Monel 400 vs Incoloy 825 - HF, Caustic and Seawater Duty
Date: 2026年9月19日 Categories: News Views: 299
By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012
Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360
Quick Answer: Should I Use Monel 400 or Incoloy 825?
Monel 400 (UNS N04400) is a nickel-copper alloy — roughly 63-70% nickel with 28-34% copper and no chromium — and it is the standard material for hydrofluoric acid, caustic and clean seawater service. Incoloy 825 (UNS N08825) is a chromium-bearing nickel-iron alloy with molybdenum and copper, and it is the standard material for sulphuric and phosphoric acid service and mildly sour oilfield duty. Monel 400 is not recommended for hydrochloric or nitric acid; Incoloy 825 is not recommended for hydrofluoric acid. The two are not interchangeable — check the medium before you swap them.
Key Takeaways
- The chemistry is completely different. Monel 400 is chromium-free nickel-copper; 825 is chromium-rich nickel-iron with molybdenum and copper. One addition (chromium) creates two different corrosion maps.
- Hydrofluoric acid belongs to Monel 400. 825 is not recommended there. Getting this wrong is a safety issue, not a cost issue.
- Caustic service favours Monel 400. 825 is susceptible to caustic stress-corrosion cracking in hot concentrated solutions.
- Sulphuric and phosphoric acid belong to 825. Monel 400 performs poorly in oxidizing acids.
- Both are excellent in seawater, but Monel 400 costs more per kilogram because of its very high nickel content.
What Are Monel 400 and Incoloy 825?
Monel 400 is a nickel-copper alloy containing 63-70% nickel and 28-34% copper, with small iron and manganese additions and no chromium at all. It is a single-phase, non-magnetic, tough material with good strength and excellent resistance to hydrofluoric acid, alkalis, and clean, fast-flowing seawater. Historically it was one of the first nickel alloys used in marine and chemical service, and it remains the reference material for HF alkylation units.
Incoloy 825 is a chromium-bearing nickel-iron alloy: 38-46% nickel, 19.5-23.5% chromium, molybdenum for reducing-acid resistance, copper for sulphuric acid resistance, and titanium for stabilisation against intergranular corrosion after welding. It was developed for sulphuric and phosphoric acid service and for oilfield environments.
Product pages: Monel alloy round bar & tube and Incoloy 825 / 800H supplier.
Composition Comparison (wt.%)
| Element | Monel 400 (N04400) | Incoloy 825 (N08825) | Per standard |
|---|---|---|---|
| Nickel | 63.0-70.0 | 38.0-46.0 | ASTM B164 / ASTM B424 |
| Copper | 28.0-34.0 | 1.5-3.0 | ASTM B164 / ASTM B424 |
| Chromium | — | 19.5-23.5 | ASTM B424 |
| Iron | 2.50 max | 22.0 min (balance) | ASTM B164 / ASTM B424 |
| Molybdenum | — | 2.5-3.5 | ASTM B424 |
| Titanium | — | 0.6-1.2 | ASTM B424 |
| Manganese | 2.00 max | 1.00 max | ASTM B164 / ASTM B424 |
| Carbon | 0.30 max | 0.05 max | ASTM B164 / ASTM B424 |
| Silicon | 0.50 max | 0.50 max | ASTM B164 / ASTM B424 |
Table note: Values are standard composition limits from the cited ASTM specifications. The immediate discriminators in a PMI check are chromium (absent in Monel 400, 19.5-23.5% in 825) and copper (28-34% in Monel 400 against 1.5-3.0% in 825). Neither alloy can be confused with the other on an OES reading.
Mechanical Properties Comparison
| Property | Monel 400, annealed | Incoloy 825, annealed | Per standard |
|---|---|---|---|
| UTS at 20°C | 517 MPa min (75 ksi) | 586 MPa min (85 ksi) | ASTM B164 / ASTM B424 |
| 0.2% yield at 20°C | ~172-345 MPa depending on temper (typical) | 241 MPa min (35 ksi) | ASTM B164 / ASTM B424 |
| Elongation at 20°C | 30% min | 30% min | ASTM B164 / ASTM B424 |
| Hardness | ~60-80 HRB (typical) | ~75-90 HRB (typical) | ASTM E18 |
| Cold-worked strength | Up to ~480 MPa yield in spring temper (typical) | Not commonly used cold-worked | — |
| Magnetic response | Essentially non-magnetic | Non-magnetic | — |
Table note: The 825 figures are standard minima from ASTM B424. Monel 400 mechanical properties vary substantially with temper: annealed bar and heavily cold-drawn bar are different products, so the temper must be stated on the purchase order, and the yield range shown reflects that variation.
Corrosion and Service Comparison
| Environment | Monel 400 | Incoloy 825 | Practical note |
|---|---|---|---|
| Hydrofluoric acid (HF) | Excellent — the reference material | Not recommended | Monel 400 is the standard HF alloy |
| Caustic (NaOH) | Excellent, incl. hot concentrated | Susceptible to caustic SCC | A genuine selection fault-line |
| Seawater, high velocity | Excellent | Good to very good | Monel 400 tolerates higher velocity |
| Seawater, stagnant or polluted | Fair (pitting risk) | Fair (pitting risk) | Neither is ideal in stagnant seawater |
| Sulphuric acid, aerated | Fair to poor | Excellent | Monel 400 needs oxygen-free conditions |
| Sulphuric acid, deaerated | Good | Good | Both acceptable in reducing conditions |
| Phosphoric acid | Fair | Very good | 825 is the traditional choice |
| Hydrochloric acid | Poor | Poor to fair | Neither is an HCl alloy |
| Nitric acid (oxidizing) | Unsuitable | Fair | Chromium is required for oxidizing acids |
| Chloride SCC | Excellent | Very good | Both are far better than 300-series stainless |
| Sour service (H2S) | Limited | Good within NACE limits | Check against ISO 15156 / MR0175 |
Table note: Ratings are qualitative engineering guidance based on published corrosion data, the NACE/ISO sour-service framework and our field experience; they are not ASTM G48/G28 results for a specific heat. For critical services, request corrosion testing in the actual medium.
The Rule That Prevents Most Failures: Never Swap These Two
Because Monel 400 and Incoloy 825 occupy adjacent display racks and look similar, they get swapped. The consequences are asymmetric:
- 825 supplied where Monel 400 was specified for HF service. This is a safety-relevant error. Incoloy 825 is not recommended for hydrofluoric acid, and a component in an HF alkylation circuit can fail quickly and dangerously.
- Monel 400 supplied where 825 was specified for sulphuric or phosphoric acid. The Monel 400 part may pass hydrostatic testing and then corrode at an unacceptable rate in aerated acid service, giving a false sense of security until wall loss becomes visible.
- Monel 400 supplied where 825 was specified for hot caustic. Generally the safer direction for caustic resistance, but the design strength and section may no longer be adequate, so it is still a specification deviation.
Our rule, and the one we recommend to buyers: specify by UNS number plus the governing ASTM specification on the purchase order, and verify chromium and copper by OES on receipt. There is no alloy pair in the nickel family where a two-element reading delivers more value for the effort.
Where Each Alloy Is the Standard Choice
| Application | Best choice | Why |
|---|---|---|
| HF alkylation unit valves, pump trim and impellers | Monel 400 | The reference material for hydrofluoric acid |
| Caustic evaporator tubes and concentrated NaOH handling | Monel 400 | Excellent caustic resistance, no caustic SCC |
| Marine pump shafts, valve trim, propeller hardware | Monel 400 | Toughness, corrosion resistance, non-magnetic |
| Sulphuric acid pickling lines and acid coolers | Incoloy 825 | Chromium plus copper chemistry for aerated acid |
| Wet-process phosphoric acid evaporators | Incoloy 825 | Handles chloride and fluoride contamination |
| Oilfield tubulars and wellhead equipment | Incoloy 825 | Sour-service capability within NACE limits |
| Seawater heat-exchanger tubing | Incoloy 825 or Monel 400 | Velocity and pollution determine the choice |
| Nuclear and fuel-cycle equipment | Monel 400 | Fluorine and fluoride service |
| Ammonia and alkali process equipment | Monel 400 | Resists the relevant environments |
Fabrication and Welding Notes
Monel 400 welds well by GTAW and SMAW with matching filler metal, but it is notch-sensitive in some conditions and the work-hardened surface layer left by machining or grinding must be removed before welding and before service in critical environments. It also has a higher thermal expansion than steel, which matters when it is joined to carbon steel components; a Monel 400 shaft in a steel housing will change fit with temperature.
Incoloy 825 is titanium-stabilised, so it resists sensitization and generally needs no post-weld heat treatment — a significant fabricated-cost advantage. It welds to itself cleanly and can be clad onto steel with an appropriate buffer layer to manage dilution.
Machining. Both grades work-harden. Monel 400 is notorious for it: a dull tool or a dwell in the cut produces a hard skin that then destroys the next tool. Positive rake angles, heavy enough feeds to stay under the work-hardened layer and constant coolant are the standard prescription, and the same discipline applies to 825 at slightly lower severity.
A practical observation from our own processing: customers occasionally ask for Monel 400 in the cold-drawn spring temper to gain strength in a seawater pump shaft, but then specify a welded construction. Cold-worked Monel 400 cannot be welded without losing the temper in the heat-affected zone. The correct answer is usually annealed Monel 400 with an increased section, or the higher-strength Monel K-500, which gets its strength from precipitation hardening rather than cold work and retains it after welding and aging. Our alloy technical knowledge center and the related technical guides section cover K-500 and the other precipitation-hardening nickel alloys.
Price Reference (2026, EXW Shanghai)
| Form | Monel 400 | Incoloy 825 | Comment |
|---|---|---|---|
| Round bar | \$35-55/kg | \$28-42/kg | Monel 400 carries 63-70% nickel |
| Plate / sheet | \$38-58/kg | \$30-45/kg | Thickness and width add spread |
| Seamless tube | \$45-70/kg | \$35-55/kg | Cold-work and testing dominate |
| Forging / flange | Quote by drawing | Quote by drawing | Yield and NDT drive cost |
Table note: Reference range only — floats with LME nickel price. 2026, EXW Shanghai, USD/kg. Monel 400 pricing is more sensitive to nickel because nickel is roughly two-thirds of its mass; 825 moves with nickel, chromium and molybdenum together.
Standard Index
| Standard | Title / scope | Covers | Form |
|---|---|---|---|
| ASTM B164 | Ni-Cu alloy rod, bar, wire | Composition + mechanical | bar, rod, wire |
| ASTM B127 | Ni-Cu alloy plate, sheet, strip | Composition + mechanical | plate, sheet, strip |
| ASTM B165 | Ni-Cu alloy seamless pipe and tube | Composition + mechanical | seamless tube |
| ASTM B424 | Ni-Fe-Cr-Mo-Cu alloy plate, sheet, strip | Composition + mechanical | plate, sheet, strip |
| ASTM B423 | Ni-Fe-Cr-Mo-Cu alloy seamless pipe and tube | Composition + mechanical | seamless tube |
| ASTM B425 | Ni-Fe-Cr-Mo-Cu alloy bar and forging billet | Composition + mechanical | bar, billet |
| ASTM B163 | Seamless Ni and Ni alloy condenser and heat-exchanger tube | Composition + mechanical | seamless tube |
| ISO 15156 / NACE MR0175 | Materials for use in H2S-containing environments | Selection limits | — |
| ASTM E8/E8M | Tension testing of metallic materials | Test method | — |
| ASTM E1476 | Standard guide for metals identification (PMI) | Test method | — |
How We Verify These Grades Before Shipment
These two alloys are the clearest case in the nickel family for mandatory composition verification, because the substitution is invisible and the service consequences are severe. Our sequence:
- OES verification of copper and chromium. Monel 400 must read Cu 28-34% with chromium absent; Incoloy 825 must read Cr 19.5-23.5% with Cu 1.5-3.0%. A single reading settles the grade.
- Temper and condition confirmation. For Monel 400 the temper (annealed, cold-drawn, spring) is confirmed against the purchase order, because strength and weldability both depend on it.
- Certificate reconciliation. EN 10204 3.1 documentation is matched line by line against the stencilled heat number, and the reconciliation is photographed into the batch record.
A case we intercepted: a customer's HF alkylation valve spares were delivered by a third party as "Monel" with Cu 2.1% and Cr 20.4% — an 825-type chemistry, indistinguishable in appearance and in hardness from the part it replaced. Had it entered the HF circuit, the outcome would have been a process-safety event rather than a warranty claim.
FAQ
Q1: Can Incoloy 825 be used for hydrofluoric acid?
No. Incoloy 825 is not recommended for hydrofluoric acid service. Monel 400 is the reference nickel-copper alloy for HF, and it remains the standard material for alkylation unit valves, pump trim and heat exchangers.
Q2: Which alloy is better for caustic service?
Monel 400, particularly in hot, concentrated sodium hydroxide. Incoloy 825 is susceptible to caustic stress-corrosion cracking, which is a well-documented failure mode, so it is not the right choice in concentrated caustic at elevated temperature.
Q3: Is Monel 400 or Incoloy 825 better for sulphuric acid?
Incoloy 825, particularly in aerated or dilute acid. Monel 400 is chromium-free and performs poorly in oxidizing conditions; it is only suitable for sulphuric acid when the solution is effectively deaerated and reducing.
Q4: Which alloy is more expensive?
Monel 400, typically 20-30% more per kilogram in 2026 EXW Shanghai reference terms, because it contains 63-70% nickel with no lower-cost iron substitution. Its price is more directly linked to the LME nickel benchmark.
Q5: Which is better in seawater?
Both are excellent in clean, fast-flowing seawater. Monel 400 tolerates higher flow velocities and is the traditional choice for pump shafts, valves and marine hardware; Incoloy 825 is commonly used for heat-exchanger tubing. Both are vulnerable to pitting and crevice attack in stagnant, polluted or biologically fouled seawater.
Q6: Can Monel 400 be welded?
Yes, with matching filler metal, and no post-weld heat treatment is normally required. The work-hardened layer produced by machining or grinding must be removed before welding, and the higher thermal expansion relative to steel has to be considered in joint design.
Q7: What is the operating temperature limit for each alloy?
Monel 400 retains useful strength and corrosion resistance to roughly 480°C in suitable environments, but it must not be exposed to sulphur or sulfur-bearing compounds at elevated temperature because of embrittlement risk. Incoloy 825 is used to roughly 550-600°C. Where higher-temperature strength is required, a different alloy family is needed — see our Inconel alloy supplier page.
Q8: Is Monel 400 magnetic?
It is essentially non-magnetic, and it remains non-magnetic even after cold working, which is a distinct advantage over austenitic stainless steels for instrument and marine applications.
Q9: Can I substitute Incoloy 825 for Monel 400 to reduce cost?
Only where the medium permits it. For sulphuric acid, phosphoric acid and mildly sour oilfield service, 825 is often the better and cheaper choice. For hydrofluoric acid, caustic and marine hardware duty, it is not a valid substitution and may create a safety risk.
Q10: What are the UNS numbers and products forms available?
Monel 400 is UNS N04400 and Incoloy 825 is UNS N08825. We supply both as round bar (6-500 mm), plate and sheet (0.1-100 mm), seamless and welded tube (OD 6-610 mm), forgings, wire and strip, with EN 10204 3.1 or 3.2 certification and third-party witness inspection by SGS, BV or TUV on request.
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Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier
Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com
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