Hastelloy C276 vs C22
Hastelloy C-276 and Hastelloy C-22 are nickel-chromium-molybdenum alloys engineered for the most aggressive chemical environments encountered in process piping. Both resist a wide spectrum of oxidizing and reducing acids, chloride-induced pitting, and stress corrosion cracking. C-22 (UNS N06022) was developed as an improvement over C-276 (UNS N10276), offering better resistance to oxidizing media and improved weld-zone performance.
Definition and Scope
C-276 (UNS N10276) is the established workhorse alloy of the Hastelloy C-family, introduced in the 1960s. It contains approximately 57% nickel, 15% chromium, 16% molybdenum, and 4% tungsten. Its very high molybdenum content provides exceptional resistance to reducing acids (hydrochloric, sulfuric) and chloride-induced localized corrosion.
C-22 (UNS N06022) is the more versatile successor, with approximately 56% nickel, 22% chromium, 13% molybdenum, and 3% tungsten. The higher chromium content improves resistance to oxidizing conditions while maintaining strong reducing-acid performance.
When Each Alloy Is Used
C-276 remains widely specified for flue gas desulfurization (FGD) systems, hydrochloric acid processing, phosphoric acid production, and chlorinated organic chemical manufacturing. It is the default “universal” corrosion-resistant alloy for strongly reducing environments.
C-22 is preferred when the service environment alternates between oxidizing and reducing conditions, or when weld-zone corrosion resistance is critical. It is the standard choice for chemical process vessels, waste treatment, pharmaceutical production, and environments containing ferric or cupric chlorides.
Specifications Table
| Property | C-276 (N10276) | C-22 (N06022) |
|---|---|---|
| Nickel | Balance (~57%) | Balance (~56%) |
| Chromium | 14.5-16.5% | 20.0-22.5% |
| Molybdenum | 15.0-17.0% | 12.5-14.5% |
| Tungsten | 3.0-4.5% | 2.5-3.5% |
| Iron (max) | 4.0-7.0% | 2.0-6.0% |
| Carbon (max) | 0.01% | 0.015% |
| PREN (approximate) | ~69 | ~65 |
| Tensile strength (min) | 690 MPa (100 ksi) | 690 MPa (100 ksi) |
| Yield strength (min) | 283 MPa (41 ksi) | 310 MPa (45 ksi) |
| Pipe spec | ASTM B622 | ASTM B622 |
| Fitting spec | ASTM B366 | ASTM B366 |
| Forging spec | ASTM B564 | ASTM B564 |
| Max service temp | 677°C (1250°F) | 677°C (1250°F) |
| Relative cost | Very high | Very high (similar to C-276) |
Comparison: Corrosion Performance
| Environment | C-276 | C-22 |
|---|---|---|
| Hydrochloric acid (reducing) | Excellent | Very good |
| Sulfuric acid (reducing) | Excellent | Good |
| Ferric chloride (oxidizing) | Good | Excellent |
| Hypochlorite solutions | Good | Excellent |
| Mixed acid (HNO3 + HCl) | Good | Excellent |
| Wet chlorine gas | Good | Very good |
| Flue gas condensate (SO2/HCl) | Excellent | Excellent |
| Seawater (crevice) | Excellent | Excellent |
| Acetic/formic acid | Excellent | Excellent |
Key Difference: Oxidizing vs. Reducing
C-276 excels in strongly reducing environments due to its higher molybdenum content (16% vs. 13%). C-22 outperforms in oxidizing environments due to its higher chromium content (22% vs. 15.5%). For environments that fluctuate between oxidizing and reducing, C-22 is the safer choice because its higher chromium provides a broader corrosion resistance envelope.
Weldability
C-22 was specifically designed to resist preferential weld-zone corrosion. The higher chromium content creates a more stable passive film in the as-welded condition, eliminating the need for post-weld annealing in most applications. C-276 welds can be susceptible to preferential corrosion in the weld heat-affected zone in certain oxidizing environments unless solution annealed after welding.
For more information on nickel alloy piping specifications, see the full guide to nickel alloy pipes.
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