Hastelloy C-22 (UNS N06022) and Hastelloy C-276 (UNS N10276) are both nickel-chromium-molybdenum alloys from the same corrosion-resistant family, and the choice between them decides how a chemical plant behaves in oxidizing media. C-22 carries roughly 21% chromium against 15.5% in C-276, while C-276 carries roughly 16% molybdenum against 13.5% in C-22. That single trade - chromium for molybdenum - explains almost every practical difference between the two alloys.
What is the chemical difference between C-22 and C-276?
| Element | Hastelloy C-22 (N06022) | Hastelloy C-276 (N10276) |
|---|---|---|
| Ni | Balance | Balance |
| Cr | 20.0-22.5 | 14.5-16.5 |
| Mo | 12.5-14.5 | 15.0-17.0 |
| W | 2.5-3.5 | 3.0-4.5 |
| Fe | 2.0-6.0 | 4.0-7.0 |
| Co (max) | 2.5 | 2.5 |
| V (max) | 0.35 | 0.35 |
| C (max) | 0.010 | 0.010 |
Chromium builds the passive oxide film that protects the alloy in oxidizing environments, and molybdenum strengthens resistance to reducing acids. C-22 is deliberately loaded toward chromium, C-276 toward molybdenum. Neither element substitutes for the other, so the two alloys are not interchangeable across the whole range of chemical service - only across the part of it where their two mechanisms overlap.
How do C-22 and C-276 compare in oxidizing and reducing media?
| Environment | C-22 (N06022) | C-276 (N10276) |
|---|---|---|
| Wet chlorine, chlorine dioxide | Preferred | Satisfactory |
| Hypochlorite, bleaching solutions | Preferred | Satisfactory |
| Ferric and cupric chloride | Preferred | Satisfactory |
| Hydrochloric acid | Satisfactory | Preferred |
| Sulfuric acid, reducing conditions | Satisfactory | Preferred |
| Acetic and formic acid | Preferred | Satisfactory |
| Seawater, chlorinated seawater | Preferred | Satisfactory |
| Oxidizing plus reducing mixed acids | Preferred | Satisfactory |
The pattern in that table is what process engineers should carry away. Where the environment is oxidizing, or fluctuates between oxidizing and reducing - chlorine handling plants, pulp and paper bleach lines, flue gas desulfurization with oxidizing additives, mixed acid pickling - C-22 is the lower-risk selection. Where the environment is firmly reducing, meaning hydrochloric or reducing sulfuric acid, C-276 uses its extra molybdenum to better effect.
Which alloy resists chloride pitting and crevice corrosion better?
Both alloys are tested for localized corrosion by immersion in acidified ferric chloride solution under ASTM G48, and the critical pitting temperature separates them. The higher chromium content of C-22 raises its resistance to pitting and crevice attack in chloride-bearing oxidizing solutions, which is why it is widely used in chlorinated seawater and hypochlorite service where C-276 would be vulnerable at flange faces and under deposits.
Crevice corrosion is the failure mode that distinguishes the two most clearly in practice. Pitting can be found by inspection; crevices hide under gaskets, lap joints and weld spatter. Selecting C-22 over C-276 for a design that cannot avoid crevices is a common and defensible decision, and it is usually cheaper than rebuilding the joint detail afterwards.
What are the mechanical properties and temperature limits?
| Property | Hastelloy C-22 | Hastelloy C-276 |
|---|---|---|
| Tensile strength (MPa) | 690 min | 690 min |
| Yield strength, 0.2% (MPa) | 283 min | 283 min |
| Elongation in 50 mm (%) | 40 min | 40 min |
| Hardness (HRB, typical) | 85-95 | 85-95 |
| Density (g/cm3) | 8.69 | 8.89 |
| Maximum continuous service temperature | approx. 650 C | approx. 650 C |
Mechanically the two alloys are close enough that a drawing usually does not need to change when one replaces the other, and the difference shows up in corrosion allowance and long-term elevated-temperature behaviour instead. Both are subject to precipitation of intermetallic phases - mu phase and sigma phase - if held in the 600 to 900 C band for long periods, which is why neither is a first choice for high-temperature structural service. Above about 540 C, Hastelloy X is normally the correct alloy instead.
How do the two alloys behave in welding and fabrication?
- Both are welded with matching nickel-base filler metals: ERNiCrMo-10 for C-22 and ERNiCrMo-4 for C-276.
- A C-276 weldment is often welded with ERNiCrMo-10 filler, because the higher-chromium weld metal resists as-welded corrosion attack at the weld toe better than matching composition.
- Neither alloy should be preheated, and interpass temperature should be kept low - below about 150 C - to limit segregation.
- Post-weld heat treatment is not used to restore corrosion resistance; it can promote precipitation, so the weld is left in the as-welded condition with the oxide removed and the surface pickled.
- Machining is slow and work-hardening is rapid, so light feeds and rigid tooling matter more than cutting speed.
The as-welded corrosion question is the one that separates experienced fabricators from the rest. Molybdenum and tungsten segregate during solidification, and the dendrite cores end up depleted in chromium. Because C-22 starts with more chromium, the depleted core still sits above the threshold needed for passivation, which is why weld procedure qualification for C-22 passes more easily in oxidizing test media. The full procedure is set out in the C-276 welding guide.
Which product forms and specifications apply to each alloy?
| Product form | Specification | Notes |
|---|---|---|
| Plate, sheet, strip | ASTM B575 | One standard covers C-22, C-276, C-4 and C-2000 - name the alloy |
| Rod, bar, forging stock | ASTM B574 | Same multi-alloy structure |
| Seamless tube | ASTM B622 | Both alloys listed in the same specification |
| Welded tube | ASTM B619 and ASTM B626 | B619 as-welded, B626 welded and annealed |
| Fittings | ASTM B366 | The WP grade designation carries the alloy |
| Forgings | ASTM B564 | Includes both nickel-chromium-molybdenum grades |
Here is the trap in the standards structure. Every specification above covers both alloys under a single number, so the specification alone never identifies the material. An order written as ASTM B575 plate is incomplete and can be satisfied with C-276, C-22, C-4 or C-2000 - four alloys with different corrosion performance. The alloy designation has to appear on the purchase order next to the standard.
How much does the C-22 premium cost?
C-22 is normally the more expensive of the two, carrying a premium over C-276 that reflects its higher chromium content and smaller production volume. The premium is usually a modest percentage of the finished component cost rather than a multiple, and it should be weighed against the cost of a premature failure in an inaccessible location.
That calculation changes with the application. For a large tank lining or a long run of ducting, the C-22 premium is significant and C-276 may be the better economic answer if the environment is genuinely reducing. For a pump casing, a valve trim set, or a flange pair that will sit in chlorinated seawater, the premium is small and the risk reduction is large. Compare the two on installed cost against failure consequence, not on material price per kilogram.
Which alloy should you order?
- Choose C-22 when the environment is oxidizing, when it alternates between oxidizing and reducing, or when crevices and deposits cannot be eliminated from the design.
- Choose C-276 when the environment is firmly reducing - hydrochloric acid, reducing sulfuric acid - or when the plant already standardises on C-276 and the specific duty suits it.
- Confirm the alloy on the order, not just the ASTM specification, because a single specification covers both materials.
- For high-temperature structural service above about 540 C, neither is correct - use Hastelloy X or an alloy selected for that duty.
Both alloys are stocked and supplied in plate, bar, seamless and welded tube, fittings and forgings. The comparison with other members of the family is worth reading alongside this one: C-2000 vs C-276 covers the newer composition, and B-2 vs C-276 covers the molybdenum-rich branch used for hydrochloric acid.
Continue reading
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Frequently asked questions
What is the main difference between Hastelloy C-22 and C-276?
C-22 contains about 21% chromium and 13.5% molybdenum while C-276 contains about 15.5% chromium and 16% molybdenum, and that trade makes C-22 the stronger choice in oxidizing media such as wet chlorine, hypochlorite and chlorinated seawater, and C-276 the stronger choice in strongly reducing acids such as hydrochloric acid.
Can C-22 replace C-276 in every application?
No, C-22 is not a universal upgrade - it is the better material wherever the environment is oxidizing or where crevices cannot be avoided, but in firmly reducing acid service the higher molybdenum content of C-276 gives it an advantage, so every substitution should be checked against the actual medium, temperature and chloride level.
Do C-22 and C-276 use the same filler metal?
They normally use matching nickel-base fillers, ERNiCrMo-10 for C-22 and ERNiCrMo-4 for C-276, but fabricators frequently weld C-276 with ERNiCrMo-10 because the higher-chromium weld metal resists as-welded attack at the weld toe better than matching composition.
Are C-22 and C-276 covered by the same ASTM standards?
Yes, and that is a common source of ordering errors: ASTM B575 covers plate for both alloys plus C-4 and C-2000, ASTM B574 covers bar, ASTM B622 covers seamless tube and ASTM B564 covers forgings, so the alloy designation must always be stated on the purchase order alongside the specification number.
Which alloy is more resistant to pitting in seawater?
C-22 is more resistant to chloride pitting and crevice corrosion because its higher chromium content builds a more stable passive film, which is why it is preferred for chlorinated seawater piping, hypochlorite service and any joint detail where a crevice cannot be eliminated by design.
Does C-22 cost more than C-276?
Typically yes, C-22 carries a premium over C-276 that reflects its higher chromium content and lower production volume, but the premium is usually a small percentage of installed component cost and should be weighed against the consequence of a failure in an inaccessible location rather than judged on material price per kilogram alone.