Alloy 400 (UNS N04400) and the C-family nickel-chromium-molybdenum alloys, best represented by Alloy C276 (UNS N10276), are often shortlisted together for marine, chemical and offshore duty. They are metallurgically unrelated: one is a nickel-copper grade built for seawater and alkalis, the other a nickel-molybdenum-chromium grade built for strong acids and chloride-rich chemistry. Recognising that split early prevents both under-specification and unnecessary cost.
Two Different Metallurgical Families
Alloy 400 is a solid-solution nickel-copper alloy: roughly 63–70% nickel with 28–34% copper, plus small additions of iron, manganese, carbon and silicon. Copper gives it excellent resistance to seawater, neutral salts and alkalis, and the nickel-rich matrix keeps it ductile and tough across a wide temperature range. The C-family grades take the opposite approach. They are nickel-molybdenum-chromium alloys with controlled carbon and silicon, and the strongest of them add tungsten. Their corrosion resistance comes from a passive chromium-rich surface combined with molybdenum that resists reducing acids, which is a completely different mechanism from Alloy 400.
Chemical Composition Compared
| Element, % | Alloy 400 / UNS N04400 | Alloy C276 / UNS N10276 |
|---|---|---|
| Nickel | 63.0 min | Balance |
| Copper | 28.0–34.0 | - |
| Chromium | - | 14.5–16.5 |
| Molybdenum | - | 15.0–17.0 |
| Tungsten | - | 3.0–4.5 |
| Iron | 2.5 max | 4.0–7.0 |
| Manganese | 2.0 max | 1.0 max |
| Carbon | 0.30 max | 0.010 max |
Corrosion Resistance: Seawater, Acids and Alkalis
Alloy 400 is exceptional in seawater and brackish water, where it resists both general and localised attack and tolerates moderately high flow velocities, which makes it a natural fit for marine hardware, pump and valve trim, heat-exchanger tubing and offshore equipment. It also performs well in caustic soda and other alkalis, and in many neutral salt solutions. Its weakness is strongly oxidising and strongly reducing acids: it is not the grade to specify for hot hydrochloric or sulfuric acid service.
The C-family alloys invert that profile. Alloy C276 handles hydrochloric and sulfuric acid across wide concentration and temperature ranges, resists wet chlorine, hypochlorite and chlorine dioxide, and remains resistant in chloride-bearing process streams where pitting and crevice corrosion threaten other materials. It resists chloride-induced stress corrosion cracking far better than austenitic stainless steels, and it is widely accepted in sour-service guidance such as NACE MR0175 / ISO 15156 for suitable product forms and conditions. In simple seawater exposure Alloy 400 is often the more economical answer; where acids or oxidising chlorides join the process, C276 becomes the safer choice.
Strength, Ductility and Magnetic Behaviour
| Property | Alloy 400 | Alloy C276 |
|---|---|---|
| Density, g/cm³ | 8.80 | 8.89 |
| Tensile strength, MPa | 550 min (annealed) | 690 min (annealed) |
| Yield strength, MPa | about 195–240 | 283 min |
| Elongation, % | 35 min | 40 min |
| Magnetic response | Magnetic at ambient temperature | Non-magnetic |
The strength gap is modest but real: C276 carries a higher minimum tensile and yield strength in the annealed condition because molybdenum and tungsten stiffen its lattice. Alloy 400 compensates with outstanding ductility and formability, which suits deep drawing and cold forming as well as fabrication by conventional welding. The magnetic difference matters in instrument and sensor applications: Alloy 400 is magnetic at room temperature, so it cannot be used where magnetic interference or minimum-permeability requirements apply, whereas the C-family grades stay non-magnetic. Alloy 400 also has roughly twice the thermal conductivity of the nickel-molybdenum-chromium grades, an advantage in heat-transfer equipment.
Cost, Fabrication and Selection Guidance
Alloy 400 is generally the less expensive material because its alloying content is simpler, while C276 carries a premium driven by molybdenum, tungsten and tight compositional control. Both are welded by gas tungsten arc and gas metal arc processes using matching nickel-base filler metals, and both are produced as seamless and welded tube and pipe, plate, sheet, strip, round, flat and hexagonal bar, wire, forgings and fasteners. Well-known specifications include ASTM B127 for plate, ASTM B164 for bar, ASTM B165 for seamless tube and ASTM B725 for welded pipe for Alloy 400, and ASTM B575, B574, B622, B619, B626 and B564 for Alloy C276, with DIN 2.4360 and DIN 2.4819 as the corresponding European designations.
A simple selection rule works well in practice: choose Alloy 400 for seawater, caustic and neutral salt service where magnetic permeability is not restricted; choose the C-family alloy when strong acids, oxidising chlorides, chlorine compounds or mixed aggressive media define the duty, or when minimum magnetic permeability is required.
FAQ
Q: Is Alloy 400 the same as a nickel-copper alloy?
Yes. Alloy 400, UNS N04400, is the standard wrought nickel-copper grade with roughly 63% nickel and 30% copper, and it is specified by ASTM B127, B164 and B165.
Q: Which material is better in seawater?
Alloy 400 is excellent in clean seawater and is often the lower-cost answer; the C-family grades are preferred when the seawater is chlorinated or combines with acids and oxidising salts.
Q: Can Alloy 400 be used in hydrochloric acid?
It is not recommended for hot or concentrated hydrochloric acid. A nickel-molybdenum-chromium grade such as C276 is the appropriate choice for that service.
Q: Why is Alloy 400 magnetic while C276 is not?
Alloy 400 is ferromagnetic at ambient temperature because of its nickel-copper composition, whereas the austenitic nickel-molybdenum-chromium structure of C276 remains non-magnetic.
Q: Which grade is stronger?
C276 has the higher guaranteed minimum strength, but Alloy 400 offers superior ductility and formability, which is often the deciding factor for formed components.
Q: Do the two alloys use the same welding consumables?
No. Each requires a matching nickel-base filler metal, and the joint area should always be evaluated in the actual service medium.
