Hastelloy B-2 and B-3 are nickel-molybdenum alloys developed for strongly reducing acids, especially hydrochloric acid. B-3 was designed to retain the family’s corrosion resistance while improving the thermal stability and fabrication tolerance that can make B-2 difficult to process.
Important: Neither alloy is a universal hydrochloric-acid solution. Oxidizing contaminants—including ferric or cupric salts—can sharply reduce performance. Use representative corrosion data and current material specifications for final selection.
The Short Answer
Choose Hastelloy B-3 (UNS N10675) for most new reducing-acid equipment when B-family corrosion resistance is needed with better thermal stability and fabrication behavior. Legacy B-2 (N10665) may remain valid under an approved design, but it is more vulnerable to embrittling phase formation during intermediate-temperature exposure. Neither should be used casually in oxidizing media.
B-2 vs B-3 Comparison
| Decision factor | Hastelloy B-2 | Hastelloy B-3 |
|---|---|---|
| UNS | N10665 | N10675 |
| Alloy type | Nickel-molybdenum | Nickel-molybdenum with chemistry optimized for stability |
| Main environment | Reducing acids, notably HCl | Similar reducing-acid service |
| Thermal stability | Can rapidly form embrittling Ni-Mo phases in critical temperature ranges | Significantly improved resistance to deleterious phase formation |
| Fabrication | Requires strict thermal control | Better forming/welding tolerance, though still controlled |
| Oxidizing contaminants | Serious limitation | Serious limitation |
| Legacy substitution | Existing drawings and procedures | Often preferred for new designs, but substitution needs approval |
Why B-3 Was Developed
The B-2 composition provides exceptional resistance to pure hydrochloric acid and other reducing media, but its nickel-molybdenum matrix can lose ductility after certain thermal exposures. Haynes reports that B-2 can rapidly form Ni4Mo around 750°C, whereas B-3 requires hours around 650°C to develop deleterious phases under cited conditions.
This wider processing window reduces the risk of cracking during fabrication and thermal cycling. It does not make B-3 immune to poor welding, uncontrolled heat treatment or long exposure in harmful ranges.
Hydrochloric Acid Performance
Both alloys are designed for reducing conditions. B-3 is resistant to pure hydrochloric acid across broad concentration and temperature ranges and also performs in hydrobromic and sulfuric acids under appropriate conditions.
Real processes are rarely pure. Oxygen ingress, chlorine, hypochlorite, nitric acid, ferric ions or cupric ions increase oxidizing potential. Zapp specifically warns that oxidizing salts can restrict B-3 use. The alloy selection study should therefore include impurities, cleaning chemicals, shutdown conditions and upstream contamination—not only the nominal acid concentration.
For a comparison against a more oxidizing-service alloy, read Hastelloy B-3 vs C-276.
Fabrication and Welding
B-3 can be formed and welded by established nickel-alloy practices. Success depends on cleanliness, low sulfur contamination, suitable filler, qualified heat input and avoiding unnecessary residence in embrittling temperature ranges.
Practical controls include:
- remove oil, paint, marking compounds and shop contamination;
- use dedicated nickel-alloy tools and abrasives;
- avoid excessive interpass temperature and broad weaving;
- do not apply an improvised stress-relief cycle;
- define repair welding before production; and
- confirm final solution heat treatment where the product specification requires it.
B-3’s improved stability makes it more forgiving than B-2, but the WPS must still demonstrate bend ductility and corrosion performance for the actual thickness.
Can B-3 Replace B-2?
Metallurgically, B-3 is often the logical successor. Contractually and mechanically, substitution is not automatic. Review:
- Design code and approved material list.
- Product-form standard and dimensions.
- Allowable stresses and design temperature.
- Welding procedure and filler qualification.
- Corrosion data for actual contaminants.
- Thermal-cycle and cleaning conditions.
- Owner/MOC approval and traceability.
An old drawing may use a trade name or obsolete revision. The engineering team should update the complete material definition rather than changing only “B-2” to “B-3.”
When a C-Type Alloy May Be Safer
If the process contains meaningful oxidizers or variable contaminants, a chromium-bearing Ni-Cr-Mo alloy such as C-276 may provide a more balanced reducing-and-oxidizing resistance. That does not mean C-276 always outperforms B-3 in pure HCl. The choice depends on the full chemistry and temperature envelope.
See the broader Nickel Alloy Corrosion Resistance Comparison.
RFQ and Materials-Selection Checklist
- UNS N10665 or N10675 and product-form specification.
- Acid species, concentration and operating temperature.
- Ferric/cupric ions, oxygen, chlorine and other oxidizers.
- Startup, shutdown, cleaning and upset chemistry.
- Pressure, stress, erosion and crevice conditions.
- Plate, pipe, bar, forging or fitting dimensions.
- Solution treatment and supplied condition.
- Welding process, filler and PWHT restrictions.
- G28 or other corrosion testing and acceptance.
- NDE, PMI and EN 10204 certification.
- Legacy substitution/MOC approval.
Frequently Asked Questions
Is Hastelloy B-3 better than B-2?
For many new fabrications, B-3 provides similar reducing-acid capability with improved thermal stability. “Better” still depends on code, specification and environment.
Can B-3 handle hydrochloric acid?
Yes, it was designed for excellent resistance to pure HCl over broad conditions. Oxidizing contaminants can change the answer dramatically.
Why is B-2 difficult to weld?
It can form embrittling ordered phases during critical thermal exposure. Welding and cooling must minimize harmful precipitation and follow qualified procedures.
Is B-3 suitable for nitric acid or wet chlorine?
Generally these are oxidizing conditions outside the B-family’s strength. Evaluate a chromium-bearing alloy using service-specific data.
Can B-2 and B-3 be mixed in one system?
Possibly, but engineering must review welding, filler, galvanic behavior, code acceptance and documentation. Do not mix them as unnamed equivalents.
Final Recommendation
For new reducing-acid equipment, evaluate B-3 first, then verify all oxidizing contaminants and fabrication cycles. Send J&A Alloy the complete process chemistry, temperature, drawing, form, code, welding route, corrosion-test clause and certificate requirements.
