Inconel 625 vs Hastelloy C-276: High-Performance Nickel Alloy Selection
When duplex 2507 or 904L are insufficient, the choice narrows to Inconel 625 or Hastelloy C-276. Both are premium nickel-based alloys with very high corrosion resistance — but they excel in different environments.
Alloy Overview
Inconel 625 (UNS N06625 / EN 2.4856, NiCr22Mo9Nb): 22Cr-9Mo-3.5Nb nickel alloy. The niobium addition stabilises the alloy against sensitisation and provides solid-solution strengthening. PRE >50. ASME P-No. 43.
Hastelloy C-276 (UNS N10276 / EN 2.4819, NiMo16Cr15W): 16Mo-15Cr-4W nickel-molybdenum-chromium alloy. The extremely high molybdenum content is the defining characteristic — it drives resistance to reducing acids, chloride pitting, and HCl to levels no other standard alloy matches. PRE >50. ASME P-No. 44.
Where Inconel 625 Excels
- Seawater and offshore: Excellent resistance to chloride pitting and crevice corrosion in ambient to moderate-temperature seawater. Used extensively in subsea wellhead components and flexible riser terminations.
- Fatigue resistance: Superior fatigue and creep-rupture strength make 625 the first choice for dynamic offshore applications — flexible pipes, expansion joints, bellows.
- Sour gas / H₂S service: NACE MR0175 / ISO 15156 qualified for sour service without special heat treatment.
- High temperature oxidation: Usable to 980°C in oxidising atmospheres.
- Cost: Typically 10–20% lower cost than C-276.
Where Hastelloy C-276 Excels
- HCl (hydrochloric acid): C-276 is the benchmark alloy for HCl service across all concentrations and temperatures. No other standard alloy performs comparably in wet HCl above ambient temperature.
- Mixed acid environments: FGD (flue gas desulphurisation) scrubbers, where HCl and H₂SO₄ combine with chlorides. C-276 was specifically developed for this environment.
- Reducing acids: Very high Mo content provides excellent resistance to sulphuric acid, phosphoric acid, and formic acid in reducing conditions.
- Wet Cl₂ and hypochlorite: Bleach production and water treatment where 625 is marginal.
Welding Considerations
Inconel 625 uses ERNiCrMo-3 filler (AWS A5.14). Hastelloy C-276 uses ERNiCrMo-4. The fillers are not interchangeable — using ERNiCrMo-3 on C-276 base metal produces an over-alloyed deposit relative to 625 but still under-alloyed in Mo relative to C-276, creating a HAZ that is significantly weaker in reducing acid environments. Both alloys require separate WPS qualifications. Neither requires PWHT (both are solid-solution-strengthened). Inter-pass temperature should be maintained below 150°C to avoid sensitisation of the weld metal.
Quick Selection Guide
| Environment | Inconel 625 | Hastelloy C-276 |
|---|---|---|
| Seawater / offshore | ✓ Preferred | ✓ Suitable |
| HCl (all concentrations) | Marginal above 50°C | ✓ Best choice |
| FGD scrubbers | Marginal | ✓ Preferred |
| Sour gas / H₂S | ✓ Preferred | ✓ Suitable |
| High temp oxidising (>800°C) | ✓ Preferred | Not suitable |
| Fatigue / dynamic | ✓ Preferred | Suitable |