When stainless steel reaches its limits — in strongly acidic environments, at high temperatures, in concentrated chloride solutions — nickel alloys are the next step. Inconel 625, Hastelloy C276, and Monel 400 are the three most frequently specified nickel alloys in chemical process, oil & gas, and marine engineering. Each has a distinct alloy chemistry that makes it suitable for specific environments. This guide helps you understand those differences and make the right material selection.
Chemical Composition Comparison
| Element | Inconel 625 (UNS N06625) | Hastelloy C276 (UNS N10276) | Monel 400 (UNS N04400) |
|---|---|---|---|
| Nickel (%) | 58 min | 57 min | 63–70 |
| Chromium (%) | 20–23 | 14.5–16.5 | — |
| Molybdenum (%) | 8–10 | 15–17 | — |
| Iron (%) | 5 max | 4–7 | up to 2.5 |
| Copper (%) | — | — | 28–34 |
| Niobium + Ta (%) | 3.15–4.15 | — | — |
| Tungsten (%) | — | 3–4.5 | — |
Inconel 625 — High-Strength, High-Temperature Alloy
Inconel 625 (ASTM B443/B444/B446) is a Ni-Cr-Mo-Nb alloy. The niobium addition provides solid-solution strengthening, giving it outstanding tensile and creep-rupture strength at temperatures up to 980°C. Its high chromium and molybdenum content delivers good resistance to oxidising acids, pitting, and crevice corrosion.
Where Inconel 625 Is Used
- Offshore oil & gas: flexible risers, umbilical tubing, subsea wellheads
- Chemical processing: heat exchangers handling strong acids and oxidising agents
- Aerospace and gas turbines: exhaust systems and thrust-reverser structures
- Nuclear: water-cooled reactor components
- Marine engineering: propeller shafts and seawater scrubbing systems
Limitations
Inconel 625 is susceptible to sensitisation at temperatures between 600–700°C during long-term service, which can reduce intergranular corrosion resistance. Not recommended for strongly reducing environments (e.g., dilute HCl) unless the corrosion rate is evaluated.
Hastelloy C276 — The Broadest Corrosion Resistance
Hastelloy C276 (ASTM B574/B575/B619) contains very high molybdenum (15–17%) and tungsten (3–4.5%), making it one of the most corrosion-resistant alloys available for aggressive chemical service. It resists both oxidising and reducing media better than almost any other nickel alloy, including concentrated sulfuric acid, hydrochloric acid, ferric chloride, and chlorine gas.
Where Hastelloy C276 Is Used
- Chemical processing: reactors, heat exchangers, columns handling mixed acids
- Pulp and paper: bleaching environments (chlorine dioxide and hypochlorite)
- Flue gas desulphurisation (FGD) scrubbers
- Pharmaceutical: vessels and piping handling halogenated compounds
- Waste treatment: incineration and acid neutralisation systems
- Oil & gas: sour gas scrubbing, downhole components in H₂S service
Limitations
Hastelloy C276 is significantly more expensive than other nickel alloys due to its high molybdenum content. It also has lower strength than Inconel 625 at elevated temperatures and is not ideal for very high-temperature service above 650°C.
Monel 400 — The Seawater Specialist
Monel 400 (ASTM B165/B164/B127) is a Ni-Cu alloy — the simplest of the three and the oldest. Its defining characteristic is exceptional resistance to seawater and brine at all temperatures and concentrations. Monel 400 is also highly resistant to hydrofluoric acid and steam, making it irreplaceable in specific industries.
Where Monel 400 Is Used
- Marine: propeller shafts, pump shafts, seawater valves, heat exchanger tubes
- Chemical: hydrofluoric acid alkylation units in refineries
- Brine systems: salt production, desalination flash evaporators
- Steam service: feed water heaters, steam turbine blading
- Petroleum refining: crude distillation overhead condensers
Limitations
Monel 400 is susceptible to stress corrosion cracking in moist, aerated hydrofluoric acid environments and in mercury-containing environments. It also performs poorly in strong oxidising acids (nitric, concentrated sulfuric) and should not be used in these environments.
Side-by-Side Summary
| Property | Inconel 625 | Hastelloy C276 | Monel 400 |
|---|---|---|---|
| Max service temp | 980°C | 650°C | 480°C |
| Min tensile strength | 827 MPa | 690 MPa | 483 MPa |
| Best corrosion resistance | Oxidising acids, seawater | Mixed acids, reducing + oxidising | Seawater, HF, brine |
| Weldability | Excellent | Good | Good |
| Relative cost | High | Very High | Moderate |
| Key standard | ASTM B443/B444 | ASTM B574/B575 | ASTM B165/B164 |
Available Product Forms from Ragnar Metals & Tubes
We supply all three alloys in pipe, tube, fittings, flanges, fasteners, sheet, plate, coil, and round bar. Standard compliance with ASTM B/ASME SB specifications. EN 10204 3.1 mill test certificates supplied with all products.