Side-by-side comparison of Incoloy 925 (UNS N09925) and Monel® K-500 (UNS N05500). Both are Special Metals age-hardenable nickel alloys, both used for high-strength corrosion-resistant bolting, both qualified under NACE MR0175 in different table sections. The fundamental difference is base chemistry: Incoloy 925 is nickel-iron-chromium-molybdenum (Cr + Mo do the corrosion work) designed for sour-service H₂S oil & gas; Monel K-500 is nickel-copper (Cu does the corrosion work) designed for seawater and marine service. They are NOT interchangeable. Incoloy 925 fails fast in chloride-rich seawater because it lacks copper; Monel K-500 fails fast in sour H₂S because it lacks chromium. The selection rule is environment-driven: pick by service medium, not by the brand or the strength.
Selecting between sour and marine alloys? Email info@torqbolt.com with the service environment (H₂S level, chloride concentration, oxygen, temperature). WhatsApp +91-22-66157017.
Chemistry Side-by-Side
| Element | Incoloy 925 (N09925) | Monel K-500 (N05500) |
|---|---|---|
| Ni | 42–46% | 63% min |
| Cr | 19.5–22.5% (signature) | not specified (typically <0.5%) |
| Fe | 22% min (bal) | 2.0 max |
| Cu | 1.5–3.0% | 27–33% (signature) |
| Mo | 2.5–3.5% (signature) | not specified |
| Ti | 1.9–2.4% (gamma-prime) | 0.35–0.85% (gamma-prime) |
| Al | 0.1–0.5% (gamma-prime) | 2.30–3.15% (gamma-prime) |
| Mn | 1.0 max | 1.5 max |
| S | 0.03 max | 0.010 max |
| C | 0.03 max | 0.25 max |
Chemistry decides the corrosion envelope. Incoloy 925's chromium + molybdenum form the passive Cr₂O₃ / MoO₃ oxide film that resists oxidising acids, sour H₂S, and chloride pitting. Monel K-500's high copper content gives it exceptional resistance to seawater (no Cl-SCC, no oxygen-cell pitting at moderate temperature) but no chromium means no passive layer, it relies on a slower-forming oxide that breaks down in oxidising or sulphide environments. Both alloys age-harden via gamma-prime Ni₃(Al, Ti) but with different ratios: 925 emphasises Ti; K-500 emphasises Al.
Mechanical Properties Compared
| Property | Incoloy 925 (Aged) | Monel K-500 (Aged) |
|---|---|---|
| Tensile Strength min | 165 ksi (1140 MPa) | 130 ksi (896 MPa) |
| 0.2% Yield Strength min | 110 ksi (760 MPa) | 85 ksi (586 MPa) |
| Elongation min | 18% | 20% |
| Hardness max (NACE) | 35 HRC | 35 HRC |
| Charpy at -50°C | 40 J typical | 50 J typical |
| Density g/cm³ | 8.08 | 8.46 |
Incoloy 925 is ~30% stronger than K-500 in yield (110 vs 85 ksi). Both are usable down to cryogenic temperatures with comparable Charpy toughness. Density is nearly the same.
Sour-Service Envelope, The Decisive Difference
| Limit | Incoloy 925 (Table A.10) | Monel K-500 (Table A.16) |
|---|---|---|
| H₂S Partial Pressure max | 1000 psi (6.9 MPa) | 15 psi (0.1 MPa), restricted |
| Hardness cap | 35 HRC | 35 HRC |
| Cl⁻ max | Saturation | Saturation |
| Temperature max | 400°F (204°C) | 250°F (121°C) |
| O₂ sensitivity | Low | HIGH, aerated brine causes accelerated attack |
Monel K-500 is restricted to 15 psi maximum H₂S in NACE MR0175. For high-H₂S sour wells, K-500 is NOT qualified and Incoloy 925 must be used. Conversely, Monel K-500 outperforms Incoloy 925 in clean, oxygen-free seawater, the marine drilling industry has used Monel K-500 for decades on subsea hardware (lock-down dogs, propeller shafts) where chloride-SCC and crevice corrosion are the dominant failure modes.
Application Selection
| Application | Recommended Alloy |
|---|---|
| API 6A wellhead bolting (sour H₂S) | Incoloy 925 |
| Subsea propeller shaft (clean seawater) | Monel K-500 |
| Subsea pump shaft (mixed seawater + light H₂S) | Incoloy 925 (more conservative) |
| Marine fastener bolting (deck, topside, splash zone) | Monel K-500 |
| Christmas tree internals (sour, high-strength) | Incoloy 925 |
| Cryogenic LNG service (low-temp toughness) | Either, K-500 slightly better at -150°F |
| Subsea connector bolting (deepwater, light H₂S) | Incoloy 925 or Inconel 725 |
| Pump shaft / impeller in HF acid service | Monel K-500 (Cu resists HF) |
Cost Comparison
- Incoloy 925: 1.0× baseline
- Monel K-500: ~1.8×, reflects 63% nickel content (vs 44% for 925) and 30% copper (which is less expensive but processed in lower volumes)
- Inconel 718: ~3.0×
Cost-driven preference: Incoloy 925 wins for sour-service production where K-500 isn't qualified anyway; Monel K-500 wins for marine service where 925 lacks the copper to perform optimally.
Frequently Asked Questions
Why does Monel K-500 fail in sour service? The high copper content (27–33%) reacts with H₂S to form copper sulphide (CuS / Cu₂S) at exposed surfaces. The resulting layer is non-adherent and breaks away, exposing fresh metal that further reacts. The corrosion rate accelerates with temperature and H₂S partial pressure. In Incoloy 925, the chromium oxide layer is stable in sour environments, there is no analogous failure mode.
Why does Incoloy 925 underperform K-500 in clean seawater? Incoloy 925's copper content is only 1.5–3.0%, not enough to provide the chloride-pitting resistance that K-500's 30% Cu delivers. In aerated seawater above 60°F, Incoloy 925 can show pitting and crevice corrosion at higher rates than K-500, particularly under deposits or in stagnant water. For pure seawater with low H₂S, K-500 is the historical choice.
Can the two alloys be welded together? Yes, but with caution. Use INCONEL Filler Metal 625 for solid-solution joints, it tolerates both base chemistries. Avoid INCO-WELD 725NDUR for K-500 side because the matching-strength filler may not bond reliably to the high-Cu Monel matrix. Post-weld heat treatment is impractical because the two alloys age at different temperatures (925: 1365°F; K-500: 1080°F).
Can Monel K-500 be used in galvanic contact with carbon steel? Yes, Monel K-500 is more noble than carbon steel but the galvanic effect is small in seawater because both alloys form their own protective films. For Incoloy 925 in contact with carbon steel in seawater, the galvanic effect is similarly small. Both alloys are commonly bolted to carbon-steel flanges in oilfield service.
Compare Incoloy 925 to Other Alloys
- Inconel vs Incoloy, What's the difference?
- Incoloy 925 vs Inconel 718
- Incoloy 925 vs Incoloy 825
- Incoloy 925 vs Incoloy 926
- Incoloy 925 vs Duplex 2205
Request a Quote
For Incoloy 925 with full MTC, or for Monel K-500 alternatives in the same enquiry:
- Email: info@torqbolt.com
- WhatsApp: +91-22-66157017
- Datasheet PDF + sample MTC available on request
Specify product type (stud bolts, nuts, pipe, plate), size, quantity, applicable standards (ASTM B805, ASTM B865 / B164 for K-500, API 6A, NACE MR0175), service medium (sour gas / seawater / marine surface) and required mechanical envelope.
References: ASTM B805 (UNS N09925); ASTM B865 / B164 (UNS N05500). Special Metals Corporation: INCOLOY® alloy 925 + MONEL® alloy K-500 Technical Bulletins. NACE MR0175 / ISO 15156-3 Tables A.10 and A.16. INCOLOY® and MONEL® are registered trademarks of Special Metals Corporation.
