Hastelloy B2 vs B3: Thermal Stability Improvement for Hydrochloric Acid Service

Hastelloy B-3 is B-2 with the embrittlement problem engineered out. Controlled chromium (1.0-3.0%), iron (1.0-3.0%) and carbon held at 0.01% maximum slow the precipitation of the brittle Ni4Mo beta phase.

In Hastelloy B-2, Ni4Mo begins forming in minutes around 700-870 degrees Celsius, with the fastest kinetics near 750 degrees Celsius. B-3 needs several hours near 650 degrees Celsius before a deleterious second phase appears, and the phase that eventually forms is the slower-growing Ni3Mo.

Both alloys deliver corrosion rates below 0.1 mm/year in pure hydrochloric acid up to about 70 degrees Celsius at any concentration. B-3 pulls ahead in boiling acid, in welded construction, and in acid carrying trace ferric ions.

Hastelloy B-3 normally enters service in the as-welded condition. B-2 frequently requires a full solution anneal near 1,066 degrees Celsius followed by a water quench.

Neither alloy tolerates oxidizers. Ferric or cupric ions, nitric acid, wet chlorine and hypochlorite require Hastelloy C-276 or C-22 instead.

B-3 typically costs 10-25% more per kilogram, but removing mandatory post-weld heat treatment usually makes the finished equipment cheaper to build.

Hastelloy B-2 vs B-3: chemical composition compared

Table 1. Compositional limits per ASTM B333 / B335 product specifications (typical published mill limits).

Equivalent designations: Hastelloy B-2 = UNS N10665, W.Nr. 2.4617, EN NiMo28, GB NS322. Hastelloy B-3 = UNS N10675, W.Nr. 2.4600, EN NiMo29Cr, JIS NW 6750, GB NS3203.

Hastelloy B-2 vs B-3: engineering properties compared

Table 2. Room-temperature minima per ASTM B333 plate specification; corrosion ratings are qualitative engineering consensus.

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

B-3 is not a different corrosion system. It is B2 steel with the metallurgy rebalanced so the alloy survives its own fabrication. Same nickel-molybdenum chemistry, essentially the same hydrochloric acid resistance, and a completely different response to the 500-900 degrees Celsius temperature window.

The B family is a three-generation story, and each generation fixed the failure mode of the one before it. The original Hastelloy B (UNS N10001, 1920s) contained 4-6% iron and 1% silicon and could not be used in the as-welded condition, because the weld heat-affected zone precipitated grain-boundary carbides and lost its corrosion resistance. B-2 (1960s) solved that by slashing carbon to 0.02%, silicon to 0.10% and iron to 2% maximum. It worked, and B-2 became the classic material for hydrochloric acid columns.

But B-2 carried a second, slower problem: because its matrix is so heavily loaded with molybdenum, it is metastable. Given a few minutes in the mid-temperature range it rearranges into ordered intermetallic compounds. B-3 (1990s) was designed specifically to attack that mechanism, using chromium and iron as deliberate alloying additions rather than as impurities to be minimised. Read the composition table again with that in mind and the design logic becomes obvious: molybdenum stayed high, carbon went down again, and chromium and iron were moved upward on purpose.

Why does Hastelloy B-2 lose ductility between 550 and 900 degrees Celsius?

Because the nickel matrix holds far more molybdenum at 1,150 degrees Celsius than it can hold at 750 degrees Celsius. The excess comes out of solution as ordered Ni4Mo, known as the beta phase. It forms as hard, brittle platelets that nucleate on grain boundaries and dislocations, and a few minutes near 750 degrees Celsius is enough to start the process.

A simple analogy helps. Molybdenum in nickel behaves like sugar dissolved in hot tea. At solution-annealing temperature the nickel holds all of it comfortably. Cool the alloy into the 500-900 degrees Celsius band and the nickel can no longer hold that much molybdenum, so the surplus separates out as crystals, the way sugar crystallises at the bottom of a cooling jar. In B-2 those crystals are Ni4Mo.

That single metallurgical event produces two different failures, which is why B-2 problems are often misdiagnosed:

1. Mechanical failure. Ni4Mo platelets are hard and brittle. They destroy room-temperature elongation and impact energy, so a sensitised B-2 component can crack during forming, during hydrotest, or during a thermal transient in service.
2. Corrosion failure. The precipitate is molybdenum-rich, so the matrix immediately around it is molybdenum-depleted. In hydrochloric acid those depleted bands corrode preferentially. In a welded joint this shows up as knife-line attack, a narrow, deep groove running alongside the fusion line in the heat-affected zone.
3. The practical consequence is that cooling rate through this window matters as much as peak temperature. A thin B-2 sheet quenched rapidly through the band can come through clean. A thick B-2 plate that cools slowly, a multipass weld that parks the heat-affected zone in the band repeatedly, or a stress-relief cycle held at 650 degrees Celsius for hours will not. This is why the standard guidance for B-2 is explicit: solution treat at about 1,066 degrees Celsius, quench rapidly, and do not place the alloy in prolonged service between roughly 538 and 816 degrees Celsius.

How does Hastelloy B-3 suppress Ni4Mo precipitation?

B-3 attacks the problem on four fronts at once. It raises the molybdenum window, adds 1-3% chromium and 1-3% iron to change the ordering pathway, drops carbon to 0.01% maximum with titanium available to tie up the remainder, and holds sulfur very low for hot ductility. The fast-forming Ni4Mo beta phase is largely replaced by the slow-forming Ni3Mo phase.

Chromium 1.0-3.0% stabilises the face-centred cubic matrix and slows the short-range ordering of nickel and molybdenum atoms that seeds Ni4Mo. Ordering is the first step of precipitation, so slowing it delays everything downstream.

Iron 1.0-3.0% changes the thermodynamics of the competing precipitates. In the B-3 balance, Ni3Mo becomes the favoured ordered phase instead of Ni4Mo, and Ni3Mo grows far more slowly.

Carbon 0.01% maximum, plus titanium up to 0.20%, removes the carbide shortcut. In B-2, grain-boundary carbides act as ready-made nucleation sites; with almost no carbon free to form them, precipitation has to start from scratch in the grain interior, which takes much longer.

Molybdenum 27.0-32.0% keeps hydrochloric acid resistance intact. This is the point most people miss: B-3 did not buy thermal stability by sacrificing molybdenum. It bought stability by rebalancing the minor elements around a high-molybdenum core.

Manganese up to 3.0% and aluminium up to 0.50% improve hot workability and deoxidation, so B-3 can be hot-formed and rolled without the tearing that plagued earlier B alloys.

Sulfur is also worth noting. B-3 caps it at 0.01% against 0.03% for B-2. Sulfur and other low-melting-point elements segregate to grain boundaries and cause hot cracking during welding and hot working, so this tightening is a fabrication margin, not a corrosion one.

What does the precipitation kinetics comparison actually show?

On a time-temperature-transformation basis, B-2 begins precipitating Ni4Mo within minutes in the 700-870 degrees Celsius range, with the fastest kinetics near 750 degrees Celsius. B-3 needs several hours of exposure near 650 degrees Celsius before a deleterious second phase appears. That is a shift of roughly one to two orders of magnitude in time, and it is the whole reason B-3 can be used as-welded.

Precipitation follows a C-curve. At high temperature there is little thermodynamic driving force; at low temperature atoms diffuse too slowly. Somewhere in the middle sits the nose of the curve, where precipitation is fastest. For B-2 that nose sits squarely on top of the temperatures a weld heat-affected zone passes through. B-3 pushes the nose to much longer times. The table below translates that into fabrication reality.

Table 3. Fabrication-relevant interpretation of the B-2 versus B-3 time-temperature-transformation behaviour.

Which alloy resists hydrochloric acid better, B-2 or B-3?

In the solution-annealed condition, in pure hydrochloric acid, they are equivalent. Both typically run below 0.1 mm/year from room temperature to about 70 degrees Celsius at any concentration, and below 0.5 mm/year in most boiling conditions. B-3 wins as soon as welds, higher temperatures or impurities enter the picture.

Hydrochloric acid defeats almost every common engineering metal. It destroys stainless steel, it attacks most nickel-chromium alloys, and it will corrode titanium. The nickel-molybdenum system survives because molybdenum forms a stable, dense passive film in a reducing (oxygen-free) acid. A chromium-based passive film, which is what makes stainless steel work, simply cannot be maintained in that environment, because there is no oxidising species present to keep repairing it.

Since both alloys carry roughly 28% molybdenum, both form the same protective film, and both give the same result. The table below gives the practical envelope shared by the two grades in pure acid.

Table 4. Representative hydrochloric acid performance for solution-annealed B-2 and B-3 in pure, deaerated acid.

How do B-2 and B-3 differ when the hydrochloric acid carries ferric contamination?

B-3 tolerates it better, but neither alloy should be specified for hydrochloric acid that contains oxidising ions. Once ferric, cupric or dissolved-oxygen contamination is part of the process stream, move to Hastelloy C-276 or Hastelloy C-22.

Ferric ions are a cathodic depolariser. They accept electrons at the metal surface and thereby destroy the molybdenum-rich passive film faster than the acid can rebuild it. The effect is dramatic: as little as 50 ppm of ferric iron can push corrosion rates from below 0.1 mm/year to above 10 mm/year for both grades. B-3 does better than B-2 for two reasons. Its 1-3% chromium gives the surface film slightly better recovery, and, critically, it has no embrittled, molybdenum-depleted heat-affected zone for the attack to concentrate in.

Table 5. Representative laboratory trend data. Rates are indicative only; always verify with coupons in your own process stream.

In real plants, ferric levels above 500 ppm are common, because hydrochloric acid readily picks up iron from carbon steel piping, pumps and storage. Unless you can guarantee ultra-pure acid and monitor it continuously, B-3 is the safer specification. And if oxidising contamination is unavoidable, neither B grade is the right answer: use C-276.

Does Hastelloy B-3 require post-weld heat treatment?

No, not for pure hydrochloric acid service, and this is the single largest fabrication advantage of B-3. B-2, by contrast, usually requires a full solution anneal near 1,050-1,100 degrees Celsius followed by a water quench.

Post-weld heat treatment on a large vessel is expensive and risky. It needs a furnace big enough for the whole fabrication, it introduces distortion that may require re-rounding and re-machining of flanges, it requires the item to be transported twice, and a slow or inadequate quench can leave the component worse than before. Every one of those steps is a schedule and quality risk. B-3 removes the step.

B-3 is still solution annealed in specific situations:

• Hydrofluoric acid service, or mixed HCl + HF streams, where a solution anneal is normally recommended.
• Boiling concentrated HCl on a high-consequence pressure boundary, where the owner wants maximum margin.
• Heavy cold work: B-3 work-hardens quickly, so cold strain above roughly 10-15% generally calls for an intermediate or final solution anneal to restore ductility.
• Code, client specification or a licensed process design that mandates it.
• Repair welds on thick sections where restraint is high and the fabricator wants to relieve residual stress.

What welding consumables and procedures should be used for B-2 and B-3?

Weld B-2 with ERNiMo-7; weld B-3 with ERNiMo-10 (DIN 2.4695 bare wire, DIN 2.4696 coated electrodes). For both grades use low heat input, keep the interpass temperature below 100-120 degrees Celsius, and do not preheat beyond what is needed to drive off moisture.

Filler and procedure both matter, because a good base metal can be ruined by a bad weld.

Table 6. Welding practice summary. Always qualify a WPS/PQR against the actual joint and thickness.

One contamination rule deserves emphasis: never grind a B-grade alloy with a wheel that has touched carbon steel, and never handle it with carbon steel slings or chains. Embedded iron particles rust in service and initiate pits that can perforate a wall surprisingly quickly.

Are Hastelloy B-2 and B-3 interchangeable in existing hydrochloric acid equipment?

Physically yes, procedurally no. Substituting B-3 for B-2 is almost always acceptable and usually beneficial. Substituting B-2 for B-3 is not, unless you re-qualify the welding procedure and reinstate post-weld heat treatment.

Work through the following checklist before any substitution.

Design code. Confirm the allowable stress for the specific UNS number at your design temperature in the current ASME Section II, Part D. The values for N10665 and N10675 are not identical, so do not assume a straight swap at elevated temperature.

Welding procedure. A change of base metal or filler metal is an essential variable. Re-qualify the WPS and PQR, and note that welding B-3 base with ERNiMo-7 filler produces a deposit with B-2-like stability, which defeats the purpose.

Heat treatment condition. If the existing B-2 item was supplied annealed and quenched, confirm the B-3 replacement is supplied the same way.

Documentation. Require a mill test certificate to EN 10204 3.1 (3.2 for critical service) and 100% positive material identification on every heat.

Galvanic isolation. B-grade alloys are cathodic to carbon steel and aluminium. Isolate supports, flanges and fasteners with non-conductive gaskets, sleeves and washers.

Process chemistry. If the stream has changed since the original specification, re-check the oxidising-ion question before you re-order anything.

When does Hastelloy B-2 still make sense?

In three situations: legacy repair and replacement, clean non-welded duty at moderate temperature, and projects where the governing specification mandates B-2. Outside those cases, B-3 is the better buy.

B-2 is a genuinely good alloy. It simply has a narrow process window, and that window is easy to respect in these applications:

• Replacement internals for an existing B-2 column, tray, or distributor, where mixing grades would create a documentation and galvanic headache.
• Machined components that never see a weld thermal cycle: valve stems, pump shafts, orifice plates, thermometer wells, small fittings.
• Thin-gauge sheet linings and expansion joints that can be fully solution-annealed and water-quenched after forming, in a shop equipped to do it.
• Projects governed by a licensed process design, an owner standard, or an existing asset specification that names B-2 explicitly.
• Clean, low-temperature, non-welded reducing-acid service where the extra cost of B-3 buys nothing.

What does the cost comparison look like over the equipment lifecycle?

B-3 costs about 10-25% more per kilogram. For welded equipment, the total installed cost usually favours B-3, because it eliminates the post-weld solution anneal and the rework risk that goes with it.

Material price is only one line on the invoice. The table below shows where the money actually moves.

Table 7. Cost drivers. Percentages are typical market observations and should be confirmed against current mill and fabricator quotations.

There is a second, harder-to-quantify saving: optionality. A B-3 vessel can be repaired in the field with a weld and returned to service. A B-2 vessel that needs a weld often needs a heat treatment that a plant turnaround cannot accommodate.

Beyond hydrochloric acid: how do B-2 and B-3 compare in other media?

In pure reducing acids the two are close, and B-3 is the safer default in every case. In oxidising media both fail, and Hastelloy C-276 or C-22 is required.

Medium by medium:

Sulfuric acid (pure, non-oxidising)

Both alloys resist pure sulfuric acid well at moderate concentration and temperature; in boiling 10% acid, rates stay below 0.1 mm/year. B-3 handles impurities better. Above roughly 60% concentration, or with any oxidising species present, move to C-276.

Phosphoric acid

Both resist pure phosphoric acid. For wet-process phosphoric acid carrying fluorides and chlorides, B-3's higher molybdenum gives it an edge in the most aggressive stages, though C-276 and C-22 remain the usual specification for oxidising process streams.

Acetic, formic and other organic acids

Both are excellent under reducing conditions. B-3 is preferred for welded reactors in acetic acid, acetic anhydride, methyl methacrylate and ethylene glycol production, precisely because those vessels are welded and B-3 tolerates the weld cycle.

Hydrobromic acid

Both are outstanding; B-3 is specified when the equipment is welded. This is a common but under-discussed application, because HBr behaves very much like HCl and eliminates almost every alternative alloy.

Hydrofluoric acid and mixed HCl + HF

B-3 can be used, but a solution anneal is normally recommended before service. Many designers prefer C-276 or Monel for fluoride-bearing streams, so verify against your specific fluoride level and temperature.

Oxidising media: nitric acid, chromic acid, hypochlorite, wet chlorine, ferric or cupric chloride

Neither alloy. The molybdenum-rich film that works so well in reducing acid is destroyed by oxidisers. Specify Hastelloy C-276 for mixed and moderately oxidising duties, or Hastelloy C-22 for strongly oxidising ones.

Table 8. Media selection matrix for the B family and its usual substitutes.

What are the hard service limits of both alloys

No oxidisers, and no long-term service in the 550-900 degrees Celsius precipitation window. In air, about 540 degrees Celsius is the practical ceiling for both grades; in reducing gas or vacuum, considerably higher temperatures are permissible.

The limits that cause real failures are these:

1. Oxidising ions. Roughly 50 ppm of ferric or cupric ion can raise hydrochloric acid corrosion rates by two orders of magnitude. Keep iron and copper out of the system: no carbon steel or copper piping upstream, no rust, no copper gaskets.
2. Intermediate-temperature service. Do not hold either alloy between about 550 and 900 degrees Celsius under load. B-2 in particular is restricted from prolonged service around 538-816 degrees Celsius.
3. Atmospheric temperature ceiling. In oxidising atmospheres such as air, plan for a ceiling near 540 degrees Celsius. In vacuum or reducing atmospheres, B-3 is used well above that, which is why it appears in vacuum furnace hardware.
4. Hot-work contamination. Heating for forging or hot forming must avoid contact with sulfur, phosphorus, lead and other low-melting-point metals, using low-sulfur fuel and a clean, slightly reducing furnace atmosphere. Flame must not impinge directly on the workpiece.
5. Galvanic coupling. B grades are noble relative to carbon steel and aluminium. Isolate them electrically, or the cheaper metal becomes the sacrificial anode.
6. Chloride stress corrosion cracking. Both alloys are effectively immune to chloride-induced SCC because of their high nickel content, which is one of their underrated advantages over stainless steel.

How should engineers specify Hastelloy B-3 in 2026?

Specify the alloy by UNS number, the product form by ASTM number, the heat treatment condition, and the documentation you require. Do not order on the trade name alone.

A complete B-3 purchase specification contains the following elements:

• Material designation: UNS N10675, W.Nr. 2.4600, GB NS3203. Cross-check all three on the mill certificate.
• Product specification by form: ASTM B333 / ASME SB333 for plate, sheet and strip; ASTM B335 / ASME SB335 for bar; ASTM B622 / ASME SB622 for seamless pipe and tube; ASTM B619 and B626 / ASME SB619 and SB626 for welded pipe and tube; ASTM B366 / ASME SB366 for fittings; ASTM B564 / ASME SB564 for forgings.
• Condition: solution annealed at 1,050-1,100 degrees Celsius, water quenched.
• Mechanical acceptance: tensile 760 MPa minimum, yield 350 MPa minimum, elongation 40% minimum.
• Documentation: mill test certificate to EN 10204 3.1, or 3.2 for pressure-retaining critical service; full heat traceability.
• Verification: 100% positive material identification by XRF or OES on every heat and every piece, performed on receipt, not only at the mill.
• Sour service: state NACE MR0175 / ISO 15156 compliance if hydrogen sulfide is present anywhere in the system.
• Welding: WPS and PQR qualified with ERNiMo-10 filler, interpass below 100 degrees Celsius, no preheat.
• NDE: radiographic or automated ultrasonic testing on pressure-retaining welds, dye penetrant on root passes, nozzle attachment welds and any area where temporary attachments were removed.
• Cleanliness: dedicated iron-free tools, grinding wheels and storage; no contact with carbon steel handling gear.
• Process verification: if the acid is not pure, run an ASTM G31 immersion coupon test in the actual process stream before final material release.

Which alloy should I choose? A six-step selection path

Answer five questions about the stream and one about the fabrication, and the choice is usually clear.

1. Does the stream contain oxidising species (Fe3+, Cu2+, dissolved oxygen, nitric acid, wet chlorine)? If yes, stop. Use Hastelloy C-276 or C-22, not a B alloy.
2. Is the equipment welded, or will it ever be welded in the field? If yes, choose B-3.
3. Does the acid approach boiling, or is it concentrated (above ~20%)? If yes, choose B-3.
4. Is there any chance of iron contamination from upstream carbon steel? If yes, choose B-3, and fix the upstream piping.
5. Is the duty clean, cool, non-welded, and already specified as B-2? B-2 is acceptable and cheaper.
6. Whichever you choose, demand the MTR and run PMI. Grade mix-ups between B-2 and B-3 are a documented industry problem, because the two look identical and cost differently.

Frequently asked questions

1. Is Hastelloy B-3 better than Hastelloy B-2?

Yes, for almost all new construction. B-3 matches B-2's hydrochloric acid resistance in the annealed condition and adds dramatically better thermal stability, easier fabrication, and the ability to enter service as-welded. B-2 remains a legitimate value choice for clean, cool, non-welded reducing-acid service and for like-for-like legacy replacement.

2. Why does Hastelloy B-2 become brittle after welding?

Welding drives the heat-affected zone through 550-900 degrees Celsius, where B-2 rapidly precipitates the ordered intermetallic compound Ni4Mo (beta phase). The precipitate is hard and brittle and leaves molybdenum-depleted bands around it, causing both ductility loss and knife-line corrosion attack. A post-weld solution anneal at about 1,066 degrees Celsius with a water quench dissolves the precipitate and restores properties.

3. Does Hastelloy B-3 need post-weld heat treatment?

Not for pure hydrochloric acid service. B-3 is specifically designed for as-welded use. A solution anneal at 1,050-1,100 degrees Celsius may still be specified for hydrochloric-hydrofluoric mixed acid service, for heavily cold-worked components, or where a code or client specification requires it.

4. At what temperature does Hastelloy B-2 precipitate Ni4Mo?

Between roughly 550 and 900 degrees Celsius, with the fastest precipitation near 750 degrees Celsius. Exposure is measured in minutes, not hours, which is why a weld thermal cycle is enough to start it. B-2 should not be used in prolonged service between about 538 and 816 degrees Celsius.

5. Can Hastelloy B-3 replace B-2 in an existing vessel?

Usually yes, and it is normally an upgrade. Before substituting, confirm the ASME Section II Part D allowable stress for N10675 at your design temperature, re-qualify the WPS and PQR with ERNiMo-10 filler, and confirm the replacement is supplied in the solution-annealed and quenched condition with full certification.

6. What filler metal is used for Hastelloy B-2 and B-3?

B-2 is welded with ERNiMo-7 (W.Nr. 2.4615) or ENiMo-7 electrodes. B-3 is welded with ERNiMo-10, with matching B-3 wire designated DIN 2.4695 and covered electrodes DIN 2.4696. Never use an ERNiMo-7 deposit on B-3 base metal if the improved thermal stability of the joint matters.

7. Is Hastelloy B-2 or B-3 magnetic?

Neither. Both are fully austenitic nickel-molybdenum alloys with a face-centred cubic crystal structure and are non-magnetic in the solution-annealed condition.

8. Which is more expensive, Hastelloy B-2 or B-3?

B-3 costs roughly 10-25% more per kilogram. However, because B-2 usually requires a post-weld solution anneal, the total fabricated cost of welded B-2 equipment is often 25-30% higher, so B-3 frequently wins on installed cost.

9. What is the Chinese GB equivalent of Hastelloy B-3?

Under GB/T 15007, Hastelloy B-3 corresponds to NS3203 and Hastelloy B-2 corresponds to NS322. Always verify the designation against the current mill test certificate, because supplier listings occasionally carry incorrect grade codes.

10. Is Hastelloy B-2 being phased out?

Not formally, but it is no longer the default. New welded designs should default to B-3. B-2 remains widely available and in widespread service for legacy maintenance, repairs and applications where the original specification names it.

11. Hastelloy B-3 versus zirconium for hot hydrochloric acid: which wins?

Zirconium outperforms both B grades in pure, hot hydrochloric acid, but it is far more sensitive to ferric and fluoride contamination and costs substantially more. Where stream purity cannot be guaranteed, B-3 is the more robust and more economical choice.

12. Can B-2 or B-3 handle hydrochloric acid containing ferric ions?

Only in very small amounts, and B-3 handles them better. As little as 50 ppm of ferric ion can raise corrosion rates above 10 mm/year for both grades. At 500 ppm in 10% acid at 60 degrees Celsius, indicative testing shows about 2.1 mm/year for B-2 against about 0.4 mm/year for B-3. Above these levels, specify Hastelloy C-276.

13. What is the maximum service temperature of Hastelloy B-3?

In oxidising atmospheres such as air, about 540 degrees Celsius is the practical ceiling. In reducing gas or vacuum, B-3 can be used considerably higher. Long-term service in the 550-900 degrees Celsius precipitation window should still be avoided for pressure-retaining components; the advantage of B-3 is that transient exposure during welding does not cause embrittlement.

14. Can Hastelloy B-3 be used in sulfuric acid?

Yes, in pure non-oxidising sulfuric acid at moderate concentration and temperature, where rates in boiling 10% acid stay below 0.1 mm/year. It is not suitable for sulfuric acid containing oxidising contaminants. For mixed or oxidising acid service, use Hastelloy C-276 or C-22.

15. Does the higher molybdenum content of B-3 improve hydrochloric acid resistance?

Not measurably in pure acid, because both grades already form the same molybdenum-rich passive film and both run below 0.1 mm/year at ambient to 70 degrees Celsius. The higher molybdenum window in B-3 exists to shift the precipitation pathway toward Ni3Mo, not to improve corrosion resistance.

16. What is the difference between Ni4Mo and Ni3Mo, in plain terms?

Both are ordered compounds of nickel and molybdenum that can form inside the metal grains as it cools. Ni4Mo is the fast-forming, highly embrittling form that plagues B-2. Ni3Mo forms far more slowly and is the phase B-3 is designed to favour. Slower precipitation means the alloy can pass through the dangerous temperature band without damage.

17. Do B-2 and B-3 require a solution anneal on the base material?

Yes. Both are supplied in the solution-annealed and quenched condition, typically 1,050-1,100 degrees Celsius followed by a rapid water quench. This dissolves any precipitate and freezes the molybdenum in solution. Any hot forming or heavy cold working must be followed by a repeat anneal.

18. Hastelloy B-3 versus C-276: how do I choose?

Choose B-3 for pure reducing acids, above all hydrochloric acid at any concentration and temperature. Choose C-276 when the stream contains oxidising species, when acids are mixed, or when ferric and cupric contamination cannot be excluded. C-276's chromium content gives it the oxidising resistance that B alloys deliberately trade away.

19. Can Hastelloy B-3 be used in hydrofluoric acid or mixed HCl + HF?

It can, but a solution anneal is normally recommended before service and the duty should be verified by testing, because fluoride behaviour is highly specific to concentration and temperature. Many designers prefer C-276 or Monel for fluoride-bearing streams.

20. How do I verify that I actually received Hastelloy B-3?

Require a mill test certificate to EN 10204 3.1 or 3.2 showing UNS N10675 with chromium in the 1.0-3.0% range and carbon at or below 0.01%, then run 100% positive material identification by XRF or OES on every piece on receipt. B-2 and B-3 are visually identical, and mislabelled material is a known industry risk.