Discover the latest trends and innovations in the nickel alloy industry. We provide expert insights and in-depth analysis on the newest developments in nickel alloys.
When specifying materials for aggressive chemical, petrochemical, or wet-process metallurgical services, engineers frequently debate between Incoloy 825 (UNS N08825 / W.Nr. 2.4858) and Hastelloy C-276 (UNS N10276 / W.Nr. 2.4819). Both are nickel-base, solid-solution strengthened, corrosion-resistant alloys, but they occupy very different positions in the alloy hierarchy. Incoloy 825 is a Ti-stabilized Ni-Fe-Cr-Mo-Cu alloy designed for cost-effective resistance to sulfuric/phosphoric acids, seawater chloride SCC, and sour oilfield environments. Hastelloy C-276 is a ultra-low-carbon, high-Mo/W, full nickel-base alloy delivering near-top-tier resistance to strong reducing acids (HCl, hot H₂SO₄), oxidizing/reducing mixed media, and extreme pitting/crevice corrosion. This article provides a rigorous, data-driven comparison to guide material selection.
1. Chemical Composition — The Root Cause of Performance Difference
|
Element |
Incoloy 825 (N08825) ASTM B424 |
Hastelloy C-276 (N10276) ASTM B575 |
Functional Impact |
|---|---|---|---|
|
Ni |
38.0 – 46.0 (typ. 42) |
Balance (52.0 – 59.0, typ. 55–57) |
C-276 higher Ni → better resistance in reducing environments & thermal stability |
|
Cr |
19.5 – 23.5 (typ. 21.5) |
14.5 – 16.5 (typ. 15.5) |
825 slightly better in oxidizing acids (HNO₃); C-276 Cr sufficient + relies on Mo/W for localized attack resistance |
|
Fe |
22.0 min / Balance (typ. 22–26) |
4.0 – 7.0 (typ. 5–6) |
825 Fe lowers cost but limits high-temp stability & max corrosion envelope |
|
Mo |
2.5 – 3.5 (typ. 3.0) |
15.0 – 17.0 (typ. 16.0) |
C-276 Mo ≈ 5× → PREN ≈ 63–69 vs 825 ≈ 32–35; decisive for pitting/crevice resistance |
|
W |
— |
3.0 – 4.5 (typ. 3.7) |
Synergistic with Mo to enhance crevice & localized corrosion resistance |
|
Cu |
1.5 – 3.0 (typ. 2.0–2.5) |
≤ 0.5 (usually ≤ 0.1) |
825 only: imparts tolerance to medium-concentration H₂SO₄ & H₃PO₄; C-276 has no Cu advantage |
|
Ti |
0.6 – 1.2 (Ti/C ≥ 8–12) |
— |
825: Ti stabilizes C as TiC → immune to intergranular sensitization after welding |
|
C |
≤ 0.05 (typ. ≤ 0.03) |
≤ 0.01 (modern grade, typ. 0.005–0.008) |
C-276: ultra-low C prevents any carbide precipitation (no Ti needed) |
|
Si |
≤ 0.5 |
≤ 0.08 (controlled low to avoid μ-phase embrittlement) |
— |
|
Co |
— |
≤ 2.5 (typ. ≤ 1.0) |
Allowed residual |
Pitting Resistance Equivalent Number (PREN = %Cr + 3.3×%Mo + 16×%N):
Incoloy 825: PREN ≈ 21 + 3.3×3 ≈ 32–35
Hastelloy C-276: PREN ≈ 15.5 + 3.3×16 ≈ 63–69
2. Corrosion Resistance by Media — Comparative Rating
|
Environment / Media |
Incoloy 825 |
Hastelloy C-276 |
Recommended Choice |
|---|---|---|---|
|
Dilute H₂SO₄ (≤ 40%, ≤ 60–80°C) |
★★★★☆ Cu provides specific tolerance |
★★★☆☆ Acceptable but no Cu advantage |
825 for economy; C-276 if chlorides/oxidizers also present at high levels |
|
Hot Conc. H₂SO₄ (> 70%, > 80°C) |
☆☆☆☆☆ Not recommended |
★★★★☆ Handles hot conc. H₂SO₄ (esp. with trace oxidant) |
C-276 (or C-22 / Si-cast iron depending on T) |
|
Hydrochloric Acid (HCl, med. conc./temp.) |
☆☆☆☆☆ Essentially unusable |
★★★★☆ Best metallic option for many HCl services (within T/% limits) |
C-276; 825 NOT suitable |
|
Wet-Process H₃PO₄ (+ F⁻ + Cl⁻) |
★★★★☆ Industry standard (extraction section) |
★★★★☆ Usable, but Cu gives 825 edge for cost/performance |
825 usually preferred on cost |
|
Seawater / Brine (Cl⁻ SCC + Pitting/Crevice) |
★★★★☆ PREN≈34, immune to Cl⁻ SCC |
★★★★★ PREN≈65, superior for stagnant crevices / hot seawater |
825 for general seawater service; C-276 for severe crevice/stagnant hot seawater |
|
Mixed Oxidizing/Reducing (e.g. HNO₃ + HCl, bleach plant) |
★★☆☆☆ Limited (Cr helps oxid., Mo low for red.) |
★★★★☆ Mo/W tolerate reductive part; Cr sufficient for oxidative |
C-276 or preferably C-22 (higher Cr) for ClO⁻ containing streams |
|
Sour Gas (H₂S + CO₂ + Cl⁻, NACE MR0175) |
★★★★☆ With soln-ann., HRC ≤ 35 |
★★★★☆ Same; also used in higher-temp/deeper wells |
Either; deeper/hotter wells may trend to C-276 or higher Ni alloys |
|
Conc. HNO₃ (hot) |
★★☆☆☆ Fair (Cr₂O₃ film) |
★☆☆☆☆ (Cr on low side vs. 300 series) |
Neither ideal → Inconel 600 / AL-6XN / high-purity 304L (low Cl⁻) |
|
Atmospheric / Fresh Water (no acid) |
★★★☆☆ Over-spec but usable |
★★★☆☆ Over-spec |
316L/2205 usually sufficient |
3. Mechanical Properties & Temperature Suitability
|
Property (Solution-Annealed Condition) |
Incoloy 825 (ASTM B424 Min) |
Hastelloy C-276 (ASTM B575 Min) |
|---|---|---|
|
Tensile Strength Rm |
≥ 586 MPa (85 ksi) |
≥ 690 MPa (100 ksi) |
|
Yield Strength Rp0.2 |
≥ 241 MPa (35 ksi) |
≥ 283 MPa (41 ksi) |
|
Elongation A₅₀ₘₘ |
≥ 30% |
≥ 40% |
|
Typical Rm / Rp0.2 (mill) |
630–720 / 270–340 MPa |
760–850 / 350–420 MPa |
|
Max. Long-Term Corrosion Service Temp. |
~ 450–480°C (strength & σ-phase consideration) |
~ 650°C (corrosion resistance retained; strength also better) |
|
High-Temp Load Bearing (> 650°C) |
Not suitable (σ-phase risk > ~650°C) |
Limited (used to ~750–815°C in some applications, but not precipitation-strengthened) |
|
Age-Hardenability |
No |
No (pure solid-solution; some modern variants have limited aging response) |
4. Welding & Heat Treatment
|
Item |
Incoloy 825 |
Hastelloy C-276 |
|---|---|---|
|
Recommended Filler |
ERNiCrMo-3 (Inconel 625 type – over-alloyed) or ERNiFeCr-1 |
ERNiCrMo-4 (AWS A5.14, matching C-276) or ERNiCrMo-10 (C-22 type if coupling with C-22) |
|
PWHT for Corrosion? |
No. Ti-stabilized; weld HAZ remains immune to IGC. Stress relief ≤ 400°C only if needed for dimensional stability. |
No. Ultra-low C prevents sensitization. Stress relief ≤ 480°C brief if required. |
|
Key Welding Caution |
Interpass ≤ 150°C; stringer beads; back purge Arg. |
Interpass ≤ 93–120°C (more crack-sensitive to heat input); strict cleaning to avoid C/Fe contamination; pickle after welding (HNO₃+HF) |
|
Base Metal Condition |
Must be solution-annealed (≥ 980°C + rapid quench) per ASTM |
Must be solution-annealed (1065–1120°C + rapid quench) per ASTM |
5. Cost Comparison (Indicative, Subject to Ni/Mo Market Fluctuation)
Material Cost (same product form, same size):
Hastelloy C-276 ≈ 1.8× – 2.5× the cost of Incoloy 825
(Primary drivers: Ni content ~55% vs ~42%, Mo content ~16% vs ~3%, tighter melting practice / VIM-VAR optional for C-276)
Machinability: C-276 work-hardens rapidly — tool wear cost slightly higher than 825 (comparable to Inconel 625).
Wall Thickness: C-276 has higher YS (≈283 vs 241 MPa min) — theoretically allows thinner walls under internal pressure, but corrosion allowance usually governs, so cost saving by thinning is rare in corroservice.
Life-Cycle Cost: In truly aggressive services (hot HCl, mixed oxidant/reductant, severe crevice), C-276 prevents early failure that 825 cannot survive → lifecycle favors C-276 despite higher CAPEX.
6. Full Comparison Summary Table
|
Characteristic |
Incoloy 825 (N08825) |
Hastelloy C-276 (N10276) |
|---|---|---|
|
Alloy Type |
Ti-stabilized Ni-Fe-Cr-Mo-Cu, solid-solution |
Ultra-low-C Ni-Cr-Mo-W, solid-solution |
|
PREN (Pitting Resistance) |
32–35 |
63–69 |
|
Key Corrosion Strength |
Dil. H₂SO₄/H₃PO₄ (Cu), seawater Cl⁻ SCC, sour gas (NACE) |
Strong reduc. acids (HCl, hot H₂SO₄), oxid./redox mix, extreme pitting/crevice |
|
Key Corrosion Limitation |
Not for hot conc. HCl/H₂SO₄; limited PREN vs. 6%Mo SS & C-276 |
No Cu → less tolerant to mid-conc. H₂SO₄ than 825 (though still resists); expensive |
|
Max. Service Temp. (Corrosion + Load) |
≤ 450–480°C (long term) |
≤ 650°C (corrosion); up to ~750°C short-time structural |
|
Weld Filler |
ERNiCrMo-3 / ERNiFeCr-1 |
ERNiCrMo-4 / ERNiCrMo-10 |
|
PWHT Needed? |
No (for corrosion) |
No (for corrosion) |
|
Relative Cost Index* |
1.0 (baseline) |
1.8 – 2.5 |
|
ASTM Product Specs |
B424(plate), B423(tube), B425(bar), B564(forgings) |
B575(plate), B622(tube), B574(bar), B564(forgings) |
*Cost index approximate; varies with market.
7. Selection Decision Guide
Medium / dilute H₂SO₄, H₃PO₄ (with/without Cl⁻), seawater with SCC concern, sour gas (NACE) at moderate T, and budget is important → Incoloy 825.
Hot concentrated HCl or H₂SO₄ (> 70%), mixed oxidizing/reducing media (bleach, nitrate+chloride), extremely severe crevice/pitting (PREN > 60 required), or where 825 has previously failed → Hastelloy C-276 (or C-22 if Cr-sensitive oxidizers present).
Concentrated hot HNO₃ → neither (use Inconel 600, AL-6XN, or 304L high-purity).
> 650°C load-bearing → neither (use Inconel 600/601/625 or Incoloy 800HT/A-286 depending on T).
Incoloy 825 is the cost-effective workhorse for Cl⁻ SCC immunity + moderate reducible acid resistance (H₂SO₄/H₃PO₄) + sour gas compliance. Hastelloy C-276 is the premium, widest-spectrum Ni-Mo-Cr-W alloy for truly aggressive reducing acids, redox swing media, and extreme localized corrosion. Choosing between them is fundamentally a question of corrosive species identity, concentration, temperature,and acceptable CAPEX vs. lifecycle risk.
