Monel 400 and the Inconel family are both nickel-based alloys for severe service. Monel is nickel-copper (67% Ni + 30% Cu); Inconel is nickel-chromium. They excel in different chemistry. Monel wins in reducing acids, seawater, and hydrofluoric acid. Inconel wins in oxidizing environments and high temperatures. Picking wrong means the part corrodes or costs 2× what was necessary.
Monel 400: 67% nickel + 30% copper + trace iron. The copper content gives it unique properties — it's the only nickel alloy where the matrix is dominated by copper behavior. This makes Monel resistant to chemistry that attacks chromium-containing alloys.
Inconel 625/718: 50-60% nickel + 15-23% chromium + variable additives (Mo, Nb, Fe). The chromium forms a passive layer that protects against oxidation and chloride attack.
Key insight: Monel performs best in reducing (non-oxidizing) environments. Inconel performs best in oxidizing environments. When you match the alloy to the chemistry, both can deliver decades of service in environments that would destroy stainless steel in weeks.
| Property | Monel 400 | Inconel 625 | Inconel 718 (aged) |
|---|---|---|---|
| Tensile strength | 550 MPa | 830 MPa | 1,300 MPa |
| Yield strength | 240 MPa | 450 MPa | 1,100 MPa |
| Max service temp | 530 °C | 980 °C | 700 °C |
| Density | 8.80 g/cm³ | 8.44 g/cm³ | 8.19 g/cm³ |
| Machinability rating | ~35% | ~15% | ~12% |
| Cost ($/kg, bar) | $35-45 | $80-110 | $95-130 |
Monel is weaker and cheaper than Inconel. It machines noticeably better (35% vs 15% rating — roughly 2× easier to machine). For non-structural corrosion applications, Monel often wins on cost-benefit.
Monel is one of the few metals that resists HF at elevated concentrations. Used in HF alkylation units, pharmaceutical HF handling, semiconductor wet benches. Inconel would not survive.
Ship propeller shafts, pump impellers, heat exchanger tubes. Monel resists erosion-corrosion from flowing seawater better than Inconel. The industry standard for marine propulsion shafts.
Monel excels in reducing acid environments. Acid cleaning systems, chemical processing handling dilute H₂SO₄ up to 80 °C.
When 316 is inadequate (warm saltwater, high-velocity flow) but titanium is overkill. Monel provides saltwater resistance at 1/3 the cost of titanium.
Monel loses strength above 530 °C. Inconel 625 holds to 980 °C. For gas turbines, exhaust manifolds, heat exchangers in hot chemistry — Inconel.
Nitric acid, chromic acid, strong oxidizers. Inconel's chromium passivation handles these; Monel's copper gets attacked.
Inconel 718 offers 1,300 MPa tensile. Monel at 550 MPa is adequate only for low-to-moderate loads. Structural applications favor Inconel.
Process streams where chemistry changes (e.g., cycling oxidizing/reducing) favor Inconel's more universal corrosion resistance.
| Environment | Monel 400 | Inconel 625 |
|---|---|---|
| Seawater (ambient, flowing) | Excellent | Excellent |
| Hydrofluoric acid | Best available | Poor |
| Hydrochloric acid (dilute, cool) | Good | Fair |
| Sulfuric acid (dilute, cool) | Good | Good |
| Sulfuric acid (concentrated, hot) | Poor | Fair |
| Nitric acid | Poor (copper attacks) | Excellent |
| Chromic acid | Poor | Excellent |
| Sodium hydroxide (caustic) | Excellent | Good |
| Hot combustion gases (oxidizing) | Moderate | Excellent |
| Reducing atmosphere high temp | Moderate | Good |
Email [email protected] with your service chemistry and temperature. For HF, seawater, or reducing acids, Monel is usually right. For high-temp or oxidizing, Inconel. We quote both.
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