Monel 400 (UNS N04400): Ni-Cu Alloy for Seawater & HF Acid
ASTM B127/B164 · Updated: 2026-05-25
Monel 400 is a solid-solution nickel-copper alloy (63-70% Ni, 28-34% Cu, ≤2.5% Fe) with approximately 70% higher strength than pure nickel while retaining excellent corrosion resistance. Immune to chloride stress corrosion cracking at all temperatures—unlike austenitic stainless steels. Resistant to seawater, hydrofluoric acid (one of the few alloys that is), sulfuric acid in reducing conditions, and alkalies. Standard material for marine pump shafts, seawater valve trim, chemical processing equipment handling HF, and hydrocarbon processing where copper-containing alloys are permitted. Not suitable for oxidizing acids (nitric, chromic) or sulfur-bearing reducing environments above 370°C. Magnetic at room temperature (Curie point 25-50°C)—may be weakly magnetic in cold-climate service.
Quick Facts
| Category | Nickel Alloy |
| Standard | ASTM B127/B164 |
| Density | 8.80 g/cm³ |
| Yield Strength | 195 MPa (28 ksi) annealed; 550 MPa (80 ksi) hard |
| Tensile Strength | 517 MPa (75 ksi) annealed; 690 MPa (100 ksi) hard |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| EN NiCu30Fe | Compare |
| DIN 2.4360 | Compare |
| JIS NW4400 | Compare |
| NCu30 | Compare |
| ISO NiCu30Fe | Compare |
Related Materials
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Frequently Asked Questions
When should Monel 400 be used instead of 316L?
Choose Monel 400 for: (1) hydrofluoric acid service where stainless steels are rapidly attacked, (2) chloride SCC immunity at all temperatures (316L is susceptible above 60°C), (3) seawater pump shafts where the higher strength and cavitation resistance extend service life, (4) reducing acid environments where the passive film on stainless steels breaks down. Not suitable for oxidizing acids (nitric, chromic) where 304/316L excel. Monel costs 10-15× more than 316L.