Material comparison
Two points of molybdenum traded for six points of chromium — and a different set of acids as a result.
Both are nickel–chromium–molybdenum alloys from the Hastelloy C family, both are excellent in aggressive chemical service, and their price is similar. The difference is the balance between chromium and molybdenum, and that balance decides which acids each one handles.
| Element | C-276 (N10276) | C-22 (N06022) |
|---|---|---|
| Nickel | balance, approx. 57% | balance, approx. 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 | 4.0 – 7.0% | 2.0 – 6.0% |
| Density | 8.89 g/cm³ | 8.69 g/cm³ |
C-22 trades about two points of molybdenum for about six points of chromium. That single shift is the whole story.
Molybdenum resists reducing conditions — hydrochloric acid, dilute sulphuric acid, media without dissolved oxygen. Chromium resists oxidising conditions — nitric acid, ferric and cupric chloride solutions, oxidising mixed acids.
| Environment | Better suited |
|---|---|
| Hydrochloric acid | C-276 |
| Dilute sulphuric acid | C-276 |
| Nitric acid and oxidising mixed acids | C-22 |
| Ferric / cupric chloride | C-22 |
| Wet chlorine, hypochlorite | C-22 |
| Phosphoric acid | comparable |
| Flue gas desulphurisation | comparable, C-276 more widely referenced |
| Mixed or variable process streams | C-22 generally more forgiving |
C-22 is the more tolerant of the two in welding. Its composition was developed partly to reduce the risk of grain-boundary precipitation in the heat-affected zone, so as-welded corrosion performance is closer to that of the base metal. For multi-pass welds in heavy sections, or where post-weld heat treatment is not practical, that is a real advantage.
For C-276, interpass temperature should be controlled and heat input kept moderate. Matched filler is ERNiCrMo-4 for C-276 and ERNiCrMo-10 for C-22.
On availability. C-276 has been in service longer and is the more widely stocked of the two across Europe, particularly in plate and pipe. For smaller quantities and shorter lead times that often decides the matter before the chemistry does.
Not a drop-in replacement. C-22 is better in oxidising media and more tolerant in welding, while C-276 is better in reducing acids such as hydrochloric. Which is preferable depends entirely on the process stream.
Both exceed 65, which puts them far beyond any stainless steel, and at that level PREN stops being a useful discriminator. Localised corrosion testing in the actual medium is more informative than the index.
They are broadly comparable. C-276 carries more molybdenum, which is the costlier element, but C-22's lower volume can offset that. In practice the quoted difference is usually small and availability matters more.
Both are used. C-276 has the longer reference list in FGD scrubbers; C-22 is sometimes preferred where the stream is more oxidising or chloride levels fluctuate. The specific chemistry of the scrubber liquor should decide.
Send us the operating temperature, the medium and the pressure. We will come back with a material recommendation and the cost difference between the two options.
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