Hastelloy C-276 (UNS N10276) and Hastelloy C-22 (UNS N06022) sit in the same nickel-chromium-molybdenum family, are covered by the same ASTM product standards, and are both sold as low-carbon, low-silicon alloys that need no post-weld heat treatment. Buyers therefore treat them as near-interchangeable. They are not. The choice between them is decided by one number - the ratio of chromium to molybdenum - and that ratio decides whether a plant survives oxidising service or reducing service.
What is the chemical difference between C-276 and C-22?
| Element | Hastelloy C-276 (N10276) | Hastelloy C-22 (N06022) |
|---|---|---|
| Nickel (Ni) | Balance | Balance |
| Chromium (Cr) | 14.5 - 16.5 | 20.0 - 22.5 |
| Molybdenum (Mo) | 15.0 - 17.0 | 12.5 - 14.5 |
| Tungsten (W) | 3.0 - 4.5 | 2.5 - 3.5 |
| Iron (Fe) | 4.0 - 7.0 | 2.0 - 6.0 |
| Cobalt (Co), max | 2.5 | 2.5 |
| Manganese (Mn), max | 1.0 | 0.50 |
| Vanadium (V), max | 0.35 | 0.35 |
| Silicon (Si), max | 0.08 | 0.08 |
| Carbon (C), max | 0.01 | 0.010 |
Read the second and third rows together, because that is the entire comparison. C-22 carries about 21% chromium against 15.5% in C-276; C-276 carries about 16% molybdenum against 13.5% in C-22. Chromium builds the oxide film that protects the alloy where the medium is oxidising. Molybdenum is what carries resistance to reducing acids and to localised attack. The two mechanisms are not substitutes for one another, which is why neither alloy covers the whole range of chemical service - only the middle of it, where both mechanisms are partly effective.
How does the chromium-molybdenum trade behave in real media?
| Medium | C-276 (N10276) | C-22 (N06022) |
|---|---|---|
| Wet chlorine, chlorine dioxide | Satisfactory | Preferred |
| Hypochlorite and bleach solutions | Satisfactory | Preferred |
| Ferric and cupric chloride | Satisfactory | Preferred |
| Nitric acid and mixed acid | Satisfactory | Preferred |
| Sulphuric acid, reducing conditions | Preferred | Satisfactory |
| Hydrochloric acid | Preferred | Satisfactory |
| Phosphoric acid, wet process | Preferred | Satisfactory |
| Organic acids with halide contamination | Satisfactory | Preferred |
| Flue-gas desulphurisation absorbers | Preferred | Preferred where chemistry swings |
| Nuclear fuel reprocessing | Satisfactory | Preferred |
The pattern is consistent: wherever the medium pushes the corrosion potential up - ferric or cupric ions, dissolved oxygen, nitrates, hypochlorite, wet chlorine - the high-chromium alloy wins. Wherever the medium is genuinely reducing - hydrochloric acid, reducing sulphuric acid - the high-molybdenum alloy wins and a B-family alloy such as B-3 wins more. When the medium swings between the two, or when nobody can guarantee what will be in the stream next month, C-22 is the safer specification because it has more of the element that handles the oxidising excursions.
What does PREN tell you about these two alloys?
| Alloy | Cr + 3.3 Mo | Cr + 3.3 (Mo + 0.5 W) |
|---|---|---|
| Hastelloy C-22 (N06022) | About 65 | About 70 |
| Hastelloy C-276 (N10276) | About 68 | About 75 |
| What the number means | Ranks chloride pitting resistance | Same ranking, weights tungsten |
Both alloys are quoted at PREN values in the mid-to-high 60s depending on which formula is used, and the formulas disagree about which of the two ranks higher. That is the useful lesson: PREN cannot separate these alloys, and it should not be used to choose between them. The decision is about oxidation state, not about a chloride-pitting ranking - PREN ignores chromium's role in oxidising media, which is precisely where C-22 earns its premium.
Which alloy is more thermally stable?
- Both alloys hold carbon at or below 0.01% and silicon at or below 0.08%, which is what allows welded structures to be used in the as-welded condition without sensitisation.
- C-22 additionally balances the Cr/Mo ratio so that nickel-chromium-molybdenum intermetallic phases precipitate more slowly, giving a wider thermal stability window.
- C-276 is the more prone of the two to secondary phase precipitation when held for long periods in the intermediate temperature range, which is why repeated weld repairs, hot forming or prolonged mid-temperature service favour C-22.
- Both resist oxidation to roughly 1040 C in oxidising atmospheres; pressure-retaining limits are set by the applicable construction code and are far lower, so never specify against the oxidation figure.
How are the two alloys welded?
| Item | C-276 | C-22 |
|---|---|---|
| Matching filler (AWS A5.14) | ERNiCrMo-4 | ERNiCrMo-10 |
| Post-weld heat treatment | Not required for corrosion service | Not required for corrosion service |
| Heat input | Keep moderate, control interpass temperature | Same discipline |
| Dissimilar joints / overlay | Choose a filler matched to or more highly alloyed than the weaker side | Same rule |
| Weld repair history | Acceptable, but repeated repairs favour C-22 | Better tolerance of repeated thermal cycles |
The filler rule matters more than the choice of alloy when dissimilar joints or cladding are involved: the weld metal has to be at least as corrosion resistant as the weaker parent metal, and the procedure should be backed by a qualified welding procedure record. Our C-276 welding guide sets out the shielding, heat input and interpass practice in detail.
Which standards and product forms apply?
| Product form | Specification |
|---|---|
| Plate, sheet and strip | ASTM B575 |
| Bar and rod | ASTM B574 |
| Seamless pipe and tube | ASTM B622 |
| Welded pipe | ASTM B619 |
| Welded tube | ASTM B626 |
| Forgings | ASTM B564 |
| Pipe fittings | ASTM B366 |
| Intergranular corrosion test | ASTM G28 |
This is the practical trap. C-276 and C-22 are covered by exactly the same set of ASTM product documents, so an order that quotes only ASTM B575 is ambiguous - it will be fulfilled in whichever alloy the mill has in stock. The UNS number must be on the order alongside the standard. Hangbo supplies both alloys in plate, sheet, strip, bar, seamless and welded tube, forgings and fittings; see the Hastelloy C-276 and Hastelloy C-22 data sheets for the supply range.
How do C-276 and C-22 compare on cost?
The two alloys are close in price because their nickel contents are similar and the difference is mostly chromium for molybdenum. C-22 is usually quoted at a modest premium. The premium is small compared with the cost of a wrong specification: an alloy chosen for the wrong oxidation state will fail long before its design life, and the failure is almost always in a weld heat-affected zone or a crevice, not in the middle of a plate.
Which alloy should you specify?
| If the duty is... | Specify | Reason |
|---|---|---|
| Oxidising media, wet chlorine or hypochlorite | C-22 | About 21% chromium builds the oxide film |
| Reducing acids - HCl, reducing H2SO4 | C-276 or B-3 | About 16% molybdenum handles reducing attack |
| Chemistry swings between oxidising and reducing | C-22 | Wider window against oxidising excursions |
| Mixed acid with oxidising impurities | C-22 | A B-family alloy would fail here |
| Hot chloride service above 100 C | C-22 | Better thermal stability |
| Repeated weld repair or hot forming | C-22 | Slower intermetallic precipitation |
| Stable reducing acid on a tight budget | C-276 | Best value where the medium is well defined |
| Hydrochloric acid with no oxidising species | B-3 | Purpose-built for reducing HCl |
One sentence summarises it: specify C-22 when you cannot guarantee the medium will stay reducing, and C-276 when you can. Where the medium contains oxidising impurities at all, a B-family alloy must not be used - see Hastelloy B-2 vs C-276 for that boundary, and C-276 vs Inconel 625 for the point at which a nickel-chromium alloy stops being the right family.
Frequently asked questions
Can C-22 fully replace C-276?
In most oxidising or mixed media yes - C-22 generally has better corrosion resistance and thermal stability. In purely reducing strong acids such as concentrated hydrochloric acid, the higher-molybdenum C-276 or the dedicated B-3 is usually more economical. Base the decision on medium composition, temperature and impurity ions.
Which filler metal is used for each alloy?
C-276 typically uses ERNiCrMo-4 and C-22 uses ERNiCrMo-10 to AWS A5.14. For dissimilar joints or overlay work, select a filler matched to or more highly alloyed than the weaker parent metal, backed by a qualified welding procedure record.
Why can't Hastelloy B alloys be used with oxidising impurities?
B-family alloys rely on a molybdenum-rich film that is stable only in reducing environments. With ferric or cupric ions, dissolved oxygen or nitric acid present, the corrosion potential shifts and that film fails, sharply raising the corrosion rate. Switch to chromium-bearing C-22 or C-2000.
Which alloy has the higher PREN?
It depends on the formula, which is the point. Cr + 3.3 Mo puts C-276 slightly ahead, and counting tungsten at half weight keeps it ahead. Either way PREN cannot separate these two alloys - what decides the outcome is the oxidation state of the medium, and PREN ignores chromium's role in oxidising media.
What is the risk of ordering only to ASTM B575?
A real one. C-276 and C-22 are covered by exactly the same ASTM product documents, so an order quoting only the standard number is ambiguous and will be filled with whichever alloy the mill has in stock. State the UNS number - N10276 or N06022 - alongside the standard.
Is post-weld heat treatment required?
Normally not. Both alloys hold carbon at or below 0.01% and silicon at or below 0.08%, so welded structures do not sensitise and can be used as welded. Heat treatment is applied only where a code or the design calls for stress relief, and long holds in the intermediate temperature range should be avoided.