Hastelloy C-276 vs Inconel 625: Corrosion Resistance Comparison

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Overview: Two Corrosion-Resistant Nickel Alloys, Two Different Purposes

When chemical plant engineers and offshore equipment designers need a nickel alloy that can handle aggressive corrosive environments, the conversation almost always comes back to two names: Hastelloy C-276 and Inconel 625. Both are nickel-based, both are specified by NACE MR0175 for sour gas service, and both offer far superior corrosion resistance compared to stainless steels. But they are not interchangeable — and choosing the wrong one can mean the difference between a heat exchanger that lasts 20 years and one that fails in six months.

The core distinction is this: Hastelloy C-276 is a specialist in reducing acids (hydrochloric acid, sulfuric acid in reducing conditions), while Inconel 625 is the versatile all-rounder that balances corrosion resistance with higher mechanical strength and lower cost. C-276 achieves its remarkable corrosion resistance through extremely high molybdenum (15-17%) plus tungsten (3-4.5%), giving it a PREN (Pitting Resistance Equivalent Number) well above 65. Inconel 625 relies on a higher chromium content (20-23%) plus niobium for strength, achieving a PREN of around 46.

In this article, we break down every dimension of the comparison: chemistry, corrosion resistance by media type, mechanical properties at room and elevated temperatures, weldability, cost, and application fit. All data is drawn from the authoritative ASTM and ASME standards that govern these alloys.

Chemical Composition Comparison

The chemistry of these two alloys tells the whole story. C-276 is formulated for maximum corrosion resistance in reducing environments — it packs nearly double the molybdenum of 625 and adds tungsten, both of which dramatically improve resistance to hydrochloric acid and other reducing media. Inconel 625 is formulated for balanced performance: higher chromium for oxidizing environments, niobium for strength, and enough molybdenum for good chloride resistance.

Element	C-276 (ASTM B575)	625 (ASTM B443)	Key Difference
Nickel (Ni)	Balance (~57%)	≥ 58.0%	Similar Ni base
Chromium (Cr)	14.5 – 16.5%	20.0 – 23.0%	625 has ~40% more Cr
Molybdenum (Mo)	15.0 – 17.0%	8.0 – 10.0%	C-276 has ~2x Mo +
Tungsten (W)	3.0 – 4.5%	—	C-276 only; key for HCl +
Niobium (Nb+Ta)	—	3.15 – 4.15%	625 only; strengthens +
Iron (Fe)	4.0 – 7.0%	≤ 5.0%	Both moderate Fe
Cobalt (Co)	≤ 2.5%	≤ 1.0%	C-276 allows more Co
Vanadium (V)	≤ 0.35%	—	C-276 only
Titanium (Ti)	—	≤ 0.40%	625 only
Aluminum (Al)	—	≤ 0.40%	625 only
Carbon (C)	≤ 0.01%	≤ 0.10%	C-276 ultra-low C +
Silicon (Si)	≤ 0.08%	≤ 0.50%	C-276 tighter Si
Phosphorus (P)	≤ 0.04%	≤ 0.015%	625 tighter P
Sulfur (S)	≤ 0.03%	≤ 0.015%	625 tighter S

The composition differences are striking. C-276's molybdenum-tungsten combination creates a passive film that is exceptionally stable in reducing acid environments where other alloys rapidly depassivate and corrode. The ultra-low carbon (max 0.01%) means C-276 is effectively immune to sensitization — chromium carbides simply cannot form at grain boundaries during welding or thermal processing. Inconel 625 counters with higher chromium for oxidizing environments and niobium for both solid-solution and precipitation strengthening.

Physical Properties Comparison

Property	C-276 (ASTM B575)	625 (ASTM B443)
Density	8.89 g/cm³ (0.321 lb/in³)	8.44 g/cm³ (0.305 lb/in³)
Melting Range	1325 – 1370°C	1290 – 1350°C
Modulus of Elasticity (RT)	205 GPa	207 GPa
Mean CTE (20–100°C)	11.2 µm/m·°C	12.8 µm/m·°C
Thermal Conductivity (RT)	11.1 W/m·K	9.8 W/m·K
Specific Heat (RT)	427 J/kg·K	410 J/kg·K
Electrical Resistivity (RT)	1.30 µΩ·m	1.29 µΩ·m

C-276 is about 5.3% denser than 625 due to its higher molybdenum and tungsten content. C-276 also has a slightly lower thermal expansion coefficient (11.2 vs 12.8 µm/m·°C at 100°C), which can be advantageous in applications with thermal cycling or tight clearances. The elastic moduli and thermal conductivities are similar enough that neither alloy has a decisive advantage on these properties alone.

Mechanical Properties Comparison

Inconel 625 is the stronger alloy, thanks to its niobium content which enables solid-solution strengthening. C-276 relies solely on solid-solution strengthening and is supplied only in the annealed condition. The yield strength difference is approximately 46% at room temperature.

Room Temperature Mechanical Properties:

Property	C-276 (Annealed)	625 (Annealed)	Comparison
Tensile Strength	≥ 690 MPa (100 ksi)	≥ 827 MPa (120 ksi)	625 is 20% stronger
Yield Strength (0.2%)	≥ 283 MPa (41 ksi)	≥ 414 MPa (60 ksi)	625 is 46% stronger +
Elongation in 50 mm	≥ 40%	≥ 30%	C-276 more ductile +
Hardness	≤ 100 HRB	≤ 25 HRC (~100 HRB)	Similar in annealed condition

Elevated Temperature Yield Strength (MPa, Typical):

Temperature	C-276	625 (Annealed)	Advantage
Room Temperature	~350	~480	625 +
200°C	~280	~380	625 +
400°C	~220	~320	625 +
600°C	~175	~300	625 +
800°C	~100	~140	625 (moderate) +

Inconel 625's higher strength is a significant advantage in pressure-containing components (valves, wellhead equipment) and high-temperature structural applications. However, C-276's higher ductility (40%+ elongation) is valuable for forming operations and expansion joints.

Corrosion Resistance: The Core of the Comparison

This is where the two alloys diverge most dramatically, and where the wrong choice can be costly.

Hydrochloric Acid (HCl)

HCl Condition	C-276 Performance	625 Performance	Winner
Dilute HCl (<5%), RT to 50°C	Excellent — <0.1 mm/yr	Fair — 0.5-2 mm/yr	C-276 Clear
Dilute HCl (<5%), boiling	Good — 0.1-0.5 mm/yr	Poor — >2 mm/yr	C-276 Clear
10% HCl, 30°C	Good — <0.5 mm/yr	Unsuitable	C-276 Clear
Concentrated HCl (>20%), RT	Fair to Good	Unsuitable	C-276 Clear

Bottom line on HCl: If your process involves hydrochloric acid at any concentration above trace levels, C-276 is the clear choice. Inconel 625 is not designed for HCl service.

Sulfuric Acid (H₂SO₄)

H2SO4 Condition	C-276 Performance	625 Performance	Winner
Dilute (<10%), up to 70°C	Excellent	Good	C-276 +
10-40%, up to 50°C	Good	Fair to Good	C-276 +
40-70%, RT	Fair	Fair	Tie ~
Concentrated (>80%), up to 50°C	Good	Excellent	625 +

In dilute (reducing) acid, C-276's Mo-W content dominates. In concentrated, oxidizing sulfuric acid, 625's higher chromium forms a more stable passive film. Dilute acid = C-276; concentrated acid = 625.

Seawater & Chloride Environments

Chloride Condition	C-276	625	Winner
Stagnant seawater, RT	Excellent	Excellent	Tie ~
Flowing seawater, RT to 50°C	Excellent	Excellent	Tie ~
Seawater crevice, 30°C+	Excellent	Good (crevice risk >40°C)	C-276 +
Acidified NaCl (pH 1-3), 50°C	Good	Fair to Good	C-276 +
FeCl3 critical pitting temp	>100°C	>85°C	C-276 +

For most seawater applications, both alloys perform excellently. Inconel 625 is more commonly used for offshore due to lower cost and higher strength. C-276 is reserved for the most demanding chloride applications: hot acidified brines, critical crevice-prone geometries, and deep sour gas wells with high chloride.

Other Corrosive Media

Media	C-276	625	Winner
Phosphoric Acid (H3PO4)	Excellent	Good	C-276 +
Nitric Acid (HNO3)	Good	Excellent	625 +
Organic Acids (Formic, Acetic)	Excellent	Excellent	Tie ~
Wet Chlorine Gas	Excellent	Good	C-276 +
Caustic Soda (NaOH) <50%	Good	Excellent	625 +
Sour Gas (H2S, NACE MR0175)	Approved	Approved	Both ~

Weldability Comparison

Welding Parameter	C-276	625
Overall Weldability	Excellent Easier	Very Good
Recommended Process	GTAW (TIG), GMAW (MIG), SMAW	GTAW (TIG), GMAW (MIG), SMAW
Filler Metal	ERNiCrMo-4 (AWS A5.14)	ERNiCrMo-3 (AWS A5.14)
Sensitization Risk	Effectively none (0.01% C) +	Low (Nb ties up C)
Post-Weld Heat Treat	Not required for corrosion	Not typically required
Hot Cracking Risk	Low	Low to Moderate
Interpass Temperature	≤ 150°C	≤ 150°C

C-276 has a reputation as one of the most weldable nickel alloys. Its ultra-low carbon content means it is virtually immune to intergranular corrosion in the as-welded condition — no post-weld heat treatment is needed. This is a significant advantage in field fabrication where heat treatment may be impractical.

Applications Comparison

Application Area	C-276	625
Chemical Reactors (HCl, H2SO4)	Excellent Preferred	Limited (HCl poor)
FGD Scrubbers	Excellent Preferred	Good
Offshore Wellhead Equipment	Good (cost limits use)	Excellent Preferred
Subsea Valve Bodies	Good	Excellent Preferred
Marine Exhaust / Scrubber	Excellent	Excellent
Pharmaceutical Equipment	Excellent Preferred	Good
Pulp & Paper Bleaching	Excellent Preferred	Good
Aerospace Ducting	Limited (weight, cost)	Excellent Preferred
Nuclear Components	Good	Excellent Preferred

Cost Comparison

Cost Factor	C-276	625
Relative Material Cost (per kg)	1.2-1.4 × (baseline = 625)	1.0 × Cheaper
Raw Material Drivers	High Mo (15-17%) + W (3-4.5%)	Mo (8-10%) + Nb (3-4%)
Typical Plate Price (2026 est.)	$45–65/kg	$35–50/kg
Availability	Good	Excellent Better
Lead Time (standard sizes)	2-4 weeks	1-3 weeks Faster

Selection Guide: Which Alloy Should You Choose?

• Choose C-276 when: Your process involves hydrochloric acid (any concentration), reducing sulfuric acid, wet chlorine gas, hot acidified chlorides, or FGD scrubber environments. You need the best possible as-welded corrosion resistance without post-weld heat treatment.
• Choose 625 when: Your application involves seawater (general), offshore oil & gas (NACE MR0175), oxidizing acids like nitric acid, or high-temperature structural components up to 650°C. You need higher mechanical strength than C-276 offers and cost is a significant factor.
• Consider both: For processes involving mixed acids or varying H₂SO₄ concentrations, run corrosion coupon tests under actual process conditions — the modest cost of testing is trivial compared to the cost of a wrong material selection.

Complete Comparison Summary

Property	C-276 (UNS N10276)	625 (UNS N06625)	Winner
Standard	ASTM B575	ASTM B443	—
Alloy Type	Ni-Cr-Mo-W	Ni-Cr-Mo-Nb	—
Density (g/cm³)	8.89	8.44	625 (lighter) +
RT Yield Strength (MPa)	≥ 283	≥ 414	625 +
RT Tensile (MPa)	≥ 690	≥ 827	625 +
Elongation (%)	≥ 40	≥ 30	C-276 +
PREN (Pitting)	> 65	~ 46	C-276 +
HCl Resistance	Excellent	Poor	C-276 +
HNO3 Resistance	Good	Excellent	625 +
As-Welded Corrosion	Excellent	Very Good	C-276 +
High-Temp Strength	Moderate	Good	625 +
Relative Cost	1.2-1.4 ×	1.0 ×	625 +
Availability	Good	Excellent	625 +

Frequently Asked Questions