Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Monel 400 Seawater Corrosion - Flowing, Stagnant, Brackish, Crevice
Monel 400 seawater corrosion (also Monel 400 in seawater, UNS N04400 marine corrosion, nickel-copper alloy seawater rate) holds below 0.025 mm per year in clean flowing seawater up to 6 m/s, climbs to 0.1 to 0.5 mm per year inside stagnant, oxygen-depleted crevices, and is immune to chloride stress corrosion cracking. The 63 to 70 percent nickel and 28 to 34 percent copper chemistry (no chromium) gives a different failure profile from austenitic stainless: no chloride pitting on bold surfaces, no chloride SCC, but susceptibility to differential-aeration attack under deposits and gaskets. Monel® is a registered trademark of Special Metals Corporation; TorqBolt is not authorized by, affiliated with or endorsed by Special Metals. This page covers flowing vs stagnant behaviour, the ambient to 60 degrees Celsius envelope, brackish to ocean salinity, biofouling, galvanic compatibility, and 316L comparison.
Flowing Seawater - Monel 400 Holds Under 0.025 mm/Year
In clean, aerated seawater at 1 to 6 m/s, Monel 400 corrodes uniformly below 0.025 mm per year over multi-year exposure. Flow keeps the nickel-copper surface film aerated, strips deposits and incipient fouling, and starves pit-nucleation sites of the differential-aeration cell needed to grow. The plateau holds across open-ocean salinity. Service: marine applications with round bar shafts and Monel 400 bolts per ASTM F468.
| Condition | Monel 400 Rate (mm/yr) | Failure Mode | Comment |
|---|---|---|---|
| Clean flowing seawater, 1 to 6 m/s, ambient | < 0.025 | Uniform general | Bold-surface baseline; chloride SCC immune |
| Stagnant seawater, ambient, no crevice | 0.025 to 0.075 | Uniform general | Rate rises with oxygen depletion |
| Stagnant seawater under crevice or deposit | 0.1 to 0.5 | Crevice / pitting | Differential aeration; design out crevices |
| Flowing seawater, 40 to 60 degrees Celsius | 0.025 to 0.05 | Uniform general | Rate doubles per 25 degrees Celsius rise |
| Seawater > 80 degrees Celsius, deaerated | 0.05 to 0.15 | Uniform + sulphide if H2S present | Specify NACE MR0175 review |
| Brackish water, 5 to 30 g/kg | < 0.025 | Uniform general | Identical envelope to open-ocean |
Stagnant Seawater - Crevice Corrosion is the Dominant Risk
Stagnant-seawater behaviour splits on geometry. On a free bold surface, the rate stays near the flowing baseline. Under deposits, gaskets, fouling overlaps or thread crevices the local environment de-oxygenates within hours, the crevice acidifies, and localised attack of 0.1 to 0.5 mm per year develops with the bold cathode supplying current. Monel 400 still beats 316L stainless in stagnant seawater (no chloride pitting on the bold surface, no chloride SCC) but is not immune to crevice geometry. Design mitigation: no dead legs, full-bottom drains, smooth weld toes, periodic flush above 1 m/s, fastener-thread inspection after lay-up. Where crevices cannot be engineered out, specify Monel K-500.
Brackish vs Ocean Salinity Envelope
Monel 400 tolerates the full marine salinity band without a step-change. Brackish at 5 to 30 g/kg (estuaries, harbours, desalination intakes) corrodes at the same < 0.025 mm/year rate as open-ocean at 33 to 37 g/kg. Hypersaline brine to 60 g/kg (evaporator concentrate, salt-works reject) lifts the rate roughly 20 percent if oxygenated, a historical Monel 400 envelope for salt-pan equipment. The chloride pitting threshold that retires 316L does not apply: Monel 400 has no chromium passive film to break down. Reference: corrosion resistance.
Temperature Effects - Ambient to 60 to 80 Degrees Celsius
The rate roughly doubles per 25 degrees Celsius rise in the ambient to 60 degrees Celsius band, staying inside the < 0.05 mm/year corridor. Above 80 degrees Celsius it accelerates; deaerated hot brine (heat-exchanger shells, evaporator concentrate, water injection) needs case-by-case review, especially with any H2S (cross-check NACE MR0175 / ISO 15156). Marine ceiling: 60 degrees Celsius continuous, short excursions to 80.
Biofouling and Copper Leaching
The 28 to 34 percent copper in the solid solution leaches at a low, sustained rate (ppb to low ppm in the boundary layer) that suppresses barnacle, mussel, tubeworm and most macro-algal settlement on the bare metal. Macro-fouling on a Monel 400 propeller or pump shaft is markedly lower than on uncoated carbon steel or 316L. The copper dose stays below the toxic threshold for receiving-water discharge. The surface profile of valve trim and shaft journals is preserved through the service interval, and differential-aeration risk under fouling deposits is suppressed at source.
Galvanic Compatibility in Seawater
Monel 400 sits near the noble end of the marine galvanic series, close to 316L, more noble than bronze, and well above carbon steel, cast iron and zinc:
| Couple with Monel 400 | Galvanic Risk in Seawater | Practice |
|---|---|---|
| Carbon steel hull, bracket, baseplate | High (carbon steel is anode) | Phenolic gasket, insulating sleeves, sacrificial zinc upstream |
| Cast iron pump body | High (cast iron is anode) | Cathodic protection; not for unprotected CI bodies |
| Aluminium bronze, gunmetal, NAB | Moderate (bronze is anode) | Acceptable at similar wetted area; avoid small-bronze / large-Monel |
| Stainless 316L | Low (similar potential) | Direct contact acceptable; watch crevice on 316L side |
| Monel K-500 (UNS N05500), titanium grade 2 | Negligible | Free coupling |
| Zinc anode | Sacrificial - protects Monel 400 | Zinc or aluminium anodes for hull-bonded trim |
Bolting: Monel 400 stud bolts and heavy hex nuts per ASTM F468 / F467 fasten 316L or bronze flanges directly. Carbon steel flanges with Monel 400 bolting demand insulating gaskets and washers, or the carbon steel corrodes preferentially around the bolt holes.
Monel 400 vs 316L Stainless in Seawater
316L pits and crevice-corrodes under chloride attack in stagnant ambient seawater and fails by chloride stress corrosion cracking above roughly 60 degrees Celsius. Monel 400 has neither failure mode. Trade-off: Monel 400 is more sensitive to differential-aeration crevice attack than 316L on its bold surface, and yield strength is 40 to 50 ksi against 316L at 30 to 35 ksi. Full comparison: Monel vs stainless steel and Monel 400 vs Hastelloy C-276.
Frequently Asked Questions
Does Monel 400 corrode in seawater? Yes, less than 0.025 mm per year in clean flowing seawater up to 6 m/s at ambient. Rate climbs in stagnant or warm seawater and inside crevices. Immune to chloride SCC, the mode that retires 304 and 316L from warm seawater.
Crevice corrosion in stagnant seawater? Yes. Under deposits, gaskets and thread overlaps the rate climbs to 0.1 to 0.5 mm per year. Mitigate by design (no dead legs, smooth welds, full drains) and periodic flush above 1 m/s.
Does Monel 400 resist biofouling? Yes. The 28 to 34 percent copper leaches at a low rate that suppresses barnacle, mussel and tubeworm settlement. Macro-fouling is lower than on carbon steel or 316L.
Temperature ceiling? 60 degrees Celsius continuous, short excursions to 80. Above 80 the rate accelerates.
Can Monel 400 bolting fasten carbon steel flanges? Only with insulating gaskets, sleeves and washers. Direct coupling makes the carbon steel anode and pits it around the bolt holes.
Related Pages
Service envelopes: Monel 400 marine applications, Monel 400 corrosion resistance, HF acid resistance, NACE MR0175 / ISO 15156.
Alloy comparison: Monel 400 vs Monel K-500, Monel vs stainless steel, Monel 400 vs Hastelloy C-276.
Product forms for marine service: Monel 400 bolts, stud bolts, heavy hex nuts, round bar for shafts, seamless pipe, flanges.
Standards: ASTM B164, ASTM B127, ASTM B165, ASTM B564, ASTM F468, ASTM F467, UNS N04400.
Request a Quote
For Monel 400 (UNS N04400) bolting, shafting, plate, pipe or flanges against a defined seawater profile (flow, salinity, temperature, galvanic context, NACE MR0175 if H2S), send drawing, quantity and certification class (EN 10204 type 3.1 or 3.2) to info@torqbolt.com or WhatsApp +91-22-66157017. Ex-Mumbai price plus heat-numbered MTC inside one working day. Domestic India 3 to 7 working days; export through JNPT.
Full Monel 400 alloy reference on the main reference.
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