Alloy B2
Alloy B2 (UNS N10665) is a nickel-molybdenum alloy with roughly 28% molybdenum and no intentional chromium addition. It was developed specifically for maximum resistance to reducing acids, particularly hydrochloric acid at all concentrations and temperatures up to boiling. It is the standard alloy for HCl service where oxidizing contaminants are absent.
1. Chemical Composition
The table below lists the elemental ranges for Alloy B2. The very high molybdenum content and near-absence of chromium are what give it strong reducing acid corrosion resistance and set it apart from C-family nickel alloys.
| Grade | C | Mn | Si | S | Cr | Mo | Fe | W | Co | Ni |
| N10665 (B-2) | ≤ 0.02 | ≤ 1.0 | ≤ 0.10 | ≤ 0.03 | ≤ 1.0 | 26.0–30.0 | ≤ 2.0 | — | ≤ 1.0 | Bal |
2. Mechanical Properties
The following minimum values apply to Alloy B2 in the solution-annealed condition for seamless pipe and tube per ASTM B622.
| Grade (UNS) | Alloy Family | Tensile Min, ksi (MPa) | Yield Min, ksi (MPa) | Elong. Min, % | Fiber Stress, psi |
| N10665 | Ni-Mo (B-2) | ≥ 110 (760) | ≥ 51 (350) | ≥ 40 % | 27,500 |
3. Equivalent Grade
The table below cross-references Alloy B2 with international equivalents. Its specific composition limits the number of direct national standard designations available.
| Designation Type | Equivalent |
| UNS | N10665 |
| Werkstoff | 2.4617 |
| Trade Name | Nimofer 6928, NiMo28, Alloy B2, Nickel B2 |
3. Key Technical Advantages
- Unmatched Resistance to Hydrochloric Acid: The 26–30% Mo content in Alloy B2 produces corrosion rates below 0.13 mm/year in hydrochloric acid at all concentrations from dilute to concentrated (37%), including at boiling temperature. C-family alloys and stainless steels cannot match this in reducing HCl service.
- Sulfuric Acid Resistance in Reducing Conditions: B2 performs well in sulfuric acid across the 0–60% concentration range at elevated temperatures in pure reducing conditions. The high molybdenum content suppresses both general and localized corrosion when oxidizing species are absent.
- Low Carbon and Silicon for Microstructural Stability: The ≤0.020% C and ≤0.100% Si limits in B2 (much tighter than original Alloy B) minimize precipitation of Ni4Mo and other harmful phases during slow cooling or extended time in the 600–900°C range. This reduces knife-line attack susceptibility in weld HAZs.
- Technical Note: Alloy B2's main limitation is its near-zero chromium content. In the presence of oxidizing contaminants, such as ferric ions (Fe3+), cupric ions (Cu2+), dissolved oxygen, or nitrates even at low concentrations, corrosion rates can increase sharply. For HCl service with occasional oxidizing contamination, Alloy B3 is the recommended upgrade. B2 should be reserved for confirmed reducing, contamination-free service.
4. Common Manufacturing Standards
- ASTM B622: Standard specification for seamless nickel and nickel-cobalt alloy pipe and tube. It covers UNS N10665 (Alloy B2) in seamless tubular form for corrosive chemical service.
- ASTM B619: Standard specification for welded nickel and nickel-cobalt alloy pipe, applicable to Alloy B2 in welded pipe form for chemical plant installations.
- ASTM B626: Standard specification for welded nickel and nickel-cobalt alloy tube, covering B2 in welded heat exchanger tube form.
- DIN EN ISO 6207 / DIN 17752: European standards for nickel alloy tube and bar. B2 is designated 2.4617 under EN material numbering.
- JIS H4552: Japanese Industrial Standard for nickel and nickel alloy tubes, covering NW0665 designation for Alloy B2.
- GB/T 15007: Chinese national standard for corrosion-resistant alloy products. B2 pipe and tube procurement generally references ASTM B622 in Chinese engineering practice.
Manufacturing Standards Comparison Table:
| Standard | GB | EN/DIN | JIS | GOST |
| ASTM B622 (Seamless Pipe/Tube) | GB/T 15007 | DIN 2.4617 | JIS H4552 (NW0665) | - |
| ASTM B619 (Welded Pipe) | - | DIN 2.4617 | - | - |
5. Primary Applications
- Hydrochloric Acid Production and Handling: Synthesis reactor tubes, absorption column piping, and storage/transfer lines in HCl manufacturing and distribution facilities. Pure HCl at all concentrations and temperatures is the governing factor in material selection here.
- Sulfuric Acid Processing (Reducing Range): Heat exchanger tubes and piping in sulfuric acid dilution, storage, and handling systems operating in the 0–60% concentration range where reducing conditions prevail.
- Phosphoric Acid Service: Reactor and evaporator components in wet-process phosphoric acid plants where reducing conditions and moderate fluoride content are present, and where C-family alloys may fall short.
- Acetic Acid and Organic Acid Reactors: Reactor vessels and heat exchanger tubes in acetic acid manufacture and organic acid concentration service in pure reducing environments free of oxidizing side streams.
- Chemical Intermediate Processing: Piping and vessels in pharmaceutical and specialty chemical intermediate manufacturing. These systems handle confirmed reducing, non-oxidizing HCl or H2SO4 streams that need more acid resistance than stainless steels or lower-molybdenum alloys can provide.