Introduction
Walk into any materials selection meeting for a chemical plant, offshore platform, or subsea system, and two alloys will almost certainly appear on the shortlist: Inconel 625 and Hastelloy C-276. Both are nickel-chromium-molybdenum alloys. Both resist chloride pitting. Both are considered “premium corrosion alloys” that carry a significant cost premium over stainless steel. And both have been used successfully in demanding environments for decades.
So why does the choice matter? Because these two alloys were designed with fundamentally different priorities — and using the wrong one in your service environment means either overpaying for performance you don’t need, or deploying a material that will fail before its design life.
The key difference: Inconel 625 was engineered for strength, weldability, and versatility across a wide temperature range. Hastelloy C-276 was engineered as a chemical-resistance specialist — maximizing corrosion performance in reducing and mixed-acid environments at the expense of some mechanical properties.
This article gives you the data to make the right call.
Composition: Where the Differences Start
Both alloys are Ni-Cr-Mo systems, but the specific balance produces dramatically different properties.
| Element | Inconel 625 (UNS N06625) | Hastelloy C-276 (UNS N10276) |
|---|---|---|
| Nickel (Ni) | ≥58% (bal.) | 57% (bal.) |
| Chromium (Cr) | 20–23% | 14.5–16.5% |
| Molybdenum (Mo) | 8–10% | 15–17% |
| Iron (Fe) | ≤5% | 4–7% |
| Niobium (Nb) | 3.15–4.15% | — |
| Cobalt (Co) | ≤1% | ≤2.5% |
| Tungsten (W) | — | 3–4.5% |
| Carbon (C) | ≤0.10% | ≤0.010% |
Three compositional differences drive most of the performance divergence:
1. Molybdenum: 9% vs 16%. Molybdenum is the dominant element controlling resistance to reducing acids (HCl, H₂SO₄) and crevice corrosion. C-276’s 16–17% Mo is among the highest of any commercial alloy — roughly double 625’s content. This is why C-276 is the go-to alloy for hydrochloric acid service.
2. Niobium: 3.15–4.15% in 625, zero in C-276. Niobium provides solid-solution strengthening and, critically, suppresses sensitization during welding by preferentially combining with carbon before chromium can. This gives 625 an inherent advantage in as-welded corrosion resistance — it doesn’t require post-weld solution annealing in most applications.
3. Carbon: 0.10% max in 625 vs 0.010% max in C-276. C-276’s ultra-low carbon is its approach to preventing sensitization — without niobium as a stabilizer, C-276 relies on low carbon content to prevent chromium carbide precipitation at grain boundaries during welding heat cycles.
4. Tungsten: 3–4.5% in C-276, none in 625. Tungsten synergizes with molybdenum in C-276 to increase resistance to oxidizing chloride environments. This contributes to C-276’s superior resistance to mixed acid (HNO₃ + HCl) environments.
Pitting and Crevice Corrosion Resistance: The PRE Numbers
The Pitting Resistance Equivalent Number (PRE or PREN) provides a standardized way to compare localized corrosion resistance. For nickel alloys, the relevant formula is:
PRE = %Cr + 3.3×%Mo + 30×%N
For these two alloys:
| Alloy | Cr | Mo | PRE |
|---|---|---|---|
| Inconel 625 | 21.5% (mid) | 9% (mid) | 21.5 + 3.3×9 = 51.2 |
| Hastelloy C-276 | 15.5% (mid) | 16% (mid) | 15.5 + 3.3×16 = 68.3 |
C-276’s PRE of ~68–70 is significantly higher than 625’s ~51. This translates to real-world performance differences:
- Critical Pitting Temperature (CPT) in ASTM G150 (3.5% NaCl): 625 ≈ 75–85°C; C-276 ≈ >120°C (often “no pitting” at test maximum)
- Critical Crevice Temperature (CCT): 625 ≈ 50–60°C; C-276 ≈ 55–65°C
For offshore and seawater applications at ambient temperature, both alloys are effectively immune to pitting — the temperature is well below either alloy’s CPT. In this range, 625 is adequate, and its cost advantage (~20–30% less than C-276 per kilogram) becomes the deciding factor.
For high-temperature seawater (>60°C, as in produced-water systems or heat exchangers), C-276’s higher CPT provides a meaningful safety margin.
Chemical Resistance: Where C-276’s High Mo Pays Off
Hydrochloric Acid (HCl)
C-276’s 16% Mo provides outstanding resistance to hydrochloric acid across a wide concentration range. Corrosion rate data:
| Environment | Inconel 625 | Hastelloy C-276 |
|---|---|---|
| 5% HCl, 25°C | <0.13 mm/yr | <0.05 mm/yr |
| 10% HCl, 50°C | 0.5–1.2 mm/yr | <0.13 mm/yr |
| 20% HCl, boiling | Not recommended | 0.3–0.5 mm/yr |
| 37% HCl, 25°C | Not recommended | 0.5–0.8 mm/yr |
For dilute HCl at ambient temperature, both alloys are acceptable. Above 10% concentration and elevated temperature, C-276 becomes the clear choice, and 625 is not recommended.
Sulfuric Acid (H₂SO₄)
| Environment | Inconel 625 | Hastelloy C-276 |
|---|---|---|
| 10% H₂SO₄, boiling | 0.5–1.0 mm/yr | <0.13 mm/yr |
| 50% H₂SO₄, 100°C | Not recommended | 0.2–0.5 mm/yr |
| 96% H₂SO₄, ambient | Acceptable | Good |
Again, C-276’s higher Mo provides superior resistance across the concentration range. 625 is acceptable only in dilute, ambient-temperature conditions.
Phosphoric Acid (H₃PO₄)
Both alloys perform well in phosphoric acid, but 625 shows slightly lower corrosion rates in high-purity phosphoric acid at moderate temperatures — an unexpected reversal driven by 625’s higher chromium content (21.5% vs 15.5%) in this less reducing environment.
Mixed Acids and Oxidizing Chlorides
Environments combining both oxidizing and reducing character (e.g., HNO₃ + HCl mixtures, bleach solutions, wet chlorine, FGD scrubber liquors) are where C-276’s tungsten content provides its greatest benefit. The W synergy with Mo suppresses anodic dissolution in mixed-potential environments where Mo alone is insufficient.
625 is not recommended for mixed acid environments above ambient temperature. C-276 or C-22 should be specified instead.
Mechanical Properties: 625 Takes the Lead
Inconel 625 was developed partly because of its outstanding mechanical properties — properties that C-276, optimized for chemistry, cannot match.
| Property | Inconel 625 (Annealed) | Hastelloy C-276 (Annealed) |
|---|---|---|
| Tensile Strength (UTS) | 827–1034 MPa | 690–779 MPa |
| Yield Strength (0.2% PS) | 414–517 MPa | 276–310 MPa |
| Elongation | 30–45% | 40–60% |
| Hardness | ~200 HB | ~180 HB |
| Max Continuous Service (oxidizing) | 870°C | 650°C |
| Max Intermittent Service | 980°C | 760°C |
Key takeaway: 625’s tensile strength is 20–30% higher than C-276 in the annealed condition. Yield strength is approximately 50% higher. For pressure-bearing components (flanges, heat exchanger tubing, downhole components), this strength advantage allows wall thickness reduction — partially offsetting the material cost difference.
C-276 is not a high-temperature structural alloy. Above ~600°C it loses mechanical strength rapidly and is not suitable for furnace or combustion applications.
Weldability: 625’s Niobium Advantage
This is the single largest practical difference between the two alloys in fabrication environments.
Inconel 625 Welding Characteristics
- Filler metal: ERNiCrMo-3 (matching composition)
- Sensitization resistance: Niobium ties up carbon → no chromium carbide precipitation → no heat-affected zone (HAZ) sensitization → no post-weld heat treatment (PWHT) required in most applications
- Weld overlay performance: Dilution of 625 into the base metal (steel) does not dramatically degrade corrosion resistance because niobium maintains the protective Cr-Mo matrix even at 30–40% dilution
- Overlay deposition: 625 is the most widely used alloy for weld overlay cladding of carbon steel vessels, piping, and subsea equipment worldwide
- Hot cracking risk: Low — 625’s compositional balance and grain boundary chemistry provide good solidification crack resistance
Hastelloy C-276 Welding Characteristics
- Filler metal: ERNiCrMo-4 (matching composition)
- Sensitization control: Relies on ultra-low carbon (≤0.010%) rather than stabilization — no niobium buffer
- PWHT requirement: Typically not required for most applications, but solution annealing at 1120°C is needed for the most aggressive corrosion environments to remove any secondary phases precipitated during welding
- Weld overlay performance: C-276 overlay is more sensitive to dilution — high dilution from the base metal can reduce Mo content in the first overlay pass, compromising corrosion resistance. Two-layer overlay is often required to ensure adequate alloy content
- Cost implication: Two-layer C-276 overlay vs. single-layer 625 overlay significantly increases fabrication cost and weld time
For weld overlay cladding applications — offshore structures, pressure vessels, riser systems, BOPs — Inconel 625 is the dominant choice not just because of cost, but because of superior weld performance and dilution tolerance.
Cost Comparison: Inconel 625 vs Hastelloy C-276
Raw material pricing fluctuates with nickel, molybdenum, and niobium markets, but the relative relationship between the two alloys is fairly stable.
| Form | Inconel 625 | Hastelloy C-276 | Premium |
|---|---|---|---|
| Plate ($/kg, indicative) | $45–60 | $60–80 | +30–40% |
| Pipe ($/kg, indicative) | $55–75 | $70–95 | +25–35% |
| Weld wire (ERNiCrMo-3 vs -4) | $80–100 | $95–130 | +20–30% |
C-276’s cost premium is driven by its higher molybdenum content (Mo is more expensive per unit than niobium) and more complex manufacturing due to tighter chemistry control requirements.
Total installed cost comparison for a heat exchanger:
A typical carbon steel + 625 weld overlay heat exchanger versus C-276 solid-wall construction:
- C-276 solid wall: high material cost, simpler fabrication
- 625 weld overlay on carbon steel: lower material cost, labor for overlay deposition
- For vessels above ~50mm wall thickness, 625 overlay on CS is almost always cheaper than solid C-276 construction
Application Decision Guide
Choose Inconel 625 When:
✅ Weld overlay cladding — subsea equipment, offshore structures, risers, BOPs, pressure vessels needing corrosion-resistant internal surfaces
✅ High-strength requirements — downhole tubing, flexible pipe armor, firewater system components, mooring hardware
✅ Seawater at ambient to moderate temperature — the PRE of 51 provides adequate pitting resistance below 60°C
✅ High-temperature service above 600°C — furnace components, heat treatment fixtures, combustion environment (C-276 not suitable here)
✅ Aerospace and turbine applications — 625’s strength and oxidation resistance up to 870°C make it NACE MR0175 and NADCAP compliant
✅ Budget-sensitive projects — where C-276’s extra corrosion resistance is not required by the service environment
Choose Hastelloy C-276 When:
✅ HCl service above 5% concentration or elevated temperature — the most reliable alloy for hydrochloric acid environments
✅ Mixed acid environments — HNO₃ + HCl, H₂SO₄ + HCl, bleach, wet chlorine
✅ High-temperature chloride service above 60°C — seawater or brine at elevated temperatures (produced water systems, evaporators)
✅ FGD scrubbers with chloride-rich flue gas — mixed acid condensate environments (see Article 28 for C-22 vs C-276 comparison)
✅ Pharmaceutical or semiconductor wet benches — high-purity aggressive cleaning acids
✅ When PRE >65 is specified — any environment where regulatory or engineering specifications require C-276’s higher pitting index
Neither Alloy Is Adequate For:
❌ Concentrated HF (hydrofluoric acid) — use Monel 400
❌ Concentrated HNO₃ (pure oxidizing, no chlorides) — use Incoloy 825 or 904L (lower cost)
❌ Concentrated alkalis at high temperature — consider Nickel 200 or Inconel 690
Weld Overlay in Practice: A Detailed Comparison
Because weld overlay is one of the most common applications where 625 and C-276 are directly compared, it’s worth examining the key parameters in detail.
625 Weld Overlay Process
The standard process for 625 weld overlay on carbon steel (CS) or low-alloy steel:
- First pass (butter layer): Deposit at lower amperage to control dilution to ~20–25%. ERNiCrMo-3 filler.
- Second pass (cap layer): Full-width overlay pass, dilution ~10–15%. Total overlay thickness: 3–5mm typical.
- Test: Ferrite number measurement; chemical analysis by XRF or spectroscopy to verify minimum Cr and Mo content.
- Result: Single-layer overlay (if dilution controlled) is often sufficient for most applications. Two-layer provides added assurance.
Why 625 is dilution-tolerant: Even at 30% dilution, the iron picked up from carbon steel reduces Mo only marginally (from 9% to ~6%) and Cr from 21.5% to ~15%. The resulting composition still provides adequate pitting resistance for most offshore/seawater service.
C-276 Weld Overlay Process
- First pass: Very sensitive to dilution. At 25% dilution, Mo drops from 16% to ~12% — below the threshold for optimal performance.
- Second pass required: To restore full composition integrity in the weld metal.
- Total overlay cost: Approximately 40–60% higher than single-layer 625 overlay for the same area coverage.
- Applications where C-276 overlay is justified: Chemical injection lines, corrosive process wetted surfaces where HCl or mixed acid exposure is confirmed.
Standards and Specifications
Inconel 625 (UNS N06625)
| Form | Standard |
|---|---|
| Plate and sheet | ASTM B443 / ASME SB443 |
| Bar and billet | ASTM B446 / ASME SB446 |
| Tube | ASTM B444 / ASME SB444 |
| Pipe | ASTM B705 |
| Weld wire | AWS A5.14, ERNiCrMo-3 |
| Fittings | ASTM B366 |
| Fasteners | ASTM F467 (grade 625) |
Hastelloy C-276 (UNS N10276)
| Form | Standard |
|---|---|
| Plate and sheet | ASTM B575 / ASME SB575 |
| Bar | ASTM B574 / ASME SB574 |
| Tube | ASTM B622 / ASME SB622 |
| Pipe | ASTM B619 |
| Weld wire | AWS A5.14, ERNiCrMo-4 |
| Fittings | ASTM B366 |
Quick Comparison: Inconel 625 vs Hastelloy C-276
| Property | Inconel 625 | Hastelloy C-276 | Advantage |
|---|---|---|---|
| UTS (annealed) | 827 MPa | 690 MPa | 625 |
| Yield strength | 517 MPa | 310 MPa | 625 |
| PREN | ~51 | ~69 | C-276 |
| HCl resistance | Limited | Excellent | C-276 |
| Mixed acid resistance | Limited | Excellent | C-276 |
| Max use temp (oxidizing) | 870°C | 650°C | 625 |
| Weld overlay dilution tolerance | High | Low | 625 |
| Post-weld treatment | Usually none | Usually none | Tie |
| Relative cost (per kg) | Lower | Higher (+30%) | 625 |
| Subsea / offshore | Primary choice | Secondary | 625 |
| Chemical plant corrosion | Secondary | Primary choice | C-276 |
Frequently Asked Questions
Q1: Can Inconel 625 replace Hastelloy C-276 in a heat exchanger handling 10% HCl at 80°C?
No — this is precisely the environment where C-276 is required and 625 will fail. At 10% HCl and 80°C, 625 would experience corrosion rates of 1–3 mm/yr, resulting in tube failure within months. C-276 in this same environment typically delivers <0.5 mm/yr and multi-year service life. The extra cost of C-276 is fully justified.
Q2: Is 625 or C-276 better for subsea weld overlay on wellhead equipment?
625 (ERNiCrMo-3) is the overwhelming industry choice for subsea weld overlay for two reasons: better dilution tolerance (single-layer often sufficient) and lower cost. C-276 overlay requires two passes to achieve adequate Mo content — increasing cost 40–60%. Unless the specific service fluid contains concentrated HCl or mixed acids, 625 overlay provides sufficient corrosion protection.
Q3: What is the difference in pitting resistance between the two alloys in seawater?
At ambient seawater temperatures (≤25°C), both alloys are essentially immune to pitting — both have CPTs well above ambient. Above 60°C seawater, C-276’s PRE of ~69 provides significantly better protection than 625’s PRE of ~51. For produced-water processing systems, heat exchangers with seawater at >60°C, or hot brine environments, C-276 is the safer specification.
Q4: Which alloy is better for high-temperature furnace applications?
Inconel 625 by a large margin. C-276 is limited to approximately 650°C in oxidizing atmospheres, while 625 maintains adequate strength and oxidation resistance up to 870°C (continuous) and 980°C (intermittent). For furnace muffles, retorts, heat treatment baskets, and combustion hardware, 625 is the correct choice — C-276 is not recommended for these applications.
Q5: How does the total installed cost compare for a chemical processing vessel?
For a solid-wall vessel in chemical processing service where C-276 properties are genuinely needed: C-276 will cost 30–40% more per kilogram than 625 in material alone. For a weld overlay construction approach (CS shell + alloy cladding), 625 overlay is typically 40–50% cheaper in total overlay cost versus C-276 two-layer overlay for the same surface area, due to both lower filler cost and reduced weld passes required.
Internal Links
- Hastelloy C-22 vs C-276: Crevice Corrosion & FGD Upgrade — When C-276 reaches its own performance ceiling in oxidizing chloride environments, C-22 is the step up.
- Incoloy 825 vs Hastelloy C-276: Sour Gas Corrosion — How a lower-cost alloy (825) competes with C-276 in H₂S-rich sour gas environments.
- Hastelloy B-3 vs C-276: Reducing vs Mixed Acid — When Mo content of C-276 is still not enough for concentrated reducing acid service.
Conclusion
Inconel 625 and Hastelloy C-276 are both outstanding alloys — but they are not competitors in most applications. They were designed for different primary use cases, and the selection decision is usually clear once you map the service environment to alloy design intent.
Spec 625 when strength, weldability, and versatile corrosion resistance in chloride environments are the primary requirements. It is the backbone of offshore and subsea construction precisely because it combines adequate corrosion resistance with the mechanical properties and welding characteristics that fabricators need.
Spec C-276 when chemical resistance to reducing acids, mixed acids, or high-concentration chlorides is the primary requirement. Its 16% Mo content is unmatched for HCl and H₂SO₄ service, and no amount of design creativity substitutes for that corrosion resistance when the environment demands it.
The practical guidance: start with the corrosion environment. If the service fluid contains HCl >5%, H₂SO₄ >30% at temperature, or mixed acid chemistry — specify C-276. For everything else in offshore, subsea, or general corrosion service — evaluate 625 first, and justify C-276 only when performance data requires it.
J&A Alloy specializes in Inconel 625, Hastelloy C-276, and related nickel alloys in plate, pipe, tube, bar, and weld wire form. Contact our technical team for material selection support, mill test report review, and competitive pricing on ASTM-certified stock.
