Introduction
Few material selection decisions carry higher stakes in chemical processing than the choice between Hastelloy B-3 and Hastelloy C-276. Both are wrought nickel-molybdenum alloys in the Hastelloy family. Both offer exceptional corrosion resistance in acidic environments. Both command premium prices and require careful fabrication procedures. And both have been responsible for premature equipment failures when specified incorrectly — sometimes with serious consequences.
The confusion between B-3 and C-276 is understandable. Their naming conventions overlap, their chemistries look superficially similar, and both alloys are marketed as “universal acid-resistant alloys.” This has led to widespread misapplication: B-3 specified in mixed-acid environments where it fails, and C-276 specified in pure HCl service where it costs far more than necessary.
This article provides a definitive technical comparison of Hastelloy B-3 and Hastelloy C-276, with specific focus on hydrochloric acid service and the broader chemical processing environments where these alloys compete. By the end, engineers will have a clear framework for matching alloy to environment — and for avoiding the most common and costly specification errors.
Understanding the Hastelloy Family
Before diving into the comparison, a brief overview of the Hastelloy family is necessary to establish context.
The Hastelloy designation, owned by Haynes International, covers a broad family of nickel-chromium-molybdenum alloys optimized for different service environments. The primary commercial grades relevant to acid service are:
- Hastelloy B-2 (legacy): Nickel-molybdenum alloy with ~30% Mo, designed for pure HCl and reducing acid service. Now largely superseded by B-3.
- Hastelloy B-3: Improved version of B-2 with enhanced thermal stability and better resistance to initiation of knife-line corrosion.
- Hastelloy C-276: Nickel-chromium-molybdenum alloy with ~16% Mo and ~15% Cr, designed for mixed oxidizing-reducing environments.
- Hastelloy C-22: Improved version of C-276 with higher Cr (22%) and better versatility.
- Hastelloy C-2000: Further improved with copper addition for enhanced sulfuric acid resistance.
The fundamental distinction is this: B-series alloys optimize for pure reducing acid environments; C-series alloys optimize for mixed oxidizing-reducing environments. This distinction drives every aspect of their corrosion behavior.
Hastelloy B-3: Composition and Metallurgy
Hastelloy B-3 (UNS N10675, ASTM B622/B619/B626) is a nickel-molybdenum alloy with the following nominal composition:
| Element | Weight % |
|---|---|
| Nickel (Ni) | 65.0 min (balance) |
| Molybdenum (Mo) | 28.5–32.0 |
| Chromium (Cr) | 1.0–3.0 |
| Iron (Fe) | 1.5–3.0 |
| Tungsten (W) | ≤3.0 |
| Cobalt (Co) | ≤3.0 |
| Manganese (Mn) | ≤3.0 |
| Aluminum (Al) | ≤0.50 |
| Titanium (Ti) | ≤0.20 |
| Carbon (C) | ≤0.01 |
| Silicon (Si) | ≤0.10 |
The defining feature of Hastelloy B-3 is its extremely high molybdenum content: 28.5–32.0%. Molybdenum is the key alloying element for resistance to reducing acids, particularly hydrochloric acid and sulfuric acid at moderate concentrations. The nickel base provides electrochemical stability in acidic environments, preventing general corrosion and stress corrosion cracking that plague iron-based alloys.
The low chromium content (1–3%) is a deliberate design choice. Chromium, while excellent for oxidizing environments, forms complex molybdic acid species in strongly reducing acids that can accelerate corrosion. By minimizing chromium, B-3 maximizes its reducing-acid performance at the cost of oxidizing-acid resistance.
Hastelloy B-3 Mechanical Properties (Solution Annealed)
| Property | Value |
|---|---|
| Ultimate Tensile Strength | 760–860 MPa |
| Yield Strength (0.2% offset) | 350–450 MPa |
| Elongation at Fracture | 40–55% |
| Hardness | 85–100 HRB |
| Max Service Temperature | 400°C (corrosion) / 425°C (vacuum only) |
| Density | 9.22 g/cm³ |
| PREN (calculated) | ~55–65 (theoretical, Mo-dominated) |
Hastelloy B-3 is delivered in the solution-annealed condition (typically 1,060–1,150°C, water quench). No precipitation hardening is required or used.
Critical Limitation: Oxidizing Environments
This is where B-3 fails catastrophically. In any environment containing oxidizing species — dissolved oxygen, ferric ions (Fe³⁺), cupric ions (Cu²⁺), nitric acid, chromic acid, or hot concentrated sulfuric acid — Hastelloy B-3 experiences rapid and severe corrosion. The low chromium content provides no passivation capability, and the alloy cannot reform a protective oxide layer in oxidizing media.
The presence of even trace amounts of oxidizing species (as low as 1 ppm Fe³⁺ in HCl) can accelerate B-3 corrosion rates by orders of magnitude. This is the single most important practical limitation of B-3.
Hastelloy C-276: Composition and Metallurgy
Hastelloy C-276 (UNS N10276, ASTM B622/B619/B626) is a nickel-chromium-molybdenum-tungsten alloy:
| Element | Weight % |
|---|---|
| Nickel (Ni) | 57.0 min (balance) |
| Molybdenum (Mo) | 15.0–17.0 |
| Chromium (Cr) | 14.5–16.5 |
| Tungsten (W) | 3.0–4.5 |
| Iron (Fe) | 4.0–7.0 |
| Cobalt (Co) | ≤2.5 |
| Manganese (Mn) | ≤1.0 |
| Vanadium (V) | ≤0.35 |
| Silicon (Si) | ≤0.08 |
| Carbon (C) | ≤0.01 |
| Phosphorus (P) | ≤0.04 |
| Sulfur (S) | ≤0.03 |
C-276’s chemistry represents a deliberate balancing act: 16% Mo + 15% Cr + 4% W. The molybdenum and tungsten together provide excellent resistance to reducing acids. The chromium content provides the critical passivation capability that allows the alloy to perform in mixed environments. The tungsten addition further enhances resistance to localized corrosion.
The Cr:Mo:W ratio is what makes C-276 the “universal” Hastelloy — it has no single dominant weakness in either oxidizing or reducing directions. This versatility comes at a cost: C-276’s reducing-acid performance is not as exceptional as B-3’s, and its oxidizing-acid performance is not as exceptional as pure chromium-based alloys.
Hastelloy C-276 Mechanical Properties (Solution Annealed)
| Property | Value |
|---|---|
| Ultimate Tensile Strength | 760–870 MPa |
| Yield Strength (0.2% offset) | 340–420 MPa |
| Elongation at Fracture | 40–55% |
| Hardness | 87–100 HRB |
| Max Service Temperature | 1,035°C (oxidation), 1,035°C (reducing) |
| Density | 8.89 g/cm³ |
| PREN (calculated) | ~68–75 |
Like B-3, C-276 is delivered in the solution-annealed condition.
Head-to-Head: Hydrochloric Acid Performance
Pure Hydrochloric Acid
In pure HCl, the comparison is clear and unambiguous: Hastelloy B-3 outperforms C-276 in virtually all concentrations and temperatures.
| HCl Concentration | Temperature | B-3 Corrosion Rate | C-276 Corrosion Rate |
|---|---|---|---|
| 1% | 80°C | <0.025 mm/yr | <0.025 mm/yr |
| 5% | 80°C | <0.025 mm/yr | 0.05–0.10 mm/yr |
| 10% | 80°C | <0.025 mm/yr | 0.15–0.30 mm/yr |
| 20% | 80°C | 0.05–0.10 mm/yr | 0.40–0.80 mm/yr |
| 37% (conc.) | 80°C | 0.15–0.25 mm/yr | 1.0–2.0 mm/yr |
| 20% | Boiling | 0.10–0.20 mm/yr | 2.0–5.0 mm/yr |
| 37% | Boiling | 0.30–0.50 mm/yr | 5.0–10.0 mm/yr |
The pattern is consistent: B-3’s higher molybdenum content provides measurably superior corrosion resistance in pure HCl at all concentrations above 5% and all temperatures above ambient. At boiling HCl (110°C), B-3 corrodes at roughly 1/20th the rate of C-276.
For pure HCl service — especially at elevated temperatures and concentrations above 10% — Hastelloy B-3 is the material of choice.
The Oxidizing Contaminant Problem
The B-3 advantage evaporates — and inverts — when oxidizing contaminants are present. This is the critical scenario where C-276 becomes necessary:
With Fe³⁺ ions: Ferric ions (Fe³⁺) are a common contaminant in HCl systems from corrosion of upstream carbon steel equipment. Even 50 ppm Fe³⁺ in 10% HCl at 80°C can increase B-3 corrosion rate from <0.025 mm/yr to >0.5 mm/yr, while C-276 remains essentially unaffected.
With Cu²⁺ ions: Copper ions from brass or copper alloy components upstream create similar catalytic acceleration of B-3 corrosion.
In air-saturated HCl: Atmospheric oxygen in open systems acts as a mild oxidizer. B-3 performs adequately in deaerated HCl but degrades in air-saturated systems.
With mixed acid streams: When HCl co-exists with HNO₃, H₂SO₄, or other oxidizing acids, B-3 is rapidly attacked. C-276 handles mixed acid environments far better.
Environmental Decision Matrix
Choose Hastelloy B-3 When:
- Pure HCl service — especially concentrations >10% and temperatures >60°C, where B-3’s corrosion rate is <0.1 mm/yr.
- Pure H₂SO₄ service — for concentrations 10–50% at moderate temperatures. Above 50% H₂SO₄, oxidizing effects dominate and C-276 becomes preferable.
- Phosphoric acid production — when processing wet-process phosphoric acid with low oxidizing impurities.
- Reducing-acid environments without oxidizing contaminants — acetic acid, formic acid, and other organic acids in reducing conditions.
- Vacuum service — B-3 has a higher maximum service temperature in vacuum (425°C) vs air (400°C).
Caveat: B-3 requires careful heat treatment control during fabrication. The alloy is sensitive to prolonged exposure in the 540–815°C range, where a ductile-to-brittle transformation (formation of ordered Ni₃Mo intermetallics) can occur in the HAZ. Post-weld heat treatment in this range must be avoided.
Choose Hastelloy C-276 When:
- Mixed oxidizing-reducing environments — the defining application. C-276 is the safest default when the exact process chemistry is uncertain or variable.
- HCl with Fe³⁺, Cu²⁺, or other oxidizing ions — any HCl service with upstream contamination.
- Seawater service — C-276 is excellent for seawater handling, particularly in heat exchangers and pumps.
- Chloride-containing oxidizing media — hypochlorite bleaching systems, chlorinated organic processing.
- Waste acid systems — waste streams with variable composition and potential oxidizing species.
- FGD (Flue Gas Desulfurization) — wet scrubbers handling acidic slurry with oxidizing conditions.
- Sour gas processing — H₂S-rich environments with HCl and CO₂.
- Pollution control equipment — wet scrubbers, acid gas absorbers, and chimney linings.
Fabrication Considerations
Weldability
Both alloys are readily weldable using conventional GTAW (TIG), GMAW (MIG), and SMAW (stick) processes, but both require careful procedure control:
Hastelloy B-3:
- Use matching filler metal (ERNiMo-3 or covered electrodes with similar composition).
- Control heat input to minimize HAZ exposure in the 540–815°C range.
- Post-weld heat treatment is generally not recommended due to the ductile-to-brittle transformation risk.
- Avoid using B-3 filler on dissimilar metal welds without careful evaluation.
Hastelloy C-276:
- More forgiving of weld heat input than B-3.
- Use matching filler metal (ERNiCrMo-4 or ENiCrMo-4).
- Post-weld heat treatment is not required for corrosion resistance in most cases.
- Superior for dissimilar metal welding (C-276 to carbon steel, to stainless, to other nickel alloys).
Machinability
Both alloys are classified as “difficult to machine” due to their high strength and tendency toward work hardening. Recommended practices:
- Use carbide or ceramic cutting tools.
- Maintain high cutting speeds with low feed rates.
- Use flood coolant to control heat and chip formation.
- Minimize tool dwell time to prevent work hardening.
- C-276 is marginally easier to machine than B-3 due to its slightly lower yield strength.
The B-3 vs C-276 Cost Question
On a raw material basis, Hastelloy B-3 and C-276 are priced similarly — both are premium nickel-molybdenum alloys. However, the total cost of ownership can differ significantly:
- B-3 equipment may be slightly less expensive when specified for pure acid service where B-3 genuinely outperforms C-276 — you pay for the Mo content you need.
- C-276 equipment may be more cost-effective overall when process chemistry is uncertain, when oxidizing contaminants are possible, or when the equipment will handle multiple acid streams over its lifetime.
- False economy: Specifying the less expensive alloy that fails prematurely is never cost-effective. The cost of unplanned shutdowns, replacement equipment, and environmental remediation almost always exceeds the material premium for the correct alloy.
Common Specification Errors
Error 1: Specifying B-3 for “HCl service” without checking for oxidizing contaminants. The most common and dangerous error. Many HCl systems have upstream carbon steel piping or vessels that introduce Fe³⁺ into the process stream. Always investigate upstream metallurgy before specifying B-3.
Error 2: Specifying C-276 for all acid service because it’s “more versatile.” C-276 is more versatile, but it is not optimal for pure HCl at high temperatures and concentrations. Paying C-276 prices for service where B-3 would perform better is unnecessary overspend.
Error 3: Assuming Hastelloy B-3 is equivalent to Hastelloy B-2. B-3 was developed specifically to overcome B-2’s susceptibility to knife-line corrosion and HAZ embrittlement. B-2 is no longer recommended for new construction. When a specification calls for B-2, update it to B-3.
Error 4: Specifying 316L stainless steel as a cost-cutting measure in HCl service. 316L corrodes at rates of 5–50 mm/yr in 10% HCl at 80°C. It is not a viable substitute. Hastelloy B-3 or C-276 are the only acceptable options for meaningful HCl concentrations above 5%.
Conclusion
The B-3 vs C-276 decision is ultimately an environmental purity question:
- Pure reducing acid, no oxidizing contaminants → Hastelloy B-3. The 30% Mo provides the best possible resistance to HCl, H₂SO₄, and phosphoric acid in clean reducing conditions.
- Mixed oxidizing-reducing acid, variable chemistry, or contaminated streams → Hastelloy C-276. The Cr + Mo + W combination handles uncertainty and oxidizing species gracefully.
The practical engineering principle: When in doubt, specify C-276. Its versatility comes at a modest premium over B-3 and prevents the catastrophic failures that occur when trace oxidizing contaminants enter a B-3 system. Conversely, do not default to C-276 for pure acid service where B-3 would provide better performance and equivalent cost — the extra corrosion margin of C-276 is wasted when there are no oxidizing species to require it.
J&A Alloy stocks Hastelloy B-3 and C-276 in sheet, plate, bar, pipe, and tube forms, with full mill test reports and traceability documentation. Our technical team supports material selection for chemical processing, oil & gas, and pollution control applications.
