Hastelloy C276 vs Inconel 625: Sample Probe Material Selection for Aggressive Service
Direct head-to-head comparison of Hastelloy C276 and Inconel 625 for sample probe assemblies. Pitting resistance equivalent (PREN), chloride limits, sour service compliance, weldability, and cost tradeoffs.
TL;DR
Hastelloy C276 and Inconel 625 are the two most-specified nickel-base alloys for aggressive sample probe service. C276 wins on wet chlorine, hypochlorite, and oxidizing chloride brines. 625 wins on high-temperature mechanical strength, sour service economics, and weldability. PREN: C276 ≈ 67, 625 ≈ 51. When in doubt for chloride pitting, choose C276; when in doubt for high-temperature creep, choose 625.
Why This Comparison Matters
Both alloys appear at the top of the sample probe material selection guide. Both are NACE MR0175-acceptable for sour service. Both cost roughly 4× to 6× a 316L probe of identical geometry. The wrong choice wastes that premium without buying any additional life.
Composition Side-by-Side
| Element | Hastelloy C276 (UNS N10276) | Inconel 625 (UNS N06625) |
| Ni | 57 (bal) | 58 min |
| Cr | 16 | 21.5 |
| Mo | 16 | 9 |
| Fe | 5 | 5 max |
| W | 4 | — |
| Nb + Ta | — | 3.65 |
| Co | 2.5 max | 1 max |
The 16% Mo + 4% W in C276 is what drives its dominance in chloride pitting resistance. The 3.65% Nb+Ta in 625 is what gives it its outstanding high-temperature strength and weldability.
Pitting Resistance Equivalent Number (PREN)
PREN = %Cr + 3.3×(%Mo) + 16×(%N)
| Alloy | PREN |
| 316L | 24-26 |
| Duplex 2205 | 35 |
| Inconel 625 | ~51 |
| Hastelloy C276 | ~67 |
| Hastelloy C22 | ~71 |
For chloride-pitting-prone service, every PREN point matters. C276 is the practical ceiling among standard nickel alloys.
Service Decision Matrix
| Service | Pick |
| Wet chlorine, hypochlorite | C276 |
| Sulfuric acid, mid-concentration | C276 |
| Hot seawater, brackish brines | C276 |
| Oxidizing acids (HNO3) | 625 |
| Sour gas, sour crude | Either; 625 preferred for cost |
| > 600 °C creep service | 625 |
| HF acid | Monel 400 (neither C276 nor 625) |
| Mixed acids | C276 |
| Welding-intensive fabrications | 625 |
Sour Service Notes
Both alloys are NACE MR0175-acceptable in solution-annealed condition. For sulfide stress cracking under high partial pressures of H2S, both materials satisfy the standard, but 625 is more frequently specified in refinery sour service because of its lower cost and superior weldability for fabricated assemblies.
Weldability
625 is the more weldable alloy. It can be TIG-welded with matching filler (ERNiCrMo-3) without significant solidification cracking risk. C276 uses ERNiCrMo-4 filler and demands stricter heat-input control to avoid Mo segregation and intermetallic precipitation.For sample probes that will be welded into a flange or hub, this difference can drive the material choice independently of corrosion considerations.
Cost Reality
At current LME pricing, a finished C276 probe runs roughly 15-25% more than the same geometry in 625. For most operators, that delta is negligible compared to the cost of an unplanned outage from a wrong-material failure. Choose by service first, by cost second.
Configurator Defaults
The SPA Configurator lists both alloys as first-class options. When the entered process media contains free chlorine, hypochlorite, or any oxidizing chloride brine, the configurator highlights C276 as the preferred selection. When the media is sour gas with high temperature, it highlights 625.