Hastelloy B-2 (UNS N10665) Technical Guide | Maximum Resistance to Hydrochloric Acid

Hastelloy B-2 (UNS N10665) Technical Guide: The Reference Alloy for Hydrochloric Acid Service at All Concentrations and Temperatures | Hangbo Alloy

Introduction

If your process engineer has ever been told that "Hastelloy B-2 is just an upgraded nickel 200," stop them right there. That statement is not merely imprecise — it is dangerously misleading, because it encourages designers to evaluate B-2 against the wrong corrosion model entirely. Hastelloy B-2, designated UNS N10665 and Werkstoff-Nr. 2.4617, belongs to the nickel–molybdenum (Ni-Mo) family of corrosion-resistant alloys, not to the commercially pure nickel grades. Its entire corrosion personality is defined by roughly 28% molybdenum and the deliberate near-absence of chromium, an alloying philosophy that is the polar opposite of what makes nickel 200 useful in caustic service. B-2 exists for one overriding purpose: to survive hydrochloric acid — the most corrosive of the common mineral acids — across essentially the whole concentration range and up to boiling temperature, in an environment where austenitic stainless steels, duplex grades, and even the chromium-bearing C-family nickel alloys fail within days or hours.

Shanghai Hangbo Alloy Group manufactures and stocks Hastelloy B-2 in a complete range of mill forms — plate, sheet, strip, round bar, forging billet, seamless and welded pipe, tube, and flanges — to ASTM B333 (plate, sheet, and strip) and ASTM B335 (rod, bar, and wire), with full EN 10204 3.1/3.2 traceability, PMI verification on every heat, and corrosion-test documentation on request. This guide explains what makes B-2 behave the way it does, where it should — and should not — be specified, and the fabrication pitfalls that separate a twenty-year vessel from a two-month failure.

Metallurgical Design: Why Molybdenum Rules and Chromium Is Banished

The corrosion behavior of B-2 is governed by one element: molybdenum. In hot, non-oxidizing (reducing) acids, and in hydrochloric acid in particular, corrosion proceeds by hydrogen-evolution cathodic reactions. Molybdenum promotes the formation of stable, protective surface films and, more importantly, resists the active dissolution that chromium-containing alloys suffer once chlorides prevent passivation. In a boiling reducing chloride environment a stainless steel cannot hold its passive film; chromium then becomes a liability rather than an asset. That is precisely why B-2 is specified with a chromium maximum of only 1.0% — every percentage point of chromium that was added to fight oxidizing media is a percentage point that accelerates attack when the acid turns reducing.

The alloy's balance is therefore austere and purposeful. Iron is held to 2.0% maximum, silicon to 0.10% maximum, and carbon to 0.01% maximum. Low carbon and low silicon are not incidental refinements; they are the metallurgical keys that suppress the precipitation of molybdenum-rich intermetallic phases (principally Ni-Mo compounds of the Ni₃Mo and Ni₇Mo₆ types) at grain boundaries during welding and thermal exposure. Uncontrolled precipitation of those phases creates chromium- and molybdenum-depleted grain boundaries that corrode preferentially — the phenomenon known as knife-line and heat-affected-zone (HAZ) attack. This weakness was the driver for the later development of Hastelloy B-3 (UNS N10675), but it can be managed in B-2 with disciplined shop practice, as discussed below.

Chemical Composition

The table below summarizes the composition limits applicable to B-2 mill products supplied to ASTM B333 and B335. Nickel is the balance element; "min/max" entries reflect the product specification ranges Hangbo Alloy certifies on every heat.

Note how tightly silicon and carbon are capped. In practice Hangbo Alloy routinely supplies B-2 heats with carbon below 0.005% and silicon below 0.05%, giving extra margin against grain-boundary sensitization during multi-pass welding. If a supplier offers "B-2" with 0.4% silicon or 0.03% carbon, that material is not fit for hydrochloric acid duty — verify the chemistry certificate before fabrication begins.

Corrosion Resistance in Hydrochloric Acid

All Concentrations, All the Way to Boiling

The defining claim for B-2, supported by decades of laboratory and plant data, is that it exhibits excellent resistance to hydrochloric acid at essentially all concentrations and temperatures up to and including the boiling point. In reagent-grade hydrochloric acid, corrosion rates below 0.1 mm/year are routinely measured across the concentration map — from 0.1% to the 20.2% azeotrope — up to the atmospheric boiling curve. This is a performance class that no stainless steel, no duplex alloy, and no chromium-bearing C-type nickel alloy approaches. Hastelloy C-276, for example, tolerates hydrochloric acid only while it remains dilute or cool; once the temperature and concentration climb, its chromium content turns against it and general corrosion accelerates steeply.

The practical service envelope for B-2 in pure hydrochloric acid is summarized below. The categories reflect published iso-corrosion experience; Hangbo Alloy recommends coupon testing in your actual acid (which will contain trace impurities) before final material selection.

The Oxidizing-Impurity Trap

Every specification engineer must understand B-2's single Achilles heel: it has virtually no tolerance for oxidizing species. Ferric chloride, cupric chloride, nitric acid, dissolved oxygen, wet chlorine, hypochlorites, and even aerated acid will destroy the protective molybdenum film and drive corrosion rates from thousandths of a millimeter to several millimeters per year. A B-2 hydrochloric acid absorber fed with acid that has been contaminated by ferric iron — for instance, from upstream carbon steel piping corrosion — can fail in weeks. The rule is simple: if the acid stream can become oxidizing, specify an alloy of the C family (or G-30/G-35) instead, or engineer the process to guarantee reducing conditions.

Beyond hydrochloric acid, B-2 performs well in sulfuric acid up to roughly 60% concentration in the reducing region at moderate temperature, in phosphoric acid at all concentrations, and in pure acetic and formic acids, and it is widely used where hot reducing organic acid mixtures contain chloride. Its resistance to chloride-induced stress-corrosion cracking (SCC) is essentially total, because the alloy operates in the active-protective regime where the crack-propagation mechanism of austenitic stainless steels cannot establish itself.

Physical and Mechanical Properties

The table below lists typical annealed values for B-2 as supplied by Hangbo Alloy; mill certificates for each heat document the actual measured values.

B-2 is supplied in the solution-annealed condition, which for this alloy means heating to approximately 1,065 °C followed by rapid quenching. Annealed B-2 is ductile and tough, and it retains useful impact properties down to cryogenic temperatures, although its commercial identity is as a corrosion alloy rather than a structural one. Designers should note that B-2 work-hardens more rapidly than austenitic stainless steel and is stiff, so cold-forming operations need more power and are best followed by re-annealing when deformation exceeds roughly 7% outer-fiber strain.

The Thermal-Stability Discipline: Welding and Heat Treatment

B-2 can be welded in the as-welded condition — this is one of its practical advantages — but only when the thermal-stability rules are respected. The alloy is susceptible to precipitation of Ni-Mo second phases when it dwells in the approximate range 538–1,093 °C (1,000–2,000 °F). Such precipitation embrittles the material and, worse, creates molybdenum-depleted zones adjacent to grain boundaries that corrode preferentially in hydrochloric acid. The practical consequences are threefold.

First, keep welding heat input low and control interpass temperature. Hangbo Alloy's fabrication guidance, consistent with industry practice, recommends interpass temperatures no higher than about 93–150 °C and stringer-bead techniques with no weaving. Second, use matching low-carbon filler: ERNiMo-7 bare wire to AWS A5.14 and ENiMo-7 coated electrodes to AWS A5.11 are the correct consumables. Third, never subject a B-2 vessel to service or stress-relief temperatures inside the 538–1,093 °C window. If a full solution anneal is required after heavy welding or forming, it is performed at approximately 1,065 °C followed by rapid water quenching. Forging is carried out in the range roughly 1,230–980 °C with finishing above 950 °C, always followed by re-solution annealing.

Because the HAZ of a properly welded B-2 joint remains free of significant precipitation, the alloy is genuinely usable as-welded in hydrochloric acid service — a major cost and schedule advantage over alloys that demand post-weld heat treatment. Where service is exceptionally severe, or where weld geometry creates high restraint, Hangbo Alloy can supply fabricated components that receive a final solution anneal and quench before dispatch, complete with corrosion test documentation per ASTM G28 Method B.

Applications

Hastelloy B-2 earns its keep in equipment that sees hot, reducing, chloride-laden acid day after day. Representative applications supplied by Hangbo Alloy include:

• Hydrochloric acid synthesis and absorption systems — absorbers, coolers, and piping handling hot HCl gas and condensate.
• Reactors, columns, and reboilers in processes where HCl is a by-product of chlorination, isomerization, or organic synthesis.
• Acid regeneration and waste-acid recovery plants, including strippers handling HCl contaminated with hydrocarbons.
• Evaporators and crystallizers for chloride brines where pH control keeps the environment reducing.
• Acetic acid and acetic anhydride plants where halide impurities preclude stainless steels.
• Pharmaceutical and fine-chemical reactors processing chlorinated intermediates.
• Pump casings, valve trim, and instrumentation hardware in HCl transfer service.

Comparing B-2 with the Alternatives

Against C-276 and other C-family alloys, B-2 wins decisively in pure reducing hydrochloric acid, typically by one to two orders of magnitude in corrosion rate. Against zirconium, which is also outstanding in HCl, B-2 offers dramatically lower cost and easier fabrication. Against B-3, the story is subtler: B-3 (UNS N10675) was developed to improve thermal stability during welding and to resist knife-line and HAZ attack even after the thermal excursions B-2 tolerates poorly. Where fabrication involves heavy multi-pass welding without the discipline described above, or where the fabricator cannot guarantee tight interpass control, B-3 is the safer engineering choice — a topic covered in Hangbo Alloy's separate B-3 technical guide. Where the welding is well controlled and pure HCl performance at minimum cost is the objective, B-2 remains a perfectly serviceable and more economical selection.

Hangbo Alloy Supply Program

Every shipment includes heat-lot traceability, EN 10204 3.1 inspection certificates, and PMI verification of molybdenum, nickel, and the critical tramp elements.

Technical FAQ

1. What is the maximum hydrochloric acid concentration Hastelloy B-2 can handle?

B-2 resists hydrochloric acid at essentially all concentrations, from trace levels up to the 20.2% azeotrope at atmospheric pressure, including boiling conditions. At ambient temperature it tolerates even the concentrated grades (30–37%) at low corrosion rates. Concentrations above the azeotrope cannot exist as boiling solutions at one atmosphere, so "all concentrations at boiling" effectively means the entire liquid-phase HCl map below ~110 °C.

2. Why does B-2 fail if the acid contains ferric or cupric ions?

Ferric and cupric ions are oxidizing species that depolarize the cathodic reaction and destroy the molybdenum-rich protective film on which B-2 depends. In pure reducing HCl the corrosion rate may be below 0.1 mm/yr; with even a few hundred ppm of FeCl₃ it can jump to several mm/yr. Process designers must exclude air, iron contamination, and oxidizing reagents from B-2 circuits.

3. What is the difference between Hastelloy B-2 and Hastelloy B-3?

B-3 (UNS N10675) retains B-2's corrosion resistance but adds compositional control — small additions of chromium, iron, and tungsten within limits — that dramatically slows the precipitation of Ni-Mo second phases. B-3 tolerates the 538–1,093 °C thermal window far better, so it is more forgiving of multi-pass welding and resists knife-line and HAZ attack after thermal exposure. B-2 is the economical choice when welding is tightly controlled.

4. Can B-2 be welded without post-weld heat treatment?

Yes, provided heat input is kept low, interpass temperature is controlled (ideally ≤ 93–150 °C), and matching ERNiMo-7/ENiMo-7 consumables are used. Properly welded B-2 is serviceable as-welded in HCl. If welding has been uncontrolled, a re-solution anneal at ~1,065 °C with water quenching restores corrosion resistance.

5. Which filler metal is used to weld B-2?

ERNiMo-7 bare wire (AWS A5.14) and ENiMo-7 coated electrodes (AWS A5.11). These are low-carbon, matching-composition Ni-Mo fillers. Using a C-type filler such as ERNiCrMo-4 in B-2 service is generally wrong because it introduces chromium that corrodes preferentially in reducing HCl.

6. Is B-2 suitable for sulfuric acid?

In the reducing concentration/temperature region — up to roughly 60% H₂SO₄ at moderate temperatures — B-2 performs well. At higher concentrations and temperatures sulfuric acid becomes oxidizing and B-2 is no longer suitable; C-family alloys or specific stainless grades are then considered. Always map your operating point against published iso-corrosion diagrams.

7. Does B-2 resist chloride stress-corrosion cracking?

Yes. B-2 does not rely on a passive film in reducing chlorides and is effectively immune to chloride SCC, which makes it attractive for chloride-bearing organic and mineral acid services where 300-series stainless steels crack unpredictably.

8. Why is the silicon and carbon limit so low in B-2?

Carbon and silicon promote the precipitation of molybdenum-rich intermetallic phases at grain boundaries during welding or thermal exposure. Those precipitates deplete adjacent molybdenum and create paths of preferential attack (knife-line/HAZ corrosion) in HCl. Capping C at 0.01% and Si at 0.10% preserves the alloy's welded integrity.

9. What is the maximum service temperature for B-2 in hydrochloric acid?

Corrosion service is limited by the boiling point of the acid at system pressure — for atmospheric HCl that is roughly 108 °C at the azeotrope. Structurally, B-2 must not be used in the 538–1,093 °C range because of second-phase precipitation and embrittlement; its corrosion-service temperature ceiling is therefore governed by the process, not by the metal.

10. How can I verify that the "B-2" plate I purchased is genuine?

Demand an EN 10204 3.1 certificate showing heat identity, plus PMI (XRF or arc-OES) confirmation of Mo 26–30%, Cr ≤ 1%, Si ≤ 0.10%, and C ≤ 0.01%. Reputable mills such as Hangbo Alloy also provide solution-annealed-condition documentation and can supply ASTM G28 corrosion-test evidence. If the certificate shows 0.4% silicon or 0.03% carbon, reject the material before it reaches your fabricator.

This page is part of the technical guide series published by Hangbo Alloy (Shanghai Hangbo Alloy Group, nickel-alloy.com). Data presented are typical engineering values compiled from recognized industry and producer sources and are provided for material selection guidance; the governing documents for any purchase are the applicable ASTM/ASME/AMS specifications and the certified mill test report. Contact Hangbo Alloy for current stock, mill certificates, PMI verification, and application engineering support.