Hastelloy C-276 (N10276) vs Hastelloy C-22 (N06022): Corrosion Resistance
Hastelloy C-276 (UNS N10276, Russian equivalent KhN65MVU / NS3304) and Hastelloy C-22 (UNS N06022, Russian equivalent KhN70MV-VI / NS3308) are two leading nickel-molybdenum-chromium alloys for extremely aggressive corrosive environments. C-276 has been the historical "gold standard" for general corrosion resistance with over 50 years of service. C-22, developed by Haynes International in the 1980s, was designed to overcome the limitations of C-276 — primarily crevice corrosion in oxidizing chloride environments and reduced corrosion resistance in the heat-affected zone (HAZ) of weld joints.
1. Chemical Composition
| Element | Hastelloy C-276 (UNS N10276) | Hastelloy C-22 (UNS N06022) | Difference |
|---|---|---|---|
| Ni | ≥57.0 | ≥56.0 | ≈ Same |
| Cr | 14.5–16.5 | 20.0–22.5 | C-22: +6% Cr |
| Mo | 15.0–17.0 | 12.5–14.5 | C-276: +3% Mo |
| W | 3.0–4.5 | 2.5–3.5 | C-276: +1% W |
| Fe | 4.0–7.0 | 2.0–6.0 | ≈ Same |
| C | ≤0.010 | ≤0.010 | Same |
| V | ≤0.35 | ≤0.35 | Same |
| Co | ≤2.5 | ≤2.5 | Same |
| Mn | ≤1.0 | ≤0.50 | C-22 slightly tighter |
| Si | ≤0.08 | ≤0.08 | Same |
| P | ≤0.04 | ≤0.02 | C-22 tighter |
| S | ≤0.03 | ≤0.02 | C-22 tighter |
Key difference: C-22 contains significantly more chromium (22% vs 16%) and less molybdenum (13.5% vs 16%). This is a strategic shift: higher chromium gives better resistance in oxidizing chloride media (wet Cl₂, HClO, FeCl₃), while molybdenum excels in reducing media (HCl, H₂SO₄ without oxidizers). C-276 is optimized for reducing environments; C-22 is optimized for universal (oxidizing + reducing) service.
2. Mechanical Properties
| Property | Hastelloy C-276 | Hastelloy C-22 |
|---|---|---|
| Yield strength σ₀.₂, MPa | ≥283 | ≥310 |
| Tensile strength σᵦ, MPa | ≥690 | ≥690 |
| Elongation δ₅, % | ≥40 | ≥45 |
| Hardness, HRB | ≤100 | ≤100 |
| Elastic modulus E, GPa | 205 | 206 |
The mechanical properties of both alloys are close. Both are solid-solution strengthened (not precipitation hardened), so strength is governed by composition and cold work. C-22 has slightly higher yield strength and elongation.
3. Physical Properties
| Property | Hastelloy C-276 | Hastelloy C-22 |
|---|---|---|
| Density, g/cm³ | 8.89 | 8.61 |
| Melting point, °C | 1325–1370 | 1300–1380 |
| Coeff. of thermal expansion (20–100°C), 10⁻⁶/°C | 12.4 | 12.4 |
| Thermal conductivity (100°C), W/(m·K) | 10.2 | 10.8 |
| Electrical resistivity, µΩ·m | 1.30 | 1.25 |
4. Corrosion Resistance — Detailed Comparison
4.1 Pitting Resistance Equivalent (PREN)
PREN = %Cr + 3.3×%Mo + 16×%N (for Ni alloys a modified formula including W is used)
PREN_W = %Cr + 3.3×(%Mo+%W/2) + 16×%N
| Alloy | Cr | Mo | W | PREN_W |
|---|---|---|---|---|
| Hastelloy C-276 | 16 | 16 | 4 | ≈72 |
| Hastelloy C-22 | 22 | 13.5 | 3 | ≈73 |
The calculated PREN_W values are close (≈72 vs ≈73), but real pitting resistance differs by environment: C-22 wins in oxidizing chlorides, C-276 in reducing media.
4.2 Critical Pitting (CPT) and Crevice (CCT) Temperatures
Per ASTM G48 (6% FeCl₃):
| Parameter | Hastelloy C-276 | Hastelloy C-22 | Advantage |
|---|---|---|---|
| CPT (pitting), °C | ≥110 | ≥120 | C-22 (+10°C) |
| CCT (crevice), °C | ≈85 | ≈120 | C-22 (+35°C) |
Key result: C-22 beats C-276 by 35°C on critical crevice corrosion temperature — the most significant C-22 advantage. Crevice corrosion is the leading cause of equipment failure in chloride environments.
4.3 Corrosion Rate in Standard Test Media
| Medium (ASTM test) | C-276, mm/year | C-22, mm/year | Advantage |
|---|---|---|---|
| G28A: 50% H₂SO₄ + Fe₂(SO₄)₃ (24 h, boiling) | <0.10 | <0.05 | C-22 |
| G28B: 23% H₂SO₄ + 1.2% HCl + FeCl₃ + CuCl₂ (24 h) | 0.43 | 0.15 | C-22 |
| 10% HCl (boiling) | 0.25 | 0.55 | C-276 |
| 10% H₂SO₄ (boiling) | 0.20 | 0.25 | C-276 (slight) |
| Wet Cl₂ (dry and wet gas) | Good | Excellent | C-22 |
| Pitting in 10% FeCl₃ (50°C) | Resistant | Resistant | Both |
4.4 Resistance in Specific Industrial Media
| Medium | Hastelloy C-276 | Hastelloy C-22 | Recommendation |
|---|---|---|---|
| Hydrochloric acid (HCl, reducing) | Excellent (to 15% boiling) | Good (to 10% boiling) | C-276 |
| Wet chlorine (Cl₂ gas) | Good | Excellent | C-22 |
| Hypochlorous acid (HClO) | Good | Excellent | C-22 |
| Sulphuric acid + chlorides | Good | Excellent | C-22 |
| Hydrofluoric acid (HF) | Excellent (to 5%) | Good (to 3%) | C-276 |
| FGD scrubbers | Good | Excellent | C-22 |
| Seawater (crevice) | Good (CCT ≈85°C) | Excellent (CCT ≈120°C) | C-22 |
| Organic acids + chlorides | Good | Excellent | C-22 |
4.5 Weld Joint Corrosion Resistance
One of C-22's key advantages is its better corrosion resistance in the HAZ of weld joints. After welding, C-276 can precipitate secondary phases (mu phase and P phase) in the HAZ at 600–1000°C, locally reducing corrosion resistance, especially in oxidizing media. C-22, with its optimized composition (Cr+Mo+W ≈ 39.5%), shows far less tendency to precipitate secondary phases and better HAZ resistance.
| Parameter | C-276 (HAZ) | C-22 (HAZ) |
|---|---|---|
| G28A corrosion rate (welded), mm/year | <0.10 | <0.05 |
| G28B corrosion rate (welded), mm/year | 0.43 | 0.15 |
| Mu-phase precipitation tendency | Medium | Low |
| Post-weld heat treatment required? | No | No |
5. Weldability
| Parameter | Hastelloy C-276 | Hastelloy C-22 |
|---|---|---|
| Welding methods | TIG, MIG, MMA, SAW, PAW | TIG, MIG, MMA, SAW, PAW |
| Welding wire | ERNiCrMo-4 (AWS A5.14) | ERNiCrMo-10 (AWS A5.14) |
| Preheat | Not required | Not required |
| Post-weld heat treatment | Not required | Not required |
| Hot-cracking tendency | Very low | Very low |
| HAZ corrosion resistance | Good | Excellent |
6. Applications
| Industry | Hastelloy C-276 | Hastelloy C-22 |
|---|---|---|
| Chemical processing | Reactors for reducing acids, HCl equipment | Scrubbers, reactors for oxidizing chloride media |
| FGD (flue gas desulfurization) | Good resistance | Preferred choice |
| Pharmaceutical | Reactors, heat exchangers | Preferred for aggressive media |
| Oil & gas (sour service) | Wellhead tie-ins, piping | Valves, flanges (crevice) |
| Pulp & paper | Bleaching equipment | Better in chlorine media |
| Marine engineering | Pipe, fasteners | Crevice joints, flanges |
| Nuclear (waste) | Storage vessels | High-level waste vessels |
7. Cost
| Factor | Hastelloy C-276 | Hastelloy C-22 |
|---|---|---|
| Relative price (3 mm sheet) | 100% (baseline) | 115–125% |
| Main cost drivers | High Mo (16%), W (4.5%) | High Cr (22%), Mo+W |
| Welding wire | ERNiCrMo-4 | ERNiCrMo-10 |
| TCO (total cost of ownership) | Lower in reducing media | Lower in oxidizing chloride media |
C-22 costs 15–25% more than C-276 at initial purchase, but on a total cost of ownership (TCO) basis in oxidizing chloride environments C-22 is often more economical because of its longer service life, especially in crevice-prone designs (flanges, gaskets, deposits).
8. Conclusion and Recommendations
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| Criterion | Choose C-276 | Choose C-22 |
|---|---|---|
| Reducing media (HCl, HF) | ✓ Optimal | Good, but trails |
| Oxidizing chlorides (Cl₂, HClO, FeCl₃) | Good | ✓ Optimal |
| Crevice corrosion critical | Good (CCT ≈85°C) | ✓ Excellent (CCT ≈120°C) |
| Welded structures in chlorides | Good | ✓ Better HAZ resistance |
| FGD / scrubbers | Good | ✓ Preferred |
| Limited budget | ✓ Cheaper by 15–25% | More expensive |
| Versatility (oxidizing + reducing) | Good | ✓ More versatile |
Summary: If your medium is predominantly reducing (hydrochloric acid, hydrofluoric acid), C-276 remains the best choice at a lower price. If the medium is oxidizing-chloride (wet chlorine, hypochlorous acid, FeCl₃, FGD scrubbers), or if your design contains crevices/gaps (flanges, gaskets), or weld-joint corrosion resistance is critical, C-22 delivers a substantial durability advantage despite its higher initial cost.
9. Frequently Asked Questions (FAQ)
1. What is the density of Hastelloy C-276 and Hastelloy C-22?
The density of Hastelloy C-276 is 8.89 g/cm³ and Hastelloy C-22 is 8.61 g/cm³. The difference (~3.2%) comes from C-276's higher nickel and molybdenum content.
2. Which alloy is more expensive, Hastelloy C-276 or C-22?
Hastelloy C-22 is typically 15–25% more expensive than C-276 due to its higher chromium (22% vs 16%) and tungsten addition. However, on a total cost of ownership basis C-22 can be more economical in wet-chlorine service thanks to better resistance.
3. Which alloy resists crevice corrosion better?
Hastelloy C-22 vastly outperforms C-276 in crevice corrosion resistance. The critical crevice temperature (CCT) per ASTM G48 is about 120°C for C-22 and about 85°C for C-276. This is driven by C-22's higher chromium (22% vs 16%) and its molybdenum+tungsten content.
4. Which alloy is more resistant in wet chlorine?
Hastelloy C-22 shows better resistance in wet chlorine (Cl₂) and hypochlorous acid (HClO) environments. This is due to its higher chromium (22% vs 16%), which forms a more stable Cr₂O₃ oxide film.
5. What is the chemical composition of Hastelloy C-276 and C-22?
C-276 (UNS N10276): Ni ≥57, Cr 14.5–16.5, Mo 15–17, W 3–4.5, Fe 4–7, C ≤0.01. C-22 (UNS N06022): Ni ≥56, Cr 20–22.5, Mo 12.5–14.5, W 2.5–3.5, Fe 2–6, C ≤0.01. The key difference: C-22 contains significantly more Cr (+6%) and less Mo (−3%).
6. What standards cover Hastelloy C-276 and C-22?
C-276: ASTM B575 (sheet), B622 (pipe), B619 (welded pipe), B574 (bar), UNS N10276, Russian equivalent KhN65MVU (NS3304). C-22: ASTM B575, B622, B619, B574, UNS N06022, Russian equivalent KhN70MV-VI (NS3308), AMS 5890 (bar).
7. Can both alloys be welded without post-weld heat treatment?
Yes. Both alloys have excellent weldability and do not require post-weld heat treatment. C-276 is welded with ERNiCrMo-4 wire, C-22 with ERNiCrMo-10. Both resist hot cracking and intergranular corrosion in the HAZ.
8. How corrosion-resistant are the weld joints?
C-22 weld joints have better HAZ corrosion resistance. In ASTM G28A testing C-22 shows a corrosion rate below 0.05 mm/year in the as-welded condition, versus below 0.10 mm/year for C-276. This reflects C-22's more uniform elemental distribution in the HAZ.
9. What is the maximum service temperature for both alloys?
For corrosion service both alloys are recommended up to 400°C for long-term use. Above 600°C secondary phases begin to precipitate, reducing corrosion resistance. For high-temperature strength, C-276 can operate up to about 1090°C in oxidizing environments.
10. In what forms are Hastelloy C-276 and C-22 supplied?
Both alloys are available as sheet (0.5–50 mm), plate (6–100 mm), bar (Ø6–250 mm), seamless pipe (Ø6–200 mm), welded pipe, wire (Ø0.1–8 mm), forgings, flanges and fittings. Welding wire: ERNiCrMo-4 (C-276), ERNiCrMo-10 (C-22).