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Incoloy is a nickel-iron-based high-temperature alloy designed for use in highly corrosive environments and extreme temperatures. These alloys combine excellent oxidation and corrosion resistance with high mechanical strength, especially at high temperatures. Unlike Hastelloy, Incoloy contains a large amount of iron and is relatively low in cost. Common incoloy include alloy 800 and alloy 825.
Incoloy 825 (UNS N08825) is an austenitic nickel-iron-chromium alloy. Alloy 825 is similar to Alloy 800, but its resistance to aqueous corrosion is improved. Alloy 825 has good corrosion resistance and can withstand a variety of corrosive media such as sulfuric acid, nitric acid and
sodium hydroxide in the same equipment. It also has good formability and weldability, suitable for most processing.
Grade | C≤ | Mn≤ | Si≤ | S≤ | Cu | Fe≥ | Ni | Cr | Al≤ | Ti | Mo |
Incoloy 825 | 0.05 | 1.0 | 0.5 | 0.03 | 1.5-3.0 | 22 | 38.0-46.0 | 19.5-23.5 | 0.2 | 0.6-1.2 | 2.5-3.5 |
Grade | Tensile, min, ksi[MPa] | Yield, min, ksi[MPa] | Elongation, %(min) |
Incoloy 825 | 85[586] | 35[241] | 30% |
Properties | Value |
Density | 8.1 g/cm3 |
Melting point | 1370 - 1400℃ |
Thermal conductivity (100℃) | 10.8 λ/(W/m*℃) |
Specific heat | 440 J/kg*℃ |
Elastic modulus | 195 GPa |
Shear modulus | - |
Resistivity | 1.37 μΩ*m |
Poisson's ratio | - |
Linear expansion coefficient (20-100℃) | 14.1 a/10-6 ℃-1 |
STANDARD | WERKSTOFFNR. | UNS | JIS | BS | GOST | AFNOR | EN | OR |
Incoloy 825 | 2.4858 | N08825 | NCF 825 | NA 16 | ЭП703 | NFE30C20DUM | NiCr21Mo | XH38BT |
Product | Standard | Show |
Pipe SMLS | ASTM B423 |  |
Tube SMLS | ASTM B423 |  |
Sheet/Plate/Strip | ASTM B424 |  |
Bar /Rod | ASTM A425 |  |
Forging /Flange | ASTM B564 |  |
Fittings | ASTM B366 |  |
Pipe Welded | ASTM B474 |  |
Incoloy 825 is one of the most widely specified nickel alloys for sour oil and gas production worldwide. Its approval under NACE MR0175 / ISO 15156 for H₂S-containing environments makes it a proven choice for downhole tubing, flowlines, and surface facilities where sulfide stress cracking (SSC) and chloride-induced corrosion are primary threats.
Why Sour Gas Wells Demand Incoloy 825
"Sour gas" refers to natural gas or reservoir fluids containing hydrogen sulfide (H₂S). When H₂S coexists with CO₂, chlorides, water, and elevated pressure/temperature, the environment creates multiple simultaneous attack vectors:
| Corrosion Mechanism | Driver | Incoloy 825 Defense |
|---|---|---|
| Sulfide Stress Cracking (SSC) | H₂S + tensile stress → brittle fracture | High Ni (38–46%) prevents hydrogen embrittlement; NACE MR0175 qualification |
| Chloride Stress Corrosion Cracking (Cl-SCC) | Cl⁻ + elevated temp > 60°C | Ni content above the SCC threshold (~20%); Ti stabilization |
| Pitting Corrosion | Cl⁻ + H₂S accelerates pit initiation | Mo (2.5–3.5%) raises PREN to ~31.5 |
| CO₂ Sweet Corrosion | CO₂ + water → carbonic acid | Cr (19.5–23.5%) forms stable passive film |
| Crevice Corrosion | Deposit-induced localized attack under gaskets/couplings | Mo + Cr synergistic protection in confined geometries |
Incoloy 825 is qualified under NACE MR0175 / ISO 15156 Part 3 for sour service, subject to the following requirements:
| Requirement | Specification |
|---|---|
| Heat Treatment Condition | Solution-annealed only; cold-worked material without subsequent annealing is NOT permitted for sour service |
| Maximum Hardness | ≤ 35 HRC (typically ≤ 190 HB) — verified per heat/lot |
| Chemical Composition | Must meet UNS N08825 limits; Ni, Cr, Mo verified on MTR |
| Traceability | Full traceability from melt to finished product required on Mill Test Report |
| Environmental Limits | Acceptable for H₂S partial pressures per ISO 15156-3 Table A.4; temperature range per application |
Typical Sour Gas Well Applications
| Application | Component | Service Conditions |
|---|---|---|
| Downhole Tubing | 2⅞"–4½" seamless tubing (ASTM B423) | H₂S partial pressure > 0.05 bar; CO₂ up to 35 mol%; chlorides > 50,000 ppm; BHT up to 230°C |
| Subsea Flowlines | 6⅝" OD flowline jumpers & pipelines | Deepwater (1,800 m+); multiphase sour fluids; external hydrostatic pressure |
| Surface Facilities | Heat exchangers, manifolds, separators | Wet sour gas; produced water with high chlorides; erosive multiphase flow |
| Wellhead Components | Christmas tree parts, valve bodies | High-pressure sour gas; cyclic loading |
| Acidizing Equipment | Coiled tubing, acid distribution manifolds | HCl / HF acid exposure during stimulation |
| Control Lines | Hydraulic & chemical injection lines | High-pressure corrosive chemicals in subsea environment |
Both Incoloy 825 and Hastelloy C-276 are NACE MR0175-compliant nickel alloys used in sour service.
Incoloy 825 covers ~80% of sour gas well applications at roughly half the cost of C276. Hastelloy C276 is the choice for the remaining ~20% — extreme chloride/H₂S conditions, strong reducing acids, or where maximum pitting resistance (PREN ~52.8 vs ~31.5) is non-negotiable.
Chemical Composition Comparison
| Element | Incoloy 825 (N08825) | Hastelloy C-276 (N10276) | Impact on Performance |
|---|---|---|---|
| Nickel (Ni) | 38.0–46.0% | ≥ 57% (balance) | C276 higher Ni → better reducing acid resistance |
| Iron (Fe) | ≥ 22% (balance) | 4–7% | 825 higher Fe → lower cost; risk of σ-phase > 650°C |
| Chromium (Cr) | 19.5–23.5% | 14.5–16.5% | 825 higher Cr → better oxidizing acid resistance |
| Molybdenum (Mo) | 2.5–3.5% | 15.0–17.0% | The key gap — C276 Mo ≈5× higher → far superior pitting/crevice resistance |
| Copper (Cu) | 1.5–3.0% | ≤ 0.5% | 825 exclusive → resistance to H₂SO₄/H₃PO₄ |
| Titanium (Ti) | 0.6–1.2% | Not significant | 825 exclusive → weld stabilization |
| Tungsten (W) | — | 3.0–4.5% | C276 exclusive → synergistic crevice corrosion resistance |
| Carbon (C) | ≤ 0.05% | ≤ 0.01% | C276 ultra-low C → no sensitization risk even without Ti |
Pitting Resistance & Key Metrics
| Property | Incoloy 825 | Hastelloy C-276 | Comparison |
|---|---|---|---|
| PREN (standard) | ~31.5 | ~52.8 | C276 +68% |
| PREN_W (with tungsten) | ~31.5 | ~74.5 | C276 +137% |
| Yield Strength (annealed) | ~241 MPa | 310–355 MPa | C276 +30–47% |
| Tensile Strength | ~550–586 MPa | ~690 MPa | C276 +15–25% |
| Density | 8.14 g/cm³ | 8.89 g/cm³ | 825 ~9% lighter |
| Max Service Temp (structural) | ~540°C | ~600°C (oxidizing) | C276 higher limit |
| Relative Cost | 1× baseline | 1.5–2× baseline | 825 significantly cheaper |
| NACE MR0175 | ✅ Qualified | ✅ Qualified | Both compliant |
Corrosion Resistance by Environment
| Environment / Media | Incoloy 825 | Hastelloy C-276 | Selection Guidance |
|---|---|---|---|
| Seawater (ambient) | Good | Excellent | C276 pitting resistance far superior; 825 adequate for calm seawater |
| Sour gas (moderate H₂S, <0.5 bar) | Good | Excellent | 825 sufficient for most moderate sour wells — cost advantage is decisive |
| Sour gas (high H₂S, >1 bar) | Adequate | Superior | C276 recommended for extreme sour conditions |
| Dilute sulfuric acid (<20%, <80°C) | Very Good | Good | 825's Cu gives it the edge in dilute H₂SO₄ |
| Hydrochloric acid (any concentration) | Limited | Excellent | C276's high Mo dominates in HCl — 825 not suitable |
| Phosphoric acid (process grade) | Very Good | Good | 825 preferred due to Cu addition |
| Chloride SCC (hot, >60°C) | Moderate | Excellent | C276 virtually immune; 825 adequate above Ni SCC threshold |
| FGD scrubber environments | Good | Excellent | 825 widely used; C276 for most aggressive scrubber zones |
Choose Incoloy 825 when:
Moderate sour conditions (pH₂S < 0.5 bar, chlorides < 50,000 ppm, T < 150°C)
Equipment handling sulfuric or phosphoric acid at moderate concentrations
Seawater heat exchangers in calm conditions
Cost is a significant factor — 825 delivers ~80% of C276's capability at ~50–60% of the price
Welded fabrication is required — Ti stabilization eliminates post-weld heat treatment
Choose Hastelloy C276 when:
Extreme sour service (pH₂S > 1 bar, chlorides > 80,000 ppm, T approaching 230°C)
Hydrochloric acid exposure of any concentration
Strong reducing environments where maximum pitting/crevice resistance is non-negotiable
Critical wellhead or subsea components where failure cost far exceeds material premium
Wet chlorine, hypochlorite, or strong oxidizer/chloride combinations
Heating tubes, containers, and chains for sulfuric acid pickling plants.
Seawater cooling heat exchangers, marine product piping systems.
Sour gas environment piping: heat exchangers, evaporators, scrubbers, dip tubes, etc. In.
Air heat exchangers in petroleum refining.
Food engineering chemical process.
Flame Retardant Alloys for High Pressure Oxygen Applications.
What is Incoloy 825 and what is it used for?
Incoloy 825 (UNS N08825, W.Nr. 2.4858) is a nickel-iron-chromium alloy with additions of molybdenum (2.5–3.5%), copper (1.5–3.0%), and titanium (0.6–1.2%). It is primarily used in sour gas well tubing and piping, sulfuric acid processing equipment, seawater heat exchangers, and pollution control systems. Its titanium stabilization prevents intergranular corrosion after welding, and its copper addition enhances resistance to reducing acids.
Is Incoloy 825 compliant with NACE MR0175 for sour service?
Yes. Incoloy 825 (UNS N08825) is an approved material under NACE MR0175 / ISO 15156 for sour service containing H₂S. The material must be supplied in the solution-annealed condition with hardness ≤ 35 HRC (typically ≤ 190 HB). Cold-worked material without subsequent annealing is not permitted for sour service applications. All Mill Test Reports must reference NACE MR0175 compliance with full heat-to-product traceability.
Incoloy 825 vs Hastelloy C276 — which is better for sour gas wells?
For moderate sour conditions (H₂S partial pressure < 0.5 bar, chlorides < 50,000 ppm, temperature < 150°C), Incoloy 825 provides sufficient corrosion resistance at roughly 50–60% of C276's cost. For extreme sour service — high H₂S partial pressures (> 1 bar), very high chloride content (> 80,000 ppm), or temperatures approaching 230°C — Hastelloy C276's PREN of ~52.8 (vs 825's ~31.5) makes it the superior choice. Many operators use 825 for downhole tubing and C276 for critical wellhead components as a balanced cost/performance strategy.
What is the PREN (Pitting Resistance Equivalent Number) of Incoloy 825?
The PREN of Incoloy 825 is approximately 31.5, calculated as Cr% + 3.3×Mo% + 16×N% = 21.5 + 3.3×3.0 + 16×0 ≈ 31.5. This is significantly higher than 316L stainless steel (PREN ~24) but lower than Hastelloy C276 (PREN ~52.8). PREN above 30 generally indicates good resistance to pitting in seawater and moderate chloride environments.
Can Incoloy 825 be welded?
Yes, Incoloy 825 has excellent weldability thanks to its titanium stabilization (0.6–1.2% Ti), which prevents sensitization and intergranular corrosion in the heat-affected zone — no post-weld heat treatment is required. Recommended filler metals:
ERNiFeCr-1 (AWS A5.14, INCOLOY 65) — matching composition filler for most applications
ERNiCr-3 (AWS A5.14, INCONEL 82) — alternative for non-critical applications
ERNiCrMo-4 (AWS A5.14, Hastelloy C-276 filler) — for maximum corrosion resistance in severe environments
Heat input should be limited to 0.5–1.5 kJ/mm with interpass temperature below 150°C. Use 100% argon shielding for GTAW.
What product forms of Incoloy 825 are available under ASTM standards?
Incoloy 825 is available across all major product forms:
Seamless pipe & tube — ASTM B423 / ASME SB-423
Sheet, plate & strip — ASTM B424 / ASME SB-424
Bar & rod — ASTM B425 / ASME SB-425
Forging & flange — ASTM B564 / ASME SB-564
Welded pipe — ASTM B474 / ASME SB-474
Fittings — ASTM B366 / ASME SB-366
All forms are covered by both ASTM and ASME (SB-series) standards for pressure vessel and piping code compliance under ASME VIII and B31.3.
What temperature range is Incoloy 825 suitable for?
Incoloy 825 is suitable for service temperatures from cryogenic conditions up to approximately 540°C (1000°F) for structural applications. For sour gas service, it performs reliably up to about 230°C (450°F) bottom-hole temperature. Above 650°C, there is a risk of σ-phase precipitation that can degrade both corrosion resistance and mechanical properties — so 825 is not recommended for prolonged high-temperature service above this limit.
Why does Incoloy 825 contain copper?
The copper addition (1.5–3.0%) in Incoloy 825 is intentionally included to enhance resistance to reducing acids, particularly sulfuric acid and phosphoric acid at moderate concentrations and temperatures. Copper promotes the formation of a more stable passive film in reducing environments where chromium-only protection is insufficient. This makes 825 uniquely effective in acid processing equipment that handles both oxidizing and reducing media — a capability that Hastelloy C276 (which contains almost no copper) does not match.
