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 Corrosion Resistance - HF, Seawater, Sour Service & More
Monel® 400 corrosion resistance (also written Monel 400 corrosion, UNS N04400 corrosion resistance, nickel-copper 400) is set by a thin nickel-copper passive film rather than chromium-rich passivation. The film holds in hydrofluoric acid, flowing seawater, caustic NaOH, sour H2S service per NACE MR0175 and dilute reducing acids. It dissolves in hot oxidising acids (nitric, chromic, ferric chloride) and degrades in aerated ammonia. Monel is a registered trademark of Special Metals Corporation; TorqBolt is not authorized by, affiliated with or endorsed by Special Metals. TorqBolt manufactures UNS N04400 fasteners and product forms per ASTM B127, B164, B564, F467 and F468 from Mumbai stock.
The Nickel-Copper Passive Film Mechanism
UNS N04400 is a solid-solution alloy at 63 to 70 percent Ni and 28 to 34 percent Cu per ASTM B127. The surface oxide is a duplex layer of nickel oxide over copper oxide that re-forms on fresh metal in air, water and most aqueous electrolytes. The film tolerates reducing conditions where chromium-passivated stainless loses passivity, and dissolves under strong oxidisers because Cu(I) oxidises to soluble Cu(II). Composition detail on chemical composition and the nickel-copper alloy family.
Corrosion Performance by Service Environment
UNS N04400 behaviour across the wetted services that drive material selection. Best, good and poor are text-only ratings.
| Environment | Monel 400 rating | Driving mechanism |
|---|---|---|
| Hydrofluoric acid, all concentrations to 120 deg C | best | Ni-Cu film stable; F- does not attack |
| Anhydrous HF storage and transport | best | No free water; no film breakdown |
| Seawater, flowing, ambient to 50 deg C | best | Erosion-corrosion resistant film |
| Seawater, stagnant, crevice geometry | good | Crevice and biofouling possible |
| Brackish and estuarine water | best | Chloride pitting resistant |
| Dilute H2SO4 / HCl, deaerated | good | Holds at low aeration only |
| NaOH caustic, all concentrations to 290 deg C | best | Industry reference alloy |
| Sour service H2S per NACE MR0175 | best (35 HRC cap) | Annex A.8 qualified |
| Aerated ammonia with O2 ingress | poor | Cu-NH3 complex formation |
| Hot HNO3, hot H2SO4 oxidising | poor | Cu(I) to Cu(II) oxidation |
| Chromic acid, ferric chloride, wet chlorine | poor | Strong oxidisers dissolve Cu |
Side-by-side comparison to austenitic stainless on the Monel 400 vs stainless steel page.
Where Monel 400 Excels
Hydrofluoric acid: the film does not react with fluoride at any concentration from dilute aqueous HF through 70 percent commercial HF up to anhydrous service. Stainless and copper alloys fail; Monel 400 is the specified material for HF alkylation reactors, valve bodies and bolting per API RP 751.
Seawater and brackish water: specified for marine pump shafts, propeller shafts, valve stems and condenser tubing; the film resists chloride pitting and erosion-corrosion at flowing velocities up to 8 m/s. In stagnant or crevice geometry the rating drops to good.
NaOH caustic: industry reference alloy for caustic soda from dilute through 73 percent up to 290 degrees Celsius. Above that, low residual stress and stress-relieved welds are specified.
Sour service H2S: qualified per NACE MR0175 / ISO 15156-3 Annex A.8 at a 35 HRC hardness cap.
Dilute reducing acids: deaerated dilute H2SO4 and HCl corrode at low rates suitable for storage and transfer. Aeration accelerates attack; not specified for aerated service.
Where Monel 400 Fails
Hot oxidising acids: HNO3 above 10 percent dissolves copper and the film fails. Chromic acid strips Monel 400 in plating-bath service. Ferric chloride etches the surface.
Wet chlorine at bleach-plant partial pressures attacks through the same mechanism. Aerated ammonia with oxygen ingress forms soluble copper-ammonium complexes; deaerated anhydrous ammonia is acceptable. Mercury induces mercury-SCC. For oxidising and mixed-acid service the specified materials are Ni-Cr-Mo alloys; selection between Inconel 625 and Hastelloy C-276 sits on vs Hastelloy C-276.
HF Acid Resistance
In refinery HF alkylation units Monel 400 is used for reactor internals, settlers, strippers, valve bodies and bolting per API RP 751. Chromium-passivated stainless fails because chromium fluoride is soluble in HF; the nickel-copper film is fluoride-stable and the corrosion rate stays below 0.1 mm/yr at typical alkylation temperatures. Bolting is ASTM F468 studs with ASTM F467 nuts. Full coverage on the HF acid resistance page.
Seawater and Brackish Service
In flowing seawater above 1 m/s the alloy resists pitting, crevice and erosion-corrosion to 50 degrees Celsius. Above 50 degrees in oxygenated seawater the rate increases but stays usable to 80 degrees. Stagnant seawater under deposits and biofouling can develop crevice attack; the mitigation is flow maintenance or cathodic protection. Brackish fasteners on docks, splash zones and offshore platforms specify the alloy. Detail on seawater corrosion and marine applications.
Sour Service H2S per NACE MR0175
UNS N04400 is qualified per NACE MR0175 / ISO 15156-3 Annex A.8 at 35 HRC maximum. The qualification covers any H2S partial pressure, any chloride and any in-situ pH; the only specified limit is the hardness cap, which constrains cold-worked product forms and weld HAZ. Sour-service flange, wellhead and downhole bolting is typically annealed N04400 to stay below the cap. Austenitic stainless is restricted to Annex A.4 limits; martensitic stainless is restricted further. Full coverage on sour-service NACE and NACE MR0175.
Stress Corrosion Cracking Envelope
Monel 400 is immune to chloride SCC across the range that limits austenitic stainless above 60 degrees Celsius; this is why the alloy displaces SS 316L on hot brackish heat-exchanger tubing and splash-zone marine fasteners. It is susceptible to mercury-SCC and to caustic SCC above 290 degrees Celsius in concentrated NaOH; mitigation is low residual stress, stress-relieved welds and hardness control. See mechanical properties and heat treatment.
Frequently Asked Questions
Does Monel 400 rust? No. The alloy contains no iron-rich phase and does not form red iron oxide. In air, water and marine atmosphere the surface darkens to a brown to olive protective patina rather than rusting.
What pH range does Monel 400 handle? Roughly pH 1 in reducing acids (HF, deaerated dilute H2SO4 and HCl) through neutral water and seawater up to pH 14 in concentrated NaOH. The alloy is oxidiser-limited rather than pH-limited; strong oxidisers dissolve the film at any pH.
How does Monel 400 compare to Inconel and Hastelloy? Monel 400 leads in HF and flowing seawater to 50 degrees Celsius. Inconel 625 (UNS N06625) leads above 540 degrees Celsius and in oxidising service. Hastelloy C-276 (UNS N10276) leads in mixed-acid and reducing service across the broadest pH band.
Is Monel 400 immune to stress corrosion cracking? Immune to chloride SCC above 60 degrees Celsius. Susceptible to mercury-SCC and to caustic SCC above 290 degrees Celsius in concentrated NaOH.
Temperature limit for corrosion service? Roughly 540 degrees Celsius in oxidising air. In sour service the limit is the 35 HRC hardness cap. In seawater the alloy holds to 80 degrees Celsius.
Related Pages
Environments: HF acid, HF alkylation, seawater corrosion, marine applications, sour-service NACE, NACE MR0175. Material: vs stainless steel, vs Hastelloy C-276, nickel-copper alloy, chemical composition, mechanical properties, heat treatment, UNS N04400. Forms: fasteners, bolts, nuts, round bar, plate, tubing, flanges. Standards: ASTM B127, ASTM B164, ASTM B564, F467, F468. Pricing, data sheet.
Request a Quote
For a Monel 400 corrosion-selection review on a specific service, send the operating temperature, fluid composition (chloride ppm, HF or H2S partial pressure, oxidiser content), product form and quantity to info@torqbolt.com or WhatsApp +91-22-66157017. TorqBolt QA-QC returns the suitability memo, EN 10204 certification type, mill source and Ex-Mumbai price within one working day. Domestic India delivery 3 to 7 working days; export through JNPT.
References
ASTM B127, B164, B564, F467 and F468. NACE MR0175 / ISO 15156-3 Annex A.8. API RP 751 for HF alkylation. Nickel Institute reference publications on Monel 400 in seawater, HF and caustic service.
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