Hastelloy C-276 and C-22 both belong to the Ni-Cr-Mo(-W) corrosion resistant family and share a low-carbon, low-silicon design that removes the need for post-weld heat treatment. The difference lies in the chromium-to-molybdenum ratio, and that ratio decides performance in oxidising versus reducing media.
1. Composition differences
| Element | C-276 (N10276) | C-22 (N06022) | Effect |
|---|---|---|---|
| Ni | Balance | Balance | Matrix; reducing-media resistance |
| Cr | 14.5–16.5 | 20.0–22.5 | Resists oxidising media |
| Mo | 15.0–17.0 | 12.5–14.5 | Resists reducing acids and pitting |
| W | 3.0–4.5 | 2.5–3.5 | Supports pitting resistance |
| Fe | 4.0–7.0 | 2.0–6.0 | — |
| C | ≤ 0.01 | ≤ 0.015 | Lower improves intergranular resistance |
2. Selection rules
- Predominantly oxidising (nitric acid, acids with Fe³⁺/Cu²⁺, hypochlorite, wet chlorine) → C-22
- Predominantly reducing (HCl, dilute sulphuric, phosphoric) → C-276 or B-3
- Alternating oxidising/reducing, fluctuating chemistry → C-22 or C-2000
- Hot (>100 °C) chloride-bearing mixed acid → C-22 for better thermal stability
- Tight budget with a stable medium → C-276 is usually more economical
3. Thermal stability and welding
Both suppress carbide and mu-phase precipitation in the weld HAZ through low carbon (≤ 0.015%) and low silicon (≤ 0.08%), so welded structures are safe in the as-welded condition. C-22 additionally balances the Cr/Mo ratio so Ni-Cr-Mo intermetallic precipitation kinetics are slower, giving better stability through repeated weld repairs, hot forming or prolonged mid-temperature service.
4. Application cross-reference
| Service | Recommended grade | Why |
|---|---|---|
| FGD absorber | C-276 / C-22 | Chlorides, low pH, variable chemistry |
| Hydrochloric acid (no oxidising impurities) | B-3 / C-276 | B-3 is dedicated to reducing HCl |
| Nitric and mixed acid | C-22 | High chromium resists oxidising acid |
| Wet chlorine and chlorine dioxide | C-276 / C-22 | Both perform excellently |
| Nuclear fuel reprocessing | C-22 | Oxidising nitric acid systems |
| Sulphuric acid with oxidising impurities | C-22 / C-2000 | B-family alloys are unsuitable |
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 HCl, 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?
C-276 typically uses ERNiCrMo-4 and C-22 uses ERNiCrMo-10 (AWS A5.14). For dissimilar joints or overlay, select a filler matched to or more highly alloyed than the weaker side, backed by a qualified welding procedure record (PQR).
Why can't Hastelloy B alloys be used with oxidising impurities?
B-family alloys (B-2/B-3) rely on a molybdenum-rich film stable only in reducing environments. With Fe³⁺, Cu²⁺, dissolved oxygen or nitric acid present, the corrosion potential shifts and the film fails, sharply raising the corrosion rate. Switch to chromium-bearing C-22 or C-2000.