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.
The B-2 Sensitisation Problem
Hastelloy B-2 (UNS N10665, 28% Mo, <1% Cr) achieves its exceptional HCl resistance through molybdenum content — Mo dissolves preferentially over Ni in reducing acid, forming a protective MoO₂ film. The problem: heat input during welding causes slow-cooled zones in the HAZ to precipitate Ni₄Mo and Ni₃Mo intermetallic phases at grain boundaries. In pure HCl these precipitates corrode preferentially, creating intergranular attack in the HAZ even though the base metal and weld metal are sound. Multiple HAZ failures in process plant within 1–3 years of installation were attributed to this mechanism.
The Critical Limitation: Oxidising Contamination
Both B-2 and B-3 share the same fundamental limitation: they fail catastrophically in oxidising conditions. Fe³⁺ ions (from dissolved iron), dissolved oxygen, HNO₃, or wet Cl₂ — even at concentrations of a few hundred ppm — eliminate the protective Mo film and cause rapid general corrosion. This makes B-3 unsuitable for any process where oxidising contamination is possible: mixed acid streams, acid that contacts air during handling, or systems where iron contamination from upstream carbon steel equipment is possible. In those environments, Hastelloy C-276 or C-22 is the safer choice — their high Cr content handles the oxidising component that destroys B-grade alloys.