Hastelloy B-3: The Reducing Acid Grade When C-276 Is Not Enough
Hastelloy C-276 is the most versatile corrosion-resistant nickel alloy — but in pure reducing acid environments (concentrated HCl, H₂SO₄ above 80%), its 15% chromium content becomes a liability rather than an asset. The B-series alloys, culminating in Hastelloy B-3 (UNS N10665 / ASTM B366 WPNB3), eliminate chromium entirely to maximise resistance in reducing conditions.
Why Low Chromium Helps in Reducing Acids
Chromium promotes passivation in oxidising environments — the Cr₂O₃ layer is what makes stainless steel corrosion-resistant in air and dilute oxidising acids. But in strongly reducing environments (concentrated HCl, hot H₂SO₄ above 70%), this passive film is destroyed and chromium does not provide protection. In C-276 (15Cr-16Mo), the chromium contributes to corrosion in pure HCl. In Hastelloy B-3 (1Cr-28.5Mo), the chromium is minimised and the very high molybdenum content (28.5%) provides corrosion resistance through a different mechanism — Mo²⁺/Mo⁶⁺ redox reactions inhibit dissolution in reducing acids.
Hastelloy B-Series Evolution
| Grade | UNS | Key Chemistry | Main Improvement |
|---|---|---|---|
| Hastelloy B | N10001 | 28Mo-5Fe-1Cr | Original — susceptible to HAZ corrosion |
| Hastelloy B-2 | N10665 | 28Mo-2Fe-1Cr | Lower Fe improves HAZ; still some sensitisation risk |
| Hastelloy B-3 | N10675 | 28.5Mo-1.5Cr-3Fe+Ti/Zr | Ti/Zr additions eliminate knife-line and HAZ attack |
B-3 is the current preferred grade. The titanium and zirconium micro-additions in B-3 suppress the precipitation of Ni₄Mo that caused knife-line corrosion in B and the HAZ sensitisation in B-2. B-3 can be used in the as-welded condition in most services without post-weld annealing — a critical practical advantage over earlier B-series grades.
Performance in Key Reducing Acids
| Environment | B-3 Performance | C-276 Performance |
|---|---|---|
| Concentrated HCl (20–37%), ambient | Excellent | Moderate — Cr attacked in concentrated HCl |
| HCl, elevated temperature (60–80°C) | Superior | Poor — unacceptable corrosion rate |
| H₂SO₄ 80–90% at 60°C | Good | Moderate |
| H₂SO₄ + oxidising contaminant (Fe³⁺, HNO₃) | Poor — oxidising conditions destroy B-3 | Excellent |
| Hydrofluoric acid (HF) | Not recommended | Not recommended — use Monel 400 |
| Wet chlorine / hypochlorite | Not suitable | Excellent |
Critical warning: Hastelloy B-3 fails catastrophically in oxidising conditions — even small amounts of dissolved iron(III) (Fe³⁺), dissolved oxygen, or nitric acid contamination in an HCl stream can accelerate corrosion many times over. This is the primary limitation of B-3 in chemical plant — if there is any risk of oxidising contamination, C-276 or C-22 is safer. Confirm the process chemistry thoroughly before specifying B-3.
Welding Hastelloy B-3
Filler: ERNiMo-10 (matching B-3 filler). Do not use ERNiCrMo-3 (Inconel 625) — it contains chromium and would deposit a weld with different reducing-acid performance. Preheat is not required. Inert gas shielding (Ar or Ar/He) is essential — B-3 is highly susceptible to oxygen and nitrogen pickup during welding, which causes embrittlement. Back purge all roots. Interpass temperature ≤150°C. B-3 can be used in the as-welded condition for most services — but a post-weld solution anneal at 1065°C followed by rapid quench is recommended for the most aggressive HCl service.
When to Specify B-3 vs C-276
Use B-3 when: the process is a pure or near-pure reducing acid (HCl above 15%, H₂SO₄ above 75%, phosphoric acid) with no oxidising species. Use C-276 when: the process contains mixed acids, oxidising contaminants, or alternates between reducing and oxidising conditions. When in doubt — C-276 is the safer default. B-3's superior reducing acid performance is only realised when the process is consistently reducing.