Inconel 625 vs Incoloy 825 in Deepwater Sour Gas: Where the Step-Up Is Justified
Deepwater sour gas production combines three challenges simultaneously: high H₂S partial pressure (NACE sour service), elevated chloride concentration from produced formation water, and elevated temperature from geothermal gradient. Both Incoloy 825 (EN 2.4858, P-No.45) and Inconel 625 (EN 2.4856, P-No.43) qualify for sour service under NACE MR0175 / ISO 15156-3 — but their performance envelopes diverge significantly as H₂S partial pressure, temperature, and chloride concentration increase. Specifying 825 where 625 is required risks undetected SCC; specifying 625 throughout where 825 is adequate adds unnecessary cost. This article defines the engineering boundary between the two grades for deepwater subsea service.
Key Differences That Govern the Boundary
| Property | Incoloy 825 (N08825) | Inconel 625 (N06625) |
|---|---|---|
| Ni content | 38–46% | 58% minimum |
| PRE | ~33–36 | ~52–56 |
| ISO 15156-3 H₂S limit (annealed) | Up to ~1.0 MPa H₂S partial pressure | No stated H₂S limit — unlimited qualification |
| Max temperature in ISO 15156-3 | 60°C (annealed condition) | 232°C (annealed) |
| Relative material cost | ~1.0× | ~1.6× |
The 60°C Temperature Ceiling on 825
The most important single limitation on 825 in sour service is the 60°C maximum temperature in ISO 15156-3 Table A.3 for the annealed condition. Deep subsea wellhead temperatures routinely exceed this threshold — a 3,000 m water depth well with a 30°C/km geothermal gradient can produce wellhead fluids at 90–120°C. At these temperatures, 825 does not meet ISO 15156-3 requirements and should not be used in the sour zone. 625 has a 232°C temperature limit for the same H₂S conditions — providing the required margin for deep, hot production systems. Any subsea sour gas fitting specification that references ISO 15156-3 must verify whether the service temperature exceeds 60°C before defaulting to 825 as the standard sour service grade.
When 825 Is Sufficient for Subsea
- Shallow water (less than 1,000 m) where produced fluid temperature at the wellhead is below 60°C
- Moderate H₂S partial pressure (below 1.0 MPa) confirmed by reservoir fluid composition analysis
- Low to moderate chloride produced water — not aggressive deep formation brine
- Non-HPHT wells where combined temperature/pressure/H₂S envelope is within 825 qualification
In all other deepwater sour gas service, 625 is the correct specification and the cost premium is justified by the significantly wider temperature and H₂S envelope it provides under ISO 15156-3.