Hastelloy C-22 vs C-276: Choosing the Right Grade for Mixed Acid and Oxidising Environments
Hastelloy C-276 is the most widely known premium nickel alloy — but Hastelloy C-22 (EN 2.4602) often outperforms it in mixed acid and strongly oxidising environments. Selecting the wrong grade in these services leads to premature failure at very high replacement cost.
The Key Compositional Difference
Hastelloy C-276 (EN 2.4819, NiMo16Cr15W, UNS N10276): 15.5–17.5% Mo, 14.5–16.5% Cr, 3–4.5% W. Very high Mo is the defining feature — optimised for reducing acid environments (HCl, H₂SO₄ in reducing conditions). PRE >50.
Hastelloy C-22 (EN 2.4602, NiCr21Mo14W, UNS N06022): 12.5–14.5% Mo, 20–22.5% Cr, 2.5–3.5% W. Lower Mo than C-276 but significantly higher Cr. This shifts the corrosion resistance profile — higher Cr provides better resistance in oxidising environments. ASME P-No. 44 (same as C-276).
Oxidising vs Reducing Environments
The critical distinction in acid service is the oxidation state of the environment. In strongly reducing environments (e.g. deaerated HCl, H₂SO₄ below the oxidising potential), Mo is the key alloying element — it inhibits active dissolution of the metal by maintaining the passive film under reducing conditions. C-276 with 16% Mo excels here.
In oxidising environments (e.g. nitric acid, wet chlorine, hypochlorite, mixed HNO₃/HCl), Cr is the critical element — it stabilises a Cr₂O₃ oxide film that is protective under oxidising conditions. C-22 with 21% Cr performs significantly better than C-276 (15% Cr) in these media. In strongly oxidising HNO₃ above approximately 50°C, C-276 may actually corrode faster than C-22.
Mixed Acid Environments
Many real industrial environments are mixtures of reducing and oxidising acids — "mixed acid" in the chemical sense. Classic examples: nitric/hydrofluoric acid (HNO₃/HF) in metal pickling; HNO₃/HCl (aqua regia) in precious metal processing; chlorinated solvents with oxidising impurities; and bleach plant piping (ClO₂, NaClO, H₂SO₄). C-22 is widely preferred for these mixed media because its higher Cr provides the oxidising-environment protection while its Mo (12.5–14.5%) remains substantial enough for reducing-media intervals.
Quick Selection Guide
| Environment | C-276 (2.4819) | C-22 (2.4602) |
|---|---|---|
| HCl — all concentrations | ✓ Preferred | ✓ Good |
| H₂SO₄ — reducing conditions | ✓ Preferred | ✓ Good |
| HNO₃ — oxidising | Marginal above 50°C | ✓ Preferred |
| HNO₃/HF (mixed acid pickling) | Marginal | ✓ Preferred |
| FGD scrubbers (HCl+H₂SO₄) | ✓ Preferred | ✓ Good |
| Wet Cl₂ / hypochlorite | Adequate | ✓ Preferred |
| Unknown / mixed service | Adequate | ✓ Better all-round |
Welding and Code Position
Hastelloy C-22 uses ERNiCrMo-10 filler (AWS A5.14) — not the same as C-276's ERNiCrMo-4. The two fillers are not interchangeable. Both grades are ASME P-No. 44, Group 2, but they require separate WPS qualifications because filler metals differ. No PWHT required for either. Inter-pass temperature ≤150°C. Arshya supplies C-22 buttweld fittings per EN 10253-4 with EN 10204 3.1 certificates; the EN designation is 2.4602 (NiCr21Mo14W).