Hastelloy B-2 vs B-3: Choosing the Right Grade for Concentrated Reducing Acid Service
The Hastelloy B-family (Ni-Mo alloys without significant chromium) represents the highest resistance to reducing acids — hydrochloric acid, sulphuric acid above 75%, and other non-oxidising mineral acids — of any commercially available piping material. The B-2 grade was widely used from the 1970s through 1990s but had a known weakness: HAZ sensitisation in the as-welded condition caused intergranular corrosion at weld seams. B-3 was developed specifically to eliminate this flaw. Understanding what B-3 fixed and what it didn't — and when C-276 remains a safer choice than either B-grade — prevents costly misapplication.
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.
What B-3 Changed
Hastelloy B-3 (UNS N10675) adds Ti (0.2%) and Zr (0.02%) and tightens the Fe, Co, and Mn limits compared to B-2. The Ti and Zr tie up the carbon and nitrogen that drive precipitation of the harmful intermetallics — the result is that B-3 can be used in the as-welded condition without post-weld solution anneal. Corrosion testing in boiling HCl showed B-3 HAZ corrosion rates 5–10× lower than B-2 in the as-welded condition. B-3 is the correct B-family grade for all new fabrications — B-2 should not be specified for new projects.
Performance Comparison
| Property | Hastelloy B-2 (N10665) | Hastelloy B-3 (N10675) |
|---|---|---|
| Mo content | 26–30% | 27–32% |
| Cr content | ≤1.0% | 1.0–3.0% |
| HAZ sensitisation | Yes — must solution anneal after weld | Eliminated — usable as-welded |
| Reducing HCl resistance | Excellent (base metal) | Excellent (base + HAZ) |
| Oxidising environment | Fails — no Cr for passivation | Fails — same low-Cr limitation |
| Filler wire | ERNiMo-7 | ERNiMo-10 |
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.