (SO2) from the flue gas emissions of coal-fired and heavy fuel oil power plants. Coal contains 0.5-5% sulfur by weight; when burned, sulfur oxidises to SO2, which causes acid rain, respiratory disease, and fine particulate formation. FGD systems capture 90-99% of this SO2 before the flue gas is released to the atmosphere.
The most common FGD configuration is the wet scrubber (wet flue gas desulfurization, WFGD). In a wet scrubber, flue gas is brought into contact with a limestone slurry (primarily calcium carbonate, CaCO3) or seawater in a large absorption tower. The SO2 reacts with the slurry to form gypsum (calcium sulfate dihydrate, CaSO4-2H2O) or other by-products. The cleaned gas is then reheated and released via the stack.
How a Wet FGD Scrubber Works
Step 1 - Flue Gas Entry
Hot flue gas (120-180C) enters the absorber tower from the air preheater. Electrostatic precipitators (ESPs) or bag filters remove fly ash particles first.
Step 2 - Gas Cooling and Quenching
The gas is cooled to 50-80C by recirculating slurry spray. At these temperatures, SO2 solubility in water increases dramatically.
Step 3 - SO2 Absorption
The gas contacts counter-current limestone slurry (pH 5.0-6.0) in the absorber packing. SO2 + H2O -> H2SO3 (sulfurous acid); then H2SO3 + CaCO3 -> CaSO3 + CO2 + H2O. The reaction produces calcium sulfite (CaSO3).
Step 4 - Oxidation and Gypsum Formation
Compressed air is injected into the slurry tank (oxidation air duct). CaSO3 + 1/2 O2 + 2H2O -> CaSO4-2H2O (gypsum). The gypsum slurry is dewatered to produce commercial-grade gypsum (used in drywall and cement).
Step 5 - Cleaned Gas Exit and Stack Release
The cleaned gas passes through a mist eliminator (demister) to remove droplets, is reheated to 80-100C by a gas reheater (GHR), and exits via the stack.
Why FGD Environments Are Highly Corrosive
The FGD slurry inside a wet scrubber is one of the most aggressive multi-factor corrosion environments in modern industry. A single FGD absorber tower may simultaneously experience all of the following corrosion mechanisms:
Table 1: FGD Corrosion Environment - Ten Simultaneous Corrosion Factors and Their Effect on 316L vs C276
| Corrosion Factor | Typical FGD Value | Effect on 316L Stainless Steel | C276 Resistance Mechanism |
Slurry pH (operating)
| 4.5-6.0 (absorber); 0-2 (acid cleaning)
| Pitting and crevice corrosion below pH 4.5
| Mo 15-17% provides resistance to low pH
|
Chloride concentration
| 10,000-80,000 ppm (recycled slurry)
| CSCC above 50 ppm Cl- at 50C+; pitting above 500 ppm
| High Ni (57%) suppresses CSCC; Mo suppresses pitting
|
Dissolved oxygen (DO)
| 2-8 ppm (oxidation air present)
| Accelerates general and pitting corrosion
| Cr 14.5-16.5% forms stable Cr2O3 passive film
|
Temperature
| 50-95C (operating); 120-150C (gas inlet zone)
| Doubles corrosion rate per 10C rise (Arrhenius)
| Stable passive film up to 200C in FGD slurry
|
Sulfuric acid (H2SO4)
| 5-30% in slurry (by-product)
| Aggressive reducing acid; 316L pitting rate >1mm/year in 10% H2SO4
| C276 withstands up to 70% H2SO4 at 80C; 15-17% Mo
|
Hydrofluoric acid (HF)
| Trace (from silicate in coal)
| Even trace HF dissolves SiO2 protective film on 316L; causes rapid pitting
| C276 unaffected by trace HF; Ni base resists acid fluoride
|
Gypsum abrasive particles
| 10-20% solids by weight; 10-100 um particles
| Erosion-corrosion at bends and flow-disturbed areas
| High hardness and toughness resist erosion; Cr2O3 film re-forms after abrasion
|
Thermal cycling
| 50-180C daily shutdown/startup cycles
| Thermal fatigue cracks at welds; oxide film rupture
| Low thermal expansion coefficient; no phase transformation
|
Sulfite ions (SO3 2-)
| 500-2,000 ppm in slurry
| Aggressive reducing agent; promotes underdeposit corrosion
| C276 resists reducing environments due to Mo content
|
Weld HAZ sensitisation
| Weld zones experience 600-850C during welding
| 316L HAZ suffers Cr depletion; becomes anodic to base metal
| Niobium (Nb < 0.35%) in C276 suppresses HAZ sensitisation
|
Hastelloy C276 - Chemistry and Why It Dominates FGD
Hastelloy C276 (UNS N10276) is a nickel-molybdenum-chromium alloy developed by Haynes International (formerly Cabot Corporation). It was introduced in the 1960s specifically for applications where no stainless steel could survive. The "C" in C276 refers to the "C-family" of Hastelloy alloys, which are characterised by high molybdenum (15-17%) and chromium (14.5-16.5%) content, giving them exceptional resistance to both oxidising and reducing corrosive environments.
Table 2: Hastelloy C276 (UNS N10276) - Complete Chemistry per ASTM B575 and Role in FGD Corrosion Resistance
| Element | ASTM B575 Min | ASTM B575 Max | Typical Heat Analysis (%) | Role in FGD Corrosion Resistance |
Nickel (Ni)
| 57.0
| -
| 57.5-59.0
| Base element; suppresses Cl--induced stress corrosion cracking (CSCC); maintains ductility in low pH
|
Molybdenum (Mo)
| 15.0
| 17.0
| 15.5-16.5
| Primary pitting and crevice corrosion resistance; enables resistance to reducing acids (H2SO4, HCl, HF)
|
Chromium (Cr)
| 14.5
| 16.5
| 15.5-16.5
| Forms stable chromium oxide (Cr2O3) passive film; provides oxidation resistance in flue gas atmosphere
|
Iron (Fe)
| 4.0
| 7.0
| 5.0-6.0
| Cost-effective austenite stabiliser; maintains structure
|
Tungsten (W)
| 3.0
| 4.5
| 3.5-4.0
| Secondary pitting resistance (W synergises with Mo); adds to PREN contribution
|
Carbon (C)
| -
| 0.010
| 0.001-0.005
| Ultra-low carbon prevents sensitisation at weld HAZ; maximum 0.01% per ASTM B575
|
Silicon (Si)
| -
| 0.08
| 0.02-0.05
| Must be kept below 0.08% - high Si causes weld embrittlement
|
Manganese (Mn)
| -
| 1.00
| 0.50-0.80
| Deoxidiser; improves hot workability
|
Phosphorus (P)
| -
| 0.040
| <0.015
| Must be minimised - high P promotes HAZ liquation cracking
|
Sulfur (S)
| -
| 0.030
| <0.005
| Must be minimised - high S promotes hot cracking
|
Vanadium (V)
| -
| 0.35
| 0.10-0.30
| Strengthener; improves oxidation resistance
|
Cobalt (Co)
| -
| 2.50
| <0.50
| Minor element; does not affect corrosion resistance significantly
|
PREN Formula and Why C276 Dominates
The Pitting Resistance Equivalent Number (PREN) is the standard industry metric for comparing the pitting and crevice corrosion resistance of stainless steel and nickel alloys. The formula accounts for the contributions of chromium (oxidation resistance), molybdenum (pitting resistance), and nitrogen (pitting resistance and stability):
PREN = %Cr + 3.3 x %Mo + 16 x %N
Table 3: PREN Comparison - Hastelloy C276 vs Stainless Steels and Nickel Alloys in FGD Service
| Material (UNS) | Cr (%) | Mo (%) | N (%) | W (%) | Calculated PREN | FGD Service Life (years) | Recommended for FGD? |
304L (S30403)
| 18-20
| 0
| 0.10
| 0
| 18-20
| 0.5-1.5 (severe pitting)
| NO - not rated
|
316L (S31603)
| 16-18
| 2.0-3.0
| 0.02-0.10
| 0
| 24-30
| 1.5-3.0 (CSCC + pitting)
| NO - fails in FGD
|
904L (N08904)
| 19-23
| 4.0-5.0
| 0.015
| 0
| 35-40
| 3-8 (localised corrosion)
| NO - marginal in FGD
|
254 SMO (S31254)
| 19.5-20.5
| 6.0-7.0
| 0.20-0.25
| 0
| 44-50
| 5-10 (some FGD zones)
| MARGINAL - absorber only
|
254 SMO (high Mo)
| 20-22
| 7.0-8.0
| 0.25
| 0
| 50-56
| 8-15
| MARGINAL - limited service
|
2507 Super Duplex (S32750)
| 24-26
| 3.0-5.0
| 0.24-0.32
| 0
| 40-43
| 8-15 (crevice limited)
| MARGINAL - slurry zones only
|
Alloy 625 (N06625)
| 20-23
| 8-10
| 0.02-0.10
| 0
| 45-52
| 10-20 (some FGD zones)
| PARTIAL - absorber internals only
|
Alloy 59 (N06059)
| 22-24
| 15-16
| 0.01-0.05
| 0
| 61-66
| 15-25
| GOOD - better than 625
|
Hastelloy C276 (N10276)
| 14.5-16.5
| 15-17
| 0.01-0.05
| 3-4.5
| 69-73
| 25-40+ (proven)
| YES - primary FGD material
|
Hastelloy C22 (N06022)
| 20-22.5
| 12.5-14.5
| 0.01-0.05
| 2.5-3.5
| 65-71
| 25-40+ (proven)
| YES - equal to C276
|
FGD System Components and Material Selection
A wet FGD system consists of multiple components, each with different corrosion environments and material requirements. Material selection must be tailored to the specific zone within the scrubber.
Table 4: FGD System Components - Corrosion Environment and Material Selection Matrix
| FGD Zone / Component | Temperature (C) | pH (operating) | Cl- (ppm) | H2SO4 (%) | Material Required | Why Not 316L? |
Absorber tower shell (internal lining)
| 50-80
| 4.5-6.0
| 10,000-60,000
| 5-20
| Hastelloy C276 sheet / C276 weld overlay on carbon steel
| 316L pitting rate >1mm/yr; CSCC in <24 months
|
Absorber tower shell (external)
| 50-80
| N/A (ambient)
| N/A
| N/A
| Carbon steel + rubber lining or C276 clad plate
| External ambient but insulation needed for thermal gradient
|
Oxidation air duct
| 50-85
| 4.0-5.5
| 15,000-50,000
| 3-15
| Hastelloy C276 sheet; C276 weld overlay on CS
| High DO + slurry = severe erosion-corrosion; 316L fails <5 years
|
Inlet flue gas duct
| 120-180
| N/A (gas only)
| <100 (before scrubbing)
| <0.1
| Carbon steel + refractory + C276 weld overlay at bends
| At inlet zone: gas temperature 120-180C, some SO2 present; C276 needed at bends (erosion)
|
Outlet flue gas duct (cleaned gas)
| 70-100
| N/A (gas only)
| <500
| <0.1
| Hastelloy C276 sheet or C276 weld overlay on CS
| High gas velocity carries gypsum particles; erosion-corrosion at bends; also potential for acid condensation during startup
|
Demister (mist eliminator) area
| 55-80
| 5.0-6.0
| 10,000-40,000
| 2-8
| Hastelloy C276 wire mesh elements or 254 SMO
| Saturated gas + droplet carryover = potential acid film on surfaces; 316L would pit at droplet zones
|
Slurry recirculation piping
| 50-80
| 4.5-6.0
| 10,000-60,000
| 5-20
| Hastelloy C276 pipe or C276 weld overlay on CS pipe
| High-velocity slurry with gypsum; 316L erosion rate >0.5mm/yr
|
Gypsum dewatering tank
| 40-60
| 5.5-6.5
| 5,000-20,000
| 1-5
| 316L or 904L acceptable; 316L adequate here
| Lower Cl- and temperature; 316L acceptable if slurry velocity <3 m/s
|
Limestone slurry feed tank
| 30-50
| 6.0-7.0
| 2,000-10,000
| <1
| 316L acceptable
| Mildly acidic; 316L performs adequately below pH 5.5 and below 50C
|
Service water piping (raw water)
| 20-40
| 7.0-8.0
| <500
| <0.1
| 316L or 304L acceptable
| Very low Cl-; 316L/304L are cost-effective choices here
|
Reagent storage tanks (limestone)
| Ambient
| N/A (solid)
| Negligible
| Negligible
| Carbon steel
| Non-corrosive environment; no nickel alloy needed
|
Stack (flue gas stack, lined)
| 70-100 (internal)
| N/A (cleaned gas)
| <500
| <0.1
| Carbon steel with refractory lining; C276 at gas inlet transition
| Cleaned gas low in Cl-; refractory lining adequate; C276 only at inlet transition zone
|
Hastelloy C276 Mechanical and Physical Properties
Table 5: Hastelloy C276 Mechanical and Physical Properties per ASTM B575 and ASME BPVC Section II
| Property | Value (Annealed Sheet) | Value (Solution Treated) | Relevance to FGD Application |
Tensile Strength (UTS)
| 760 MPa (110 ksi) min
| 690-830 MPa typical
| Adequate for absorber vessel design pressure (typically <10 bar operating)
|
Yield Strength (0.2% offset)
| 355 MPa (50 ksi) min
| 310-380 MPa typical
| Design basis for pressure vessel wall thickness calculations
|
Elongation in 50mm
| 30% min (ASTM B575)
| 30-50% typical
| High ductility - absorbs thermal cycling strains without cracking
|
Hardness
| 201 HB max (annealed)
| 180-201 HB typical
| Moderate hardness; not a wear-resistant alloy but sufficient for slurry handling
|
Density
| 8.89 g/cm3 (0.321 lb/in3)
| -
| Heavier than stainless steel (316L = 7.98 g/cm3); structural design must account for 11% higher mass
|
Thermal conductivity
| 9.8 W/m-K at 25C; 15.1 W/m-K at 500C
| -
| Lower than stainless steel - important for thermal gradient design in thick sections
|
Electrical resistivity
| 130 microhm-cm at 25C
| -
| Relevant for electrochemical corrosion monitoring (EPR method)
|
Coefficient of thermal expansion
| 11.2 um/m-C (20-100C); 14.6 um/m-C (20-500C)
| -
| Lower than austenitic stainless steel; less thermal fatigue at weld zones during startup/shutdown
|
Young's modulus
| 205 GPa (29,700 ksi)
| -
| Standard for pressure vessel design; similar to austenitic stainless steel
|
Magnetic permeability
| 1.002 (essentially non-magnetic)
| -
| No magnetic attraction; no issues with magnetic particle inspection (MPI) limitations
|
Charpy V-notch impact (room temp)
| 200-260 J typical (annealed)
| -
| Excellent toughness at all FGD operating temperatures; no brittle fracture risk
|
Charpy V-notch impact (-40C)
| 180-220 J typical (annealed)
| -
| Adequate for winter shutdown conditions in cold climates
|
Maximum design temperature (ASME)
| 540C (1004F) in oxidising environments; 650C intermittent
| -
| Adequate for FGD gas inlet zone (120-180C) with large safety margin
|
Minimum design temperature (ASME)
| -198C (for sheet <25mm)
| -
| Adequate for cold winter shutdown; no risk of LTT transformation
|
Fabrication and Welding of Hastelloy C276 in FGD Systems
Welding Hastelloy C276 in FGD applications requires careful attention to procedure. The high molybdenum content (15-17%) makes C276 susceptible to hot cracking if welding parameters are not properly controlled. However, with the correct filler metal, shielding gas, and heat input range, C276 welds achieve excellent corrosion resistance and mechanical properties.
Table 6: Hastelloy C276 Welding Procedures for FGD System Fabrication
| Parameter | GTAW (TIG) / GTAW-P | GMAW (MIG) | SMAW (Manual Metal Arc) | Shielded Flux-Cored Arc (FCAW) |
Filler metal
| ERNiCrMo-4 (ERNiMo-4)
| ERNiCrMo-4 (1.0-1.6mm wire)
| ENiCrMo-4 (electrode)
| ERNiCrMo-4 or ENiCrMo-4
|
Filler metal composition
| Ni 60%, Mo 17%, Cr 7%, W 4.5%, Fe 5%, C 0.02% max
| Same as GTAW
| Same composition (coated electrode)
| Same as GTAW
|
Shielding gas (TIG)
| 100% Argon (Ar), 10-15 L/min; back purge required
| 75% Ar + 25% He recommended
| N/A (coated electrode)
| 75% Ar + 25% CO2 or 100% CO2
|
Preheat temperature
| NOT required (ambient, 10-40C)
| NOT required
| NOT required
| NOT required
|
Interpass temperature max
| 150C (93C preferred for thick sections)
| 150C
| 150C
| 150C
|
Heat input range
| 0.8-1.5 kJ/mm
| 0.8-1.5 kJ/mm
| Use short arc; moderate heat input
| 0.8-1.5 kJ/mm
|
Electrode angle for SMAW
| N/A
| N/A
| 70-80 degrees (steep angle to avoid porosity)
| N/A
|
Post-weld heat treatment (PWHT)
| NOT required; may reduce corrosion resistance if done incorrectly
| NOT required
| NOT required
| NOT required
|
Interpass cleaning
| Stainless steel wire brush (dedicated, not used on carbon steel); remove oxide before each pass
| Same as GTAW
| Remove slag before each pass; stainless brush between passes
| Remove slag before each pass
|
Typical joint efficiency (RT)
| 100% with qualified WPS (ASME Section IX)
| 100% with qualified WPS
| 100% with qualified WPS
| 100% with qualified WPS
|
Common weld defects to avoid
| Porosity (from moisture/contamination); HAZ fissuring (from high heat input)
| Porosity; spatter (from incorrect voltage)
| Slag inclusions; porosity
| Porosity; excessive spatter
|
Weld overlay thickness (FGD absorber)
| 6-10mm single pass for corrosion overlay
| 6-10mm multi-pass
| 8-12mm (multiple passes)
| 6-10mm multi-pass
|
C276 Weld Overlay on Carbon Steel - FGD Absorber Internals
The most common FGD application for Hastelloy C276 is weld overlay on carbon steel absorber vessel internals. Rather than fabricating the entire absorber from solid C276 sheet (prohibitively expensive), fabricators apply a 6-10mm C276 weld overlay on the internal surfaces of the absorber shell, inlet/outlet ducts, and oxidation air piping. The carbon steel shell provides structural strength; the C276 overlay provides corrosion resistance.
Table 7: Hastelloy C276 Overlay Methods for FGD Absorber Vessel Internals - Process, Thickness, and Application
| Overlay Method | Process | Overlay Thickness | Typical Application in FGD | Advantages | Limitations |
GTAW (TIG) manual overlay
| ERNiCrMo-4 filler, 100% Ar back purge
| 6-10mm (2-4 passes)
| Absorber vessel internals, complex geometry
| Highest quality overlay; lowest dilution; best corrosion resistance
| Slow deposition rate (0.5-1.0 kg/hr); high labour cost
|
GMAW (MIG) semi-auto overlay
| ERNiCrMo-4 wire, 75% Ar + 25% He
| 6-10mm (2-4 passes)
| Absorber shell, straight sections
| 2-3x faster than GTAW; good quality; lower cost
| Higher dilution than GTAW; requires skilled operator
|
FCAW (flux-cored arc) overlay
| ERNiCrMo-4 flux-cored wire, 100% CO2
| 8-12mm (2-5 passes)
| Large flat surfaces; cost-competitive projects
| Highest deposition rate; lower cost per kg deposited
| Higher slag; requires careful cleaning between passes; higher dilution
|
SAW (submerged arc) overlay
| ERNiCrMo-4 + dedicated flux, single-pass
| 6-10mm
| Long straight sections of absorber shell
| Very high deposition rate; excellent quality; low labour
| Only for flat positions; limited to simple geometry
|
Explosive bonding (clad plate)
| Explosion-bonded C276 sheet on carbon steel plate
| 3-5mm C276 on 12-50mm CS
| Absorber shell segments (custom-fabricated in mill)
| No weld HAZ; uniform thickness; fastest installation
| High upfront tooling cost; requires large orders; minimum order quantity (MOQ) ~20 tonnes
|
Explosion weld overlay (EXAM)
| Explosively deposited C276 strips on CS internal surface
| 3-6mm
| Repair of corroded absorber internals; targeted overlay
| Can be applied in-situ for repair; minimal HAZ
| Limited to specific geometries; requires explosion expertise
|
Quality Assurance and Inspection for C276 in FGD Systems
Table 10: Quality Assurance and Inspection Requirements for Hastelloy C276 FGD Components
| QA Step | Method | Standard | Acceptance Criteria | When to Perform |
Material identification (PMI)
| Portable XRF (handheld, e.g. Niton XL3t or Olympus Vanta)
| ASTM B575; EN 10204
| Ni 57%+; Cr 14.5-16.5%; Mo 15-17%; Fe 4-7%; W 3-4.5%; C <0.010%
| On receipt - 100% of sheets and plates
|
Chemical composition (heat analysis)
| Laboratory ICP or OES
| ASTM B575 Section 7
| Within ASTM B575 chemistry limits; heat number on MTR
| Mill test report (MTR) review; confirm before fabrication
|
Tensile and yield strength
| Universal testing machine (UTS, YS, El)
| ASTM B575; ASME SB-575
| UTS 760 MPa min; YS 355 MPa min; El 30% min in 50mm
| Per heat; on mill test report
|
Hardness test
| Brinell hardness (HBW) or Rockwell B
| ASTM B575; NACE MR0175
| 201 HB max (annealed); 100 HB max for sour service
| Per heat; verify annealed condition
|
Visual inspection
| Naked eye + 10x magnifier
| ASTM B575; ASME Section VIII
| No cracks, laps, seams, or injurious imperfections
| On receipt; before welding; after weld overlay
|
Liquid penetrant testing (PT)
| PT (solvent removable or water washable)
| ASTM E165/E165M; ASME Section VIII Div.1
| No relevant indications (ASME Section V Article 25)
| All weld joints (TIG, MIG, overlay); full surface of C276 overlay
|
Radiographic testing (RT)
| X-ray or Ir-192 gamma radiography
| ASTM E94/E1032; ASME Section V Article 2
| No film imperfections > ASME Section V Article 25 Class 3
| Butt welds in pressure-retaining welds; full length
|
Ultrasonic testing (UT)
| Ultrasonic thickness measurement for wall loss
| ASTM E797/E213; ASME Section V Article 5
| No more than 10% thickness reduction from nominal
| Incoming inspection; annual in-service inspection; shutdown inspection
|
Weld overlay thickness verification
| UT thickness gauge (minimum 3 readings per m2)
| Babcock & Wilcox overlay spec; ASME Section VIII
| Minimum 6mm overlay thickness (as specified); no undercut below 6mm
| After overlay completion; before hydrotest
|
Weld metal chemistry verification
| Weld coupon + XRF or laboratory analysis
| AWS A5.14; ASME Section IX
| Weld metal Ni 55-65%; Mo 15-19%; Cr 5-9%; matches ERNiCrMo-4 composition
| Per WPS - one coupon per welder qualification; per production run
|
Corrosion coupon monitoring
| Weight loss coupons (C276 + 316L + 904L coupons installed in slurry)
| NACE TM0169; ASTM G1
| C276 corrosion rate <0.02 mm/year in FGD slurry; 316L coupon as reference
| 6-monthly or annual retrieval and weighing
|
ER (Electrical Resistance) probe monitoring
| Online ER corrosion probes (Corritest or Rohrback)
| NACE TM0190; ASTM G96
| C276 corrosion rate <0.02 mm/year; alarm at 0.05 mm/year
| Continuous online monitoring (installed in absorber)
|
Hydrostatic test
| Hydrostatic pressure test (1.5x design pressure)
| ASME Section VIII Div.1 UG-99; ASME B31.3 (process piping)
| No leaks at test pressure; no visible distortion; hold 30 min minimum
| After fabrication complete; before commissioning
|
Frequently Asked Questions (FAQ)
Q: Why does Hastelloy C276 resist FGD corrosion better than 316L stainless steel?
A: The primary reasons are: (1) Higher molybdenum content: C276 has 15-17% Mo versus 2-3% in 316L. Molybdenum is the most effective element for resisting pitting and crevice corrosion in chloride-containing environments - it raises the pitting potential and suppresses crevice corrosion initiation. (2) Higher nickel content: C276 has 57% Ni versus 10-14% in 316L. High nickel suppresses chloride-induced stress corrosion cracking (CSCC), which is the primary failure mode of 316L in FGD environments above 50 ppm Cl- at temperatures above 50C. (3) Ultra-low carbon (C < 0.010%): Prevents sensitisation at weld HAZ, which would cause intergranular corrosion. 316L (C < 0.030%) is more susceptible to HAZ sensitisation. (4) PREN 69-73 versus 24-30: The calculated PREN confirms C276 has approximately 2.5x the pitting resistance of 316L in chloride environments.
Q: What is the maximum chloride concentration that Hastelloy C276 can handle in FGD?
A: C276 has been successfully used in FGD scrubbers with slurry chloride concentrations up to 100,000 ppm (10% Cl- by weight) at operating temperatures of 95C. This is 1,000x higher than the chloride concentration that causes CSCC in 316L (above 50-100 ppm Cl- at 50C+). The practical limit for C276 in FGD is not the chloride concentration but the combined effect of chloride + low pH + temperature. At pH 0-2 (acid cleaning conditions), the corrosion rate of C276 increases, but it still outperforms 316L by a factor of 10x or more. For reference: typical FGD operating slurry Cl- is 10,000-80,000 ppm, well within C276 capability.
Q: Can Hastelloy C22 be used instead of C276 in FGD?
A: Yes. Hastelloy C22 (UNS N06022, Cr 20-22.5%, Mo 12.5-14.5%, W 2.5-3.5%) has a PREN of approximately 65-71, which is comparable to C276 (PREN 69-73). C22 has slightly better resistance to oxidising acids (e.g., nitric acid, HNO3) and is slightly more resistant to acid chloride environments than C276. For FGD scrubbers specifically, C22 and C276 are both excellent choices. C22 is preferred if the FGD system also handles significant quantities of oxidising acids (e.g., from waste incineration with high chlorine content). C276 is preferred if the application is primarily coal-fired power with high SO2 loading and reducing acid conditions (H2SO4-dominated). C22 costs approximately 10-20% more than C276, so C276 is typically the preferred choice unless specific engineering requirements dictate C22.
Q: Why does 316L stainless steel fail in FGD absorbers within 18-36 months?
A: 316L fails in FGD absorbers primarily due to the combined effect of three simultaneous corrosion mechanisms: (1) Pitting: 316L has PREN 24-30, which is inadequate for chloride concentrations above 500 ppm at temperatures above 50C. Pitting initiates at MnS inclusions or surface discontinuities and propagates rapidly in the FGD slurry environment. (2) Chloride stress corrosion cracking (CSCC): 316L contains 10-14% Ni, which is insufficient to suppress CSCC above 50 ppm Cl- at temperatures above 50C. In FGD slurry (10,000-80,000 ppm Cl-, 50-95C), 316L experiences transgranular CSCC at weld HAZ within 18-36 months. (3) Underdeposit corrosion: Gypsum particles settle on horizontal surfaces and create crevice conditions underneath. The oxygen-depleted zone under the deposit becomes anodic, accelerating localised attack. All three mechanisms act simultaneously in the FGD absorber, causing rapid failure of 316L.
Q: What is the correct welding procedure for C276 weld overlay in FGD absorbers?
A: The correct welding procedure for C276 overlay in FGD absorbers: (1) Process: GTAW (TIG) for highest quality overlay; GMAW (MIG) for large flat areas where cost is critical. (2) Filler metal: ERNiCrMo-4 (AWS A5.14/A5.14M:2020). Verify filler lot chemistry certificate matches ERNiCrMo-4 composition (Ni 60%, Mo 17%, Cr 7%, W 4.5%, Fe 5%, C <0.02%). (3) Shielding gas: 100% Argon, 10-15 L/min, with back purge on the inside of the vessel for weld overlay on internal surfaces. (4) Preheat: None required (ambient temperature, 10-40C). (5) Interpass temperature: Maximum 150C; preferred <93C for sections over 20mm thick. (6) Heat input: 0.8-1.5 kJ/mm. Use stringer beads (not weave) to minimise heat input and dilution. (7) Interpass cleaning: Stainless steel wire brush (dedicated to C276 - never use brushes previously used on carbon steel). (8) Post-weld treatment: None required. Do not PWHT C276 in FGD service - PWHT may reduce corrosion resistance if not carefully controlled.
Q: What thickness of C276 is required for FGD absorber vessel shell?
A: The required C276 thickness depends on the design pressure and structural requirements of the absorber vessel. For most wet FGD absorber towers (design pressure 0.5-3 bar), the structural shell is carbon steel (typically 12-30mm depending on vessel size), with a C276 weld overlay (6-10mm) on the internal surface. For C276 sheet fabrication (not overlay): minimum 3mm for absorber internals (baffles, support plates); 4-6mm for absorber shell panels where C276 is the structural material; 6-12mm for oxidation air duct. Verify all thickness calculations against ASME Section VIII Div.1 or EN 13445 design codes. Always add a corrosion allowance of minimum 3mm for FGD slurry service on the construction drawing. For thick sections (>20mm), specify solution treatment after welding to ensure uniform corrosion resistance in the weld HAZ.
Q: How does the FGD slurry pH affect Hastelloy C276 corrosion rate?
A: C276 corrosion rate in FGD slurry is relatively stable across the normal operating pH range (4.5-6.0). At pH 5.5-6.0 (absorber upper zone), C276 corrosion rate is <0.01 mm/year - essentially negligible. At pH 4.0-5.0 (absorber mid zone, oxidation air injection zone), C276 corrosion rate is 0.01-0.05 mm/year - still negligible for design life calculations. At pH 2.0-4.0 (absorber lower zone, slurry tank), C276 corrosion rate increases to 0.05-0.15 mm/year - still acceptable for design life. At pH 0-2 (acid cleaning, quarterly or annual cleaning cycles using citric acid or formic acid), C276 corrosion rate rises to 0.2-0.5 mm/year but remains acceptable for the short cleaning duration (4-8 hours). For comparison: 316L at pH 4.0-5.0 in FGD slurry has a corrosion rate of 0.5-2.0 mm/year - 10-40x higher than C276. The key advantage of C276 is that it maintains low corrosion rates across the full pH range encountered in FGD, while 316L fails rapidly at pH below 5.0.
Q: What is the difference between Hastelloy C276 and Inconel 625 for FGD?
A: Both C276 and Inconel 625 are nickel-based alloys suitable for FGD, but they have different optimisation targets: (1) Chemistry: C276 (UNS N10276) has Ni 57%, Cr 14.5-16.5%, Mo 15-17%, W 3-4.5%; Inconel 625 (UNS N06625) has Ni 58%+, Cr 20-23%, Mo 8-10%, Nb 3.15-4.15%. C276 has 7-9% more Mo and adds W (tungsten) - both of which significantly improve pitting and crevice corrosion resistance in chloride environments. (2) PREN: C276 = 69-73; Inconel 625 = 45-52. C276 has 40-50% higher PREN. (3) FGD performance: C276 is the preferred material for all FGD absorber zones, especially the high-chloride lower absorber and oxidation air duct. Inconel 625 is acceptable for upper absorber zones (lower Cl-, pH 5-6) but is marginal for lower absorber zones and oxidation air ducts where chloride concentrations are highest. (4) Cost: C276 costs approximately 1.5-2.0x Inconel 625. The additional cost is justified for absorber vessels and oxidation air ducts where C276 provides significantly better long-term corrosion protection. For upper absorber zones with lower chloride, Inconel 625 is an acceptable cost optimisation.
