Alloy System
Ni-Fe-Cr-Mo-Cu
Strengthening
Solid Solution + Ti-Stabilized
What is Incoloy 825? Incoloy 825 (UNS N08825 / W.Nr. 2.4858) is a nickel-iron-chromium alloy with deliberate additions of molybdenum (2.5–3.5%), copper (1.5–3.0%), and titanium (0.6–1.2%) for superior resistance to reducing acids and chloride stress-corrosion cracking. The high nickel content (38–46%) provides chloride SCC resistance, the chromium (19.5–23.5%) resists oxidizing environments, the molybdenum resists pitting and crevice corrosion, and the copper provides sulfuric acid passivation. This multi-element chemistry makes 825 a versatile all-around alloy for chemical processing, oil & gas, marine, and nuclear applications handling sulfuric, phosphoric, hydrochloric, and organic acids. Titanium stabilization prevents sensitization in the as-welded condition. Incoloy 825 is qualified for pressure vessel service to 540°C and is widely used in sulfuric acid pickling equipment, phosphoric acid evaporators, oilfield sour service, and seawater-cooled heat exchangers. HT PIPE supplies Incoloy 825 in seamless pipes, plates, welded pipes, forged fittings, flanges, and round bars with full EN 10204 3.1 certification.
Nickel (Ni) ★
38.0 – 46.0%
SCC resistance + austenitic matrix
Chromium (Cr)
19.5 – 23.5%
Oxidizing acid resistance
Molybdenum (Mo) ★
2.5 – 3.5%
Pitting & crevice resistance
Copper (Cu) ★
1.5 – 3.0%
H₂SO₄ passivation
Tensile Strength (min)
≥590 MPa
86 ksi
Yield Strength (min)
≥240 MPa
35 ksi
Elongation (min)
≥30%
Highly ductile
Density
8.14 g/cm³
0.294 lb/in³
Melting Range
1370–1400°C
2500–2550°F
Incoloy 825: Multi-Element Chemistry for Multi-Environment Corrosion
Incoloy 825 (UNS N08825) was developed as a general-purpose corrosion-resistant alloy capable of handling both oxidizing and reducing environments in a single material. While the Incoloy 800 family is optimized for high-temperature structural and oxidation resistance, 825 is optimized for aqueous corrosion resistance across a broad range of chemical process streams.
The alloy’s chemistry reflects this purpose. The high nickel content (38–46%) provides chloride stress-corrosion cracking resistance superior to 300-series stainless steels and raises the threshold for chloride pitting. Chromium (19.5–23.5%) provides oxidizing acid resistance. Molybdenum (2.5–3.5%) provides pitting and crevice corrosion resistance and contributes to reducing acid performance. Copper (1.5–3.0%) provides self-passivation in sulfuric acid through the same electrochemical mechanism used in Hastelloy C-2000. Titanium (0.6–1.2%) stabilizes against sensitization.
This multi-element approach makes 825 a versatile workhorse for chemical plants handling diverse process streams. It is qualified for pressure vessel service to 540°C per ASME Code and is widely specified for sulfuric acid pickling equipment, phosphoric acid evaporators, oilfield sour service (NACE MR0175), seawater-cooled heat exchangers, nuclear fuel reprocessing, and pollution control equipment. For pure reducing acid service where 825’s performance is insufficient, Hastelloy B-3 is the upgrade path. For pure oxidizing acid service, Hastelloy C-22 provides superior performance.
Five Defining Advantages of Incoloy 825
1. Multi-Environment Versatility
Handles oxidizing acids, reducing acids, chlorides, and alkaline solutions in a single material. Reduces inventory complexity.
2. Sulfuric Acid Passivation
Cu addition (1.5–3.0%) self-passivates in H₂SO₄. Resists dilute to 60% concentrations at moderate temperatures.
3. Chloride SCC Resistance
38–46% Ni provides superior chloride SCC resistance. Suitable for seawater, brine, and chloride-contaminated service.
4. Pitting & Crevice Resistance
Mo addition (2.5–3.5%) raises PREN to ~32. Resists pitting in chloride environments better than 316L.
5. Ti Stabilization
0.6–1.2% Ti prevents sensitization. As-welded corrosion resistance retained. Polythionic-acid resistant for refinery service.
Chemical Composition — ASTM B424 UNS N08825
825’s defining feature is the deliberate combination of four corrosion-fighting elements: Ni (SCC), Cr (oxidizing), Mo (pitting), and Cu (H₂SO₄). Each element plays a specific role, and their combined effect delivers broader corrosion resistance than any single-element approach:
| Element |
Min (%) |
Max (%) |
Role |
| Nickel (Ni) ★ |
38.0 |
46.0 |
Austenitic matrix stabilizer. Primary chloride SCC resistance driver. Higher than 800 family (30–35%). |
| Chromium (Cr) |
19.5 |
23.5 |
Forms protective Cr₂O₃ film. Oxidizing acid resistance (HNO₃). |
| Iron (Fe) |
22.0 |
Balance |
Cost-effective balance element. |
| Molybdenum (Mo) ★ |
2.5 |
3.5 |
Pitting and crevice corrosion resistance. Reducing acid protection. Raises PREN. |
| Copper (Cu) ★ |
1.5 |
3.0 |
Self-passivation in H₂SO₄ through cathodic H₂ evolution. Same mechanism as Hastelloy C-2000. |
| Titanium (Ti) |
0.60 |
1.20 |
Stabilizer. Forms TiC preferentially over Cr₂₃C₆. Prevents sensitization in as-welded condition. |
| Carbon (C) |
— |
0.05 |
Ultra-low. Minimizes carbide precipitation in HAZ. |
| Manganese (Mn) |
— |
1.00 |
Deoxidizer; improves hot workability. |
| Silicon (Si) |
— |
0.50 |
Deoxidizer. Controlled low for corrosion resistance. |
| Aluminum (Al) |
— |
0.20 |
Residual. Deoxidizer. |
| Sulfur (S) |
— |
0.015 |
Ultra-low for hot workability. |
★ The four corrosion-fighting elements (Ni, Cr, Mo, Cu) define 825’s multi-environment versatility.
PREN (Pitting Resistance Equivalent Number): PREN = %Cr + 3.3 × %Mo = 21 + 3.3 × 3 ≈
31 (nominal). This is significantly higher than 316L (~25) and 304 (~18), reflecting superior pitting resistance. For applications requiring higher pitting resistance (PREN > 40), upgrade to a Ni-Cr-Mo alloy such as
Hastelloy C-276 (PREN ~66) or
C-22 (PREN ~65).
Mechanical Properties — Room Temperature (Annealed)
825 is a solid-solution strengthened alloy. The combination of Ni, Cr, Mo, and Cu in the austenitic matrix provides good room-temperature strength with excellent ductility. Properties are stable after solution annealing and do not degrade with moderate temperature exposure.
| Condition |
UTS (MPa) |
YS (MPa) |
Elongation (%) |
Hardness |
| Annealed (min, ASTM) |
≥590 |
≥240 |
≥30 |
≤200 HB |
| Typical (20°C) |
~690 |
~325 |
~40 |
~165 HB |
Elastic Constants (Annealed at 20°C)
| Elastic Constant |
Value |
Unit |
| Modulus of Elasticity (E) |
196 |
GPa (28.4 × 10³ ksi) |
| Shear Modulus (G) |
75 |
GPa (10.9 × 10³ ksi) |
| Poisson’s Ratio (ν) |
0.31 |
— |
Physical Properties — UNS N08825
825’s density of 8.14 g/cm³ is higher than the 800 family (7.94) due to the heavier Mo and Cu additions. The alloy is non-magnetic in all conditions.
| Property |
Value |
Unit |
| Density |
8.14 |
g/cm³ (0.294 lb/in³) |
| Melting Range |
1370 – 1400 |
°C (2500 – 2550 °F) |
| Thermal Conductivity (at 20°C) |
10.8 |
W/m·K |
| Specific Heat (at 20°C) |
440 |
J/kg·K |
| Electrical Resistivity (at 20°C) |
1.13 |
μΩ·m |
| Coeff. of Thermal Expansion (20–100°C) |
13.7 |
μm/m·°C |
| Magnetic Permeability |
< 1.001 |
Non-magnetic |
Heat Treatment Procedures
825 is supplied in the solution-annealed condition. The annealing temperature is lower than the 800 family because 825 does not require coarse grain for high-temperature service. Stress relief is performed at low temperature to avoid carbide precipitation.
Step 1 — Solution Anneal
920 – 980 °C + Rapid WQ/AC
Hold ~1 hr per 25 mm section thickness. Water quench or rapid air cool. Dissolves carbides and restores full corrosion resistance after cold work or welding.
Step 2 — Stress Relief (optional)
400 – 450 °C, 1–2 hr, air cool
For machined components requiring dimensional stability. Below carbide precipitation range — does not affect corrosion resistance.
Step 3 — Post-Weld
As-Welded Acceptable
Ti stabilization prevents sensitization. As-welded corrosion resistance is acceptable for most service. Solution anneal recommended for severe environments or thick sections.
CRITICAL NOTES:
Avoid dwelling at 540–760°C for extended periods. This is the carbide precipitation range. Prolonged exposure can cause sensitization and intergranular corrosion. For service above 540°C, choose Incoloy 800H/800HT instead.
Do not exceed 980°C during solution annealing. Excessive temperature causes grain growth and may degrade corrosion resistance.
Cold work >10% requires re-annealing before service to restore full corrosion resistance.
Corrosion Resistance — Multi-Environment Performance
825’s defining advantage is multi-environment versatility. The combination of Ni (SCC), Cr (oxidizing), Mo (pitting/reducing), and Cu (H₂SO₄ passivation) creates a corrosion resistance envelope that spans both oxidizing and reducing media, with particular strength in sulfuric, phosphoric, and organic acid service.
Sulfuric Acid (H₂SO₄) ★ — DEFINING STRENGTH
Excellent resistance across a wide concentration range. The Cu addition (1.5–3.0%) provides self-passivation in intermediate concentrations through cathodic H₂ evolution, shifting the corrosion potential into the passive range where a protective Cr₂O₃ film forms. At room temperature, 825 resists H₂SO₄ up to 60% concentration. At 80°C, resistance extends to ~50%. At boiling, resistance is limited to dilute (<10%) and concentrated (>90%) ranges. 825 is the standard material for sulfuric acid pickling equipment, dilution systems, and heat exchangers.
Phosphoric Acid (H₃PO₄) ★
Excellent resistance to wet-process phosphoric acid at all concentrations and temperatures up to 85°C. Widely used in phosphoric acid evaporators, heat exchangers, and piping in fertilizer plants. The Mo content provides resistance to fluoride and chloride contamination common in wet-process acid.
Hydrochloric Acid (HCl)
Good resistance at room temperature to dilute concentrations (<10%). Performance degrades rapidly at elevated temperatures. For pure HCl service at high temperature, Hastelloy B-3 is the correct choice.
Nitric Acid (HNO₃)
Good resistance at all concentrations up to boiling. The 19.5–23.5% Cr provides oxidizing acid protection. However, for pure HNO₃ service where maximum resistance is required, 304L stainless steel is actually preferred (lower cost, no Cu dilution of Cr effect).
Chloride Stress-Corrosion Cracking (SCC) ★
Superior to 304/316 stainless steels. The 38–46% Ni content raises the chloride SCC threshold significantly. 825 is widely used in seawater-cooled heat exchangers, marine piping systems, and oilfield sour service where chloride SCC is a primary failure mode for stainless steels.
Pitting & Crevice Corrosion
Good pitting resistance with PREN ≈ 31 (nominal). The Mo addition (2.5–3.5%) raises the Critical Pitting Temperature (CPT) to ~50°C in ASTM G48 Method C — significantly better than 316L (~30°C). Suitable for seawater and brine service at moderate temperatures. For higher pitting resistance, upgrade to a Ni-Cr-Mo alloy.
Intergranular Corrosion (IGC)
Ti stabilization (0.6–1.2%) prevents sensitization in the as-welded condition. The Ti/C ratio > 12:1 ensures carbon is bound as TiC rather than forming Cr₂₃C₆ at grain boundaries. Passes ASTM A262 Practice A and E in solution-annealed condition. Polythionic-acid resistant for refinery service.
Corrosion Rate Comparison — 825 vs 316L vs 800 (mm/yr)
| Environment |
Incoloy 825 |
316L SS |
Incoloy 800 |
| 50% H₂SO₄ at 60°C |
0.05 |
2.5 |
1.8 |
| 20% HCl at 25°C |
0.15 |
1.5 |
2.0 |
| 85% H₃PO₄ at 85°C |
0.1 |
1.2 |
0.5 |
| Chloride SCC (boiling MgCl₂) |
Resistant |
Cracks |
Resistant |
| G48 Pitting CPT |
~50°C |
~25°C |
~35°C |
Green = excellent; Red = unacceptable; Yellow = moderate. For severe reducing acid service, upgrade to Hastelloy B-3 or C-2000.
Incoloy 825 vs Other Corrosion-Resistant Alloys
825 is positioned as a versatile mid-tier corrosion alloy. It outperforms 316L and 904L stainless across most environments at lower cost than Hastelloy grades. For the most severe service, upgrade to a Ni-Cr-Mo alloy:
| Alloy |
H₂SO₄ Resistance |
Chloride SCC |
Pitting (PREN) |
Max Temp |
Relative Cost |
| 316L SS |
★ |
★ |
~25 |
425°C |
$ |
| Incoloy 825 |
★★★★ |
★★★★ |
~31 |
540°C |
$$$ |
| Hastelloy C-276 |
★★★★★ |
★★★★★ |
~66 |
1040°C |
$$$$$ |
| Selection Guide |
825 strong |
825 strong |
Upgrade for >50°C |
Use 800H above 540°C |
825 mid-tier |
Incoloy 825 Positioning: "The Versatile Mid-Tier Corrosion Alloy"
Choose 825 for chemical process equipment handling sulfuric, phosphoric, organic acids, and chloride-containing media up to 540°C. It is the cost-effective choice when 316L is insufficient but Hastelloy grades are over-specified. For pure reducing acid service (HCl), upgrade to
Hastelloy B-3. For the broadest corrosion resistance, choose
Hastelloy C-276 or
C-2000. For high-temperature structural service, choose
Incoloy 800.
Key Applications
825 is the workhorse alloy for chemical processing, oil & gas, marine, and nuclear applications handling diverse corrosive media:
| Application |
Components & Conditions |
| Sulfuric Acid Pickling ★ |
Heating coils, pickling tanks, baskets, chains, and pickling line components for steel and titanium pickling in 20–60% H₂SO₄ at 60–90°C. |
| Phosphoric Acid Production ★ |
Heat exchangers, evaporators, washers, and immersion tubes for wet-process phosphoric acid in fertilizer plants. Resists fluoride/chloride contamination. |
| Oil & Gas Sour Service |
Downhole tubing, wellhead components, gas lift piping, and process equipment in H₂S-containing service. NACE MR0175 qualified. |
| Seawater Heat Exchangers |
Coolers, condensers, and piping systems using seawater as coolant. Chloride SCC and pitting resistance essential. |
| Nuclear Fuel Reprocessing |
Dissolvers, evaporators, and process piping handling HNO₃/H₂SO₄/NaOH mixtures. Multi-environment capability essential. |
| Pollution Control |
Flue gas desulfurization (FGD) components, scrubbers, and wastewater treatment systems handling acidic chloride media. |
Applicable Standards & Specifications
| Standard |
Product Form / Scope |
Notes |
| ASTM B424 / ASME SB-424 |
Plate, Sheet, Strip — UNS N08825 |
Primary flat-product spec. |
| ASTM B423 / ASME SB-423 |
Seamless Pipe & Tube — UNS N08825 |
Seamless pipe primary spec |
| ASTM B163 / ASME SB-163 |
Seamless Condenser & Heat-Exchanger Tubes |
Heat exchanger tube spec |
| ASTM B425 / ASME SB-425 |
Rod, Bar — UNS N08825 |
Round bar and wire products |
| ASTM B704 / B705 |
Welded Pipe & Tube — UNS N08825 |
Welded pipe/tube products |
| ASTM B366 / ASME SB-366 |
Wrought Welded Fittings |
Butt-weld and socket-weld fittings |
| ASTM B564 / ASME SB-564 |
Nickel Alloy Forgings |
Flanges, fittings, pressure parts |
| NACE MR0175 / ISO 15156 |
Sour Service — H₂S environments |
Oil & gas sour service qualification |
| DIN W.Nr. 2.4858 |
NiCr21Mo — European Werkstoff equivalent |
European project specification |
Available Product Forms from HT PIPE
We supply Incoloy 825 in solution-annealed condition with EN 10204 3.1 certification on every shipment. ASTM A262 intergranular corrosion testing per heat is available on specification. NACE MR0175 qualification available for oil & gas sour service.
| Product Form |
Specification & Range |
| Seamless Pipes & Tubes |
½″–12″ NB, Sch 10S–Sch 80S. ASTM B423 UNS N08825. Solution-annealed + WQ. |
| Plates & Sheets |
0.5–50 mm thickness. ASTM B424 UNS N08825. Solution-annealed. Custom cut-to-size available. |
| Welded Pipes |
ASTM B704/B705 UNS N08825. Full range of sizes. As-welded or solution-annealed per requirement. |
| Forged Fittings |
ASTM B366 UNS N08825. Butt-weld and socket-weld. Elbows, tees, reducers, caps, stub ends. |
| Flanges |
ASTM B564 UNS N08825. 150#–1500#. Weld neck, blind, slip-on, lap joint, long weld neck, orifice. |
| Round Bars |
6–300 mm diameter. ASTM B425 UNS N08825. Hot-finished and cold-drawn. Solution-annealed. |
HT PIPE Project Experience — Incoloy 825
Case Study: Phosphoric Acid Evaporator Bundle — 5.6 Tons Incoloy 825 Seamless Tube
HT PIPE supplied 5.6 tons of Incoloy 825 seamless tubes (ASTM B423 UNS N08825, OD 38.1 mm × WT 2.77 mm, lengths 7.3 m) to a North African fertilizer plant for phosphoric acid evaporator bundle replacement in 2024. The evaporator concentrates wet-process phosphoric acid from 28% to 54% P₂O₅ at 85°C, with chloride contamination (2,000 ppm) and fluoride contamination (1,500 ppm) typical of wet-process acid.
Material selection justification: Incoloy 825 was selected over 316L and 904L stainless based on three factors:
316L suffered chloride pitting after 18 months (PREN 25 insufficient for 2,000 ppm Cl− at 85°C);
904L would provide similar pitting resistance but lacks the Cu addition for H₃PO₄ passivation;
Incoloy 825’s Cu (1.5–3.0%) + Mo (2.5–3.5%) combination handles both the H₃PO₄ and chloride contamination simultaneously;
Ti stabilization provides IGC resistance in as-welded condition during tube-to-tubesheet expansion;
Delivery: Full EN 10204 3.1 certification including chemical analysis (Cu verified 2.1%, Mo 3.0%, Ti 0.9%), mechanical testing, hydrostatic test (250 bar), and PMI on every tube. ASTM A262 Practice A IGC test passed on all 7 heats. Solution-annealed at 950°C with rapid air cool. Delivered in 14 weeks. Evaporator has operated 20 months with no tube failures, extending planned run length from 12 to 30 months between cleanings.
For additional project references for sulfuric acid, phosphoric acid, oil & gas, or seawater applications, contact our technical sales team.
Frequently Asked Questions — Incoloy 825
Q1: What is the difference between Incoloy 825 and Incoloy 800?
825 is optimized for aqueous corrosion resistance, while 800 is optimized for high-temperature structural and oxidation resistance. 825 has higher Ni (38–46% vs 30–35%), and adds Mo (2.5–3.5%) and Cu (1.5–3.0%) for reducing acid and pitting resistance. 800 has higher carbon and Ti/Al for high-temperature creep and oxidation. Choose 825 for chemical process equipment handling acids and chlorides up to 540°C. Choose 800 for furnace components and heat exchangers operating above 600°C. Full Incoloy 800 material guide →
Q2: Can Incoloy 825 replace 316L stainless steel?
Yes — 825 is a direct upgrade over 316L for service involving sulfuric acid, phosphoric acid, chlorides above 60°C, or sour gas (H₂S). It provides superior chloride SCC resistance (38–46% Ni vs 10–14%), better pitting resistance (PREN ~31 vs ~25), and H₂SO₄ passivation through the Cu addition. The trade-off is 3–4× higher material cost. For routine ambient or mild aqueous service, 316L remains more economical. For aggressive media or elevated temperature chloride service, 825 is the standard upgrade.
Q3: How does the copper addition improve sulfuric acid resistance?
The Cu addition (1.5–3.0%) works through an electrochemical self-polarization mechanism. In sulfuric acid, copper cathodically reduces H+ to H₂ gas (2H+ + 2e− → H₂↑), consuming electrons at the metal surface. This consumption shifts (polarizes) the alloy’s corrosion potential into the passive range where a protective Cr₂O₃ film is thermodynamically stable. Essentially, the Cu acts as an internal electrochemical catalyst that forces the alloy surface into passivation. This is the same mechanism used in Hastelloy C-2000, though at lower Cu content than C-2000 (1.3–1.9%).
Q4: Does Incoloy 825 require post-weld heat treatment?
No — 825 can be used in the as-welded condition for most applications. The Ti stabilization (0.6–1.2%) prevents sensitization by forming TiC preferentially over Cr₂₃C₆ at grain boundaries. For severe service (strong oxidizing media, boiling concentrated acids, or critical pressure boundaries), a full solution anneal at 920–980°C + rapid cool is recommended. Use matching filler metal ERNiFeCr-1 (AWS A5.14) or ERNiCrMo-3 for welding. Always qualify welding procedures with ASTM A262 corrosion testing for critical applications.
Q5: Is Incoloy 825 suitable for sour service (H₂S)?
Yes — 825 is qualified for sour service per NACE MR0175 / ISO 15156 for oil & gas applications. The high nickel content provides resistance to sulfide stress cracking (SSC), and the alloy resists the combination of H₂S, CO₂, and chloride contamination common in oilfield environments. 825 is specified for downhole tubing, wellhead components, gas lift piping, and process equipment in sour service. Maximum hardness qualification is 22 HRC per NACE MR0175. Contact us for NACE-compliant supply with full certification.
Request a Quote for Incoloy 825 (UNS N08825)
Send your specification — sizes, quantities, product form (pipe, plate, fittings, flanges), service environment (acid type, concentration, temperature), certification requirements (EN 10204 3.1/3.2, ASTM A262 IGC, NACE MR0175). We typically quote within 24 hours with full MTC details, mill lead times, and technical recommendations for your specific corrosion environment.
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