Hastelloy C-276 (UNS N10276) is one of the most universally corrosion-resistant wrought nickel alloys available for chemical processing, pollution control, and pulp and paper operations. Its nominal composition — approximately 57% Ni, 16% Mo, 15.5% Cr, and 5.5% Fe, with tungsten additions — gives it exceptional resistance to both oxidizing and reducing media simultaneously. This dual-characteristic resistance is unusual: most alloys excel in one environment but fail in the other. C-276 does not require this trade-off. This article provides a systematic technical review of Hastelloy C-276’s corrosion performance across the full range of environments it encounters in industrial service, with specific data to support alloy selection decisions.
Chemical Composition and the Metallurgical Basis for Corrosion Resistance
Hastelloy C-276 derives its corrosion resistance from a carefully balanced multi-element composition. Understanding why this alloy performs the way it does requires examining each element’s role.
Nominal composition of Hastelloy C-276 (UNS N10276):
| Element | ASME/ASTM Specified Range | Typical Heat |
|---|---|---|
| Nickel | Balance | ~57% |
| Molybdenum | 15.0–17.0% | 16.0% |
| Chromium | 14.5–16.5% | 15.5% |
| Iron | 4.0–7.0% | 5.5% |
| Tungsten | 3.0–4.5% | 3.8% |
| Cobalt | ≤2.5% | 0.2% |
| Manganese | ≤1.0% | 0.5% |
| Vanadium | ≤0.35% | 0.15% |
| Carbon | ≤0.010% | 0.001% |
| Silicon | ≤0.08% | 0.03% |
| Phosphorus | ≤0.040% | 0.010% |
| Sulfur | ≤0.030% | 0.002% |
The role of each element:
Molybdenum (15–17%): The primary driver of C-276’s reducing-acid resistance. Molybdenum forms stable oxychloride surface films in reducing environments, suppressing anodic dissolution in HCl, H₂SO₄, and HF. It also suppresses pit initiation by raising the pitting potential.
Chromium (14.5–16.5%): Provides oxidation resistance. Chromium forms a passive Cr₂O₃ film that resists oxidizing acids (HNO₃, FeCl₃) and atmospheric oxidation. The combination of high chromium (for oxidizing resistance) and high molybdenum (for reducing resistance) is what gives C-276 its unique dual-characteristic behavior.
Tungsten (3–4.5%): Acts synergistically with molybdenum to improve resistance to localized corrosion in chloride environments. Tungsten incorporation into the passive film increases its stability and reduces pit propagation rates.
Ultra-low carbon (≤0.010%, typically 0.001%): Critical for weld decay resistance. Low carbon prevents chromium carbide (Cr₂₃C₆) precipitation at grain boundaries, which would cause sensitization and intergranular corrosion — a common failure mode in as-welded C-276 components.
The PREN value: Pitting Resistance Equivalent Number (PREN) is calculated as: PREN = %Cr + 3.3×%Mo + 16×%N
For Hastelloy C-276 (15.5% Cr, 16% Mo, N ≈ 0): PREN = 15.5 + 3.3×16 + 16×0 = 68.3
This places C-276 well above super duplex stainless steels (PREN ~35–40) and significantly above 316L stainless (PREN ~24). The high PREN reflects C-276’s excellent resistance to chloride-induced pitting and crevice corrosion.
Pitting and Crevice Corrosion Resistance
Critical Pitting Temperature (CPT)
Pitting corrosion in C-276 is controlled by two primary factors: chloride concentration and temperature. The Critical Pitting Temperature (CPT) is the temperature above which pits will initiate and propagate in a given environment.
For Hastelloy C-276 in 6% FeCl₃ solution (per ASTM G48 Practice C):
- CPT: >85°C (185°F) — pitting initiation suppressed even at elevated temperatures
- For comparison: 316L stainless steel CPT is approximately 10–15°C; 904L is approximately 25–30°C; 254 SMO super austenitic is approximately 45–50°C
In practical process environments:
| Environment | Maximum Service Temperature for C-276 |
|---|---|
| Aerated seawater (static) | 40–50°C (with CPA management) |
| Stagnant seawater | 30–40°C |
| 10% FeCl₃ solution | 50–60°C |
| 1% HCl,aerated | 60–70°C |
| 5% HCl, deaerated | 80–90°C |
Critical Crevice Corrosion Temperature (CCCT)
Crevice corrosion is more aggressive than pitting because it is self-propagating: the crevice geometry creates an oxygen-depleted, low-pH environment that accelerates metal dissolution. The Critical Crevice Corrosion Temperature (CCCT) for C-276 in 6% FeCl₃ per ASTM G48 Practice F is approximately 25–35°C (77–95°F) — substantially lower than the CPT due to the galvanic acceleration within the crevice.
This distinction is critical for design: even if the process temperature is below the CPT, if the component has crevices (under gaskets, bolt heads, deposit accumulation, flange faces), the effective corrosion temperature is governed by the CCCT.
Design implications for minimizing crevice corrosion:
- Specify spiral-wound or ring gaskets with adequate compression
- Avoid dead-legs, sediment traps, and deposit-prone geometries
- Use weld overlay or cladding rather than solid C-276 for very large vessels where economics favor cladding
- Ensure complete drainage and avoid stagnant conditions
Performance in Specific Acid Environments
Hydrochloric Acid (HCl)
Hastelloy C-276 is the material of choice for HCl service across a wide concentration and temperature range. Unlike 316L stainless (rapidly attacked above 0.1% HCl at room temperature) or even 904L (limited to ~5% at 50°C), C-276 handles concentrated HCl at elevated temperatures.
General corrosion rates in HCl (mm/y, stirred conditions):
| HCl Concentration | 20°C | 50°C | 70°C | 100°C |
|---|---|---|---|---|
| 1% | <0.01 | <0.01 | 0.02 | 0.10 |
| 5% | <0.01 | 0.02 | 0.08 | 0.35 |
| 10% | <0.01 | 0.05 | 0.20 | 0.80 |
| 20% | 0.01 | 0.10 | 0.40 | 1.5 |
| 37% (concentrated) | 0.05 | 0.30 | 0.90 | 2.5 |
Notes: Rates are for deaerated solutions. Aeration significantly accelerates corrosion. For aerated HCl, C-276 performance degrades substantially above 5% concentration. In practice, nitrogen blanketing of HCl storage and process vessels is standard when using C-276.
The upper practical limit for Hastelloy C-276 in HCl is approximately 10% concentration at boiling temperature, or 20% at 70°C, with full deaeration. Above these limits, Hastelloy C-22 or C-2000 should be evaluated.
Sulfuric Acid (H₂SO₄)
C-276 performs well in sulfuric acid, though its performance is more concentration-sensitive than its HCl performance. Sulfuric acid is oxidizing at high concentrations (>85%) and reducing at intermediate concentrations (10–85%).
General corrosion rates in H₂SO₄ (mm/y, deaerated, unstirred):
| H₂SO₄ Concentration | 30°C | 60°C | 80°C | Boiling |
|---|---|---|---|---|
| 5% | <0.01 | <0.01 | 0.02 | 0.10 |
| 10% | <0.01 | 0.01 | 0.05 | 0.20 |
| 25% | 0.01 | 0.02 | 0.08 | 0.40 |
| 50% | 0.01 | 0.03 | 0.10 | 0.50 |
| 75% | 0.02 | 0.05 | 0.15 | 0.60 |
| 96% (fuming) | 0.10 | 0.50 | 1.2 | — |
C-276 is suitable for sulfuric acid service from dilute (~5%) to approximately 50% concentration at temperatures up to 80°C. For high-temperature (>80°C) concentrated sulfuric acid (>70%), evaluate Hastelloy D-205 or specialized silicon brass alloys. For hot (>100°C) 98% sulfuric acid, Hastelloy B-3 or C-2000 may be more appropriate depending on aeration.
Phosphoric Acid (H₃PO₄)
Phosphoric acid is produced by two routes — wet-process (from phosphate rock, containing HF, F⁻, and SiF₆²⁻ as impurities) and thermal-process (high purity). C-276 performs well in both, though wet-process acid is significantly more corrosive due to fluoride complexes.
General corrosion rates in H₃PO₄ (mm/y):
| H₃PO₄ Concentration | 60°C | 85°C | 100°C |
|---|---|---|---|
| 30% (thermal process) | <0.01 | <0.01 | 0.02 |
| 54% (merchant grade) | <0.01 | 0.02 | 0.05 |
| 85% (thermal, hot) | 0.02 | 0.05 | 0.15 |
| 85% (wet process, with F⁻) | 0.10 | 0.30 | 0.80 |
C-276 is widely used in phosphoric acid cooling coils, evaporators, and piping for wet-process plants, where its fluoride resistance is essential. 316L stainless steel fails rapidly in wet-process acid; C-276 is the baseline solution.
Hydrofluoric Acid (HF)
C-276 has moderate resistance to hydrofluoric acid. Unlike Hastelloy B alloys (which are designed specifically for HF service), C-276’s resistance to HF is limited by the fluoride ion’s aggressive attack on chromium oxide films.
Practical limits for C-276 in HF:
- Concentration: up to approximately 10% HF
- Temperature: up to approximately 50–60°C
- Aeration: must be deaerated
- Above these limits: Hastelloy B-3 (UNS N10675) or Monel 400 are preferred
Mixed Acids and Process Streams
The most demanding applications for C-276 are mixed-acid environments — where two or more corrosive species act simultaneously. These conditions typically defeat single-alloy solutions:
HCl + HF mixtures: C-276 handles concentrations up to approximately 5% HCl + 1% HF at 60°C. Above these levels, Hastelloy B-3 is required.
H₂SO₄ + HCl mixtures: C-276 is suitable for mixtures containing up to approximately 15% total acidity at 60°C. For highly aggressive mixed acid streams (e.g., pickling baths), C-22 or C-2000 outperform C-276.
HNO₃ + HF (mixed acid for nuclear fuel reprocessing): C-276 is generally unsuitable; Zircaloy or specialized austenitic stainless steels are preferred.
Oxidizing vs. Reducing Environments: Where C-276 Excels and Where It Doesn’t
One of C-276’s defining advantages is its balanced resistance to both oxidizing and reducing media. However, “balanced” does not mean “infinite.” There are environments where C-276 performs exceptionally and others where alternative alloys are superior.
C-276 performs exceptionally in:
- Deaerated HCl (all concentrations at moderate temperatures)
- Deaerated H₂SO₄ (up to ~50% at 80°C)
- Wet-process phosphoric acid (fluoride-containing)
- Seawater (with CP and low-temperature operation)
- Pulp and paper bleach plant environments
- SO₂ scrubber environments
- Acetic acid and most organic acids
C-276 has limited performance in:
- Aerated HCl (corrosion accelerates significantly)
- High-temperature (>200°C) strong oxidizers (HNO₃, hot concentrated H₂SO₄)
- Aqua regia (hot, unsuited to any single Ni alloy)
- Ferric chloride (above ~50°C, C-22 preferred)
- Cupric chloride (stress corrosion cracking risk)
- Nitric acid (316L or 904L more cost-effective)
The ferric chloride test (ASTM G28 Practice A): This is the standard test for evaluating C-276’s resistance to oxidizing chlorides. C-276 shows corrosion rates < 0.25 mm/y in the standard test. C-22 (UNS N06022) shows even better performance in this test — approximately 5× better than C-276 — which is why C-22 is often specified for ferric chloride and mixed oxidizer-chloride environments where C-276 would be marginal.
Stress Corrosion Cracking (SCC) Resistance
Hastelloy C-276 is generally resistant to chloride stress corrosion cracking (SCC) due to its austenitic nickel-base microstructure and low stacking fault energy. Unlike 304/316 stainless steels, which crack in tensile-stressed chloride environments above approximately 60°C, C-276 maintains its integrity to much higher temperatures.
SCC performance in chloride solutions:
- In 22% NaCl at 100°C: C-276 shows no SCC failure under U-bend stress specimens after 1,000 hours. 304 stainless fails within 24 hours.
- Upper temperature limit for C-276 in chloride environments: approximately 200°C for unstressed components; for stressed components in standard service, temperatures above 150°C should be evaluated carefully.
Where SCC risk remains for C-276:
- Low-pH chloride brines (acid chloride, HCl vapor condensate)
- Simultaneous presence of chloride + oxygen + tensile stress
- Crevices creating concentration cells that produce acidic conditions
- Weld residual stresses in heat-affected zones
For maximum SCC resistance in chloride service, specify C-276 in the solution-annealed condition (1,121–1,177°C, rapid cool), which provides the softest, most ductile microstructure. Cold work increases yield strength but can reduce SCC resistance.
Comparison with Hastelloy C-22 and Inconel 625
For engineers evaluating C-276 against alternative alloys, the most common comparison is C-22 (UNS N06022) and Inconel 625 (UNS N06625).
Hastelloy C-276 vs. C-22:
C-22 has higher chromium (20.5–22.5% vs. 14.5–16.5%) and no tungsten, giving it superior resistance to oxidizing environments (particularly ferric chloride and mixed oxidizer-chlorides). C-22 also has lower molybdenum, making it slightly less resistant in strong reducing acids.
Selection guide:
- Choose C-276 for: HCl, deaerated H₂SO₄, wet-process phosphoric acid, seawater, mixed reducing-oxidizing media at moderate temperatures
- Choose C-22 for: ferric chloride, mixed oxidizer-chlorides, seawater with oxidizing biocide treatment, aqua regia
Hastelloy C-276 vs. Inconel 625:
Inconel 625 has 3.15–4.15% niobium, which C-276 lacks. Niobium improves mechanical strength and weldability but slightly reduces corrosion resistance in some environments. Inconel 625 is the right choice for aerospace and high-temperature (>500°C) structural applications; C-276 is the right choice for concentrated acid environments.
| Property | C-276 | Inconel 625 |
|---|---|---|
| PREN (approx.) | 68 | 53 |
| HCl max service temp (deaerated) | 80–100°C | 60–70°C |
| Crevice corrosion resistance | Superior | Good |
| Yield strength (solution annealed) | 310–350 MPa | 380–450 MPa |
| Weldability | Excellent (ERNiCrMo-4) | Good (ERNiCrMo-3) |
| High temperature (>600°C) | Good to 650°C | Excellent to 900°C |
Common Application Scenarios
Based on corrosion performance data, C-276 is typically specified for:
- Chemical processing equipment: Reactor vessels, heat exchangers, columns, and piping for HCl, H₂SO₄, and phosphoric acid service
- Pollution control: Flue gas desulfurization (FGD) scrubber internals, waste acid neutralization systems
- Pulp and paper: Bleach plant washers, digesters, and piping in chlorine dioxide bleaching environments
- Pharmaceutical: High-purity chemical reactors where corrosion contamination is unacceptable
- Seawater: Condensate coolers, saltwater piping, offshore platform process systems (with cathodic protection for crevice management)
- Metal cleaning: Acid pickling racks, acid circulation systems, electrolytic cleaning cells
Five Key Facts About Hastelloy C-276 Corrosion Resistance
- PREN ~68 gives C-276 outstanding chloride pitting resistance — among the highest of any commercial wrought alloy, significantly above super duplex stainless steels (PREN ~40) and austenitic stainless grades.
- Dual-characteristic resistance comes from the Cr-Mo-W balance — high chromium (15.5%) for oxidizing environments + high molybdenum (16%) for reducing environments + tungsten (3.8%) for localized corrosion suppression.
- Aeration is the single biggest practical factor reducing C-276 HCl performance — nitrogen blanketing and deaerated service can double or triple the effective temperature/ concentration range.
- Crevice corrosion governs service temperature limits more than pitting — always specify gasket materials, bolt loading, and drainage to minimize crevice geometry. The CCCT is typically 50–60°C lower than the CPT.
- For mixed oxidizer-chloride environments (FeCl₃, CuCl₂), C-22 outperforms C-276 — the higher chromium in C-22 provides better resistance to oxidizing chloride attack where C-276 is marginally resistant.
Related articles: How to Inspect Nickel Alloy Materials: PMI and Beyond
