Monel 400 seamless pipe (UNS N04400) is a nickel-copper alloy produced to ASME SB-165 with a nominal composition of 67% nickel and 33% copper. It is not a chromium alloy, so it does not rely on a passivation film for its corrosion resistance. Instead it is intrinsically resistant to reducing acids and alkalis, and it is essentially immune to chloride stress-corrosion cracking.
That combination explains where Monel 400 is still specified today: hydrofluoric acid handling, alkalis, sour service where H2S is present, seawater instrumentation lines, and any application where chloride SCC would be a concern with a stainless.
Specification and Designation
| Designation | Value |
|---|---|
| UNS number | N04400 |
| ASME specification | SB-165 |
| ASTM equivalent | B163 (tube) / B165 (pipe) |
| Material class | Nickel-copper alloy |
Chemical Composition (ASME SB-165, Table 2)
| Element | Composition, % | Variation under min / over max |
|---|---|---|
| Nickel (by difference) | 63.0 min | 0.45 |
| Copper | 28.0 min – 34.0 max | 0.15 under / 0.20 over |
| Iron | 2.5 max | 0.05 |
| Manganese | 2.0 max | 0.04 |
| Carbon | 0.3 max | 0.02 |
| Silicon | 0.5 max | 0.03 |
| Sulfur | 0.024 max | 0.005 |
Limits quoted directly from ASME BPVC Section II Part B, SB-165 Table 2. Nickel is determined arithmetically by difference, which is why the certificate reports it as Ni (by difference).
Mechanical Properties (ASME SB-165, Table 1)
| Condition and size | Tensile, min psi (MPa) | Yield (0.2%), min psi (MPa) | Elongation in 2 in. / 50 mm, min % |
|---|---|---|---|
| Annealed, 5 in. (127 mm) OD and under | 70 000 (480) | 28 000 (195) | 35 |
| Annealed, over 5 in. (127 mm) OD | 70 000 (480) | 25 000 (170) | 35 |
| Stress relieved, all sizes | 85 000 (585) | 55 000 (380) | 15 |
Note the trade-off: the stress-relieved condition gives markedly higher strength but drops elongation from 35% to 15%. Where subsequent forming is required, specify annealed.
Corrosion Behaviour
- Reducing acids. Excellent in hydrochloric acid, sulphuric acid and phosphoric acid, which is the classic reason for choosing Monel over stainless.
- Hydrofluoric acid. One of very few alloys that handles HF well, which makes it standard in HF alkylation units and glass etching.
- Alkalis. Resistant to caustic soda and potash at all concentrations and temperatures.
- Chloride SCC. Immune. This is the single strongest reason to keep Monel 400 in chloride service today.
- Oxidising media. Poor. Do not use in aerated nitric acid, wet chlorine, or oxidising acid mixtures.
Supply Range
| Parameter | Range |
|---|---|
| Outer diameter | 6 mm – 610 mm (1/4″ – 24″) |
| Wall thickness | 0.5 mm – 40 mm |
| Length | Random length, or cut to customer requirement |
| Condition | Annealed or stress relieved |
Testing and Inspection
- EN 10204 3.1 mill test certificate
- Hydrostatic or nondestructive electric test on every tube with OD 1/8 in. (3 mm) and over and wall 0.015 in. (0.38 mm) and over
- Chemical analysis by E76 methods where disputes arise; the certificate shows actual heat values
- Dye penetrant inspection on all pipe; ultrasonic on heavy wall
- Intergranular corrosion testing to ASTM G28 available on request
Surface, Marking and Packing
Supplied annealed with a pickled or bright surface, capped and sleeved, marked with heat number, standard, size and works mark. Export packing in wooden cases or on steel pallets with plastic end caps.
Applications
- Hydrofluoric acid and alkylation plant piping
- Caustic and alkalis handling
- Sour gas and H2S service where chloride SCC is a risk
- Seawater instrumentation, control lines and impulse tubing
- Oil and gas refining, sour water handling
- Food processing and chlor-alkali equipment
Related Products
For chromium-bearing alloys with different corrosion behaviour, compare our Inconel 625 seamless pipe for chloride pitting and crevice service, or Hastelloy C-276 seamless pipe for strongly oxidising acids.
Technical Enquiries
Send the grade, dimensions, quantity and service condition, plus any third-party inspection requirement, to our technical sales desk. We will confirm the ASTM or EN line item, delivery condition and delivery terms, FOB Shanghai or Ningbo.
Frequently Asked Questions
Is Monel 400 suitable for seawater?
Yes, and this is one of its classic uses. It resists seawater corrosion and does not suffer chloride stress-corrosion cracking. For high-velocity seawater, especially with entrained sand or oxidising biocides, Hastelloy C-276 or titanium is generally preferred because of better pitting margin.
Monel 400 or Monel K-500?
Monel 400 is solid solution annealed, giving good corrosion resistance and moderate strength. Monel K-500 is precipitation hardened, giving roughly twice the strength of 400 at the cost of slightly lower corrosion resistance in strong acid. Choose 400 for pure corrosion duty and K-500 where mechanical loading is significant.
Can Monel 400 be fabricated?
Yes. It welds readily and can be formed, though it work-hardens rapidly. Restrict interpass temperature to about 150°C and use a matching or ENiCrMo filler. Avoid annealing after cold forming, which can embrittle the material.
What is the difference between annealed and stress relieved Monel 400?
Stress relieved material is heated to relieve residual cold-work stresses, giving higher tensile and yield strength with much lower elongation. It suits rigid, pre-stressed installations. Annealed material is the correct choice for anything that will be subsequently formed or bent.
Do you supply Monel 400 with EN 10204 3.1 certification?
Yes, as standard. Third-party inspection by Lloyd’s, BV, DNV, SGS or TUV can be arranged with release applied before shipment.
