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 vs Stainless Steel (304, 316, 316L) for Wetted Service and Bolting
Monel 400 (UNS N04400, ASTM B164) is a nickel-copper alloy at 63 to 70 percent Ni and 28 to 34 percent Cu, balance Fe and Mn, with no chromium. SS 304 (UNS S30400) is the lean austenitic stainless at 18 to 20 Cr and 8 to 10.5 Ni; SS 316 (UNS S31600) adds 2 to 3 Mo to the same austenitic base; SS 316L (UNS S31603) is the low-carbon 316 variant at 0.030 percent C max for welded plant. Stainless is supplied to ASTM A240, A276 and A479; Monel 400 is supplied to ASTM B127 (plate, sheet), B164 (bar), B165 (pipe, tube), B564 (forgings) and F468 (bolts). Stainless 304, 316 and 316L are the default selection in ambient and non-aggressive service where cost and machinability dominate. Monel 400 is specified for hydrofluoric acid at any concentration, flowing seawater, brackish heat exchangers, sour gas at NACE MR0175 H2S thresholds, and non-magnetic naval hardware. In those services the chromium-rich passive film on stainless pits, crevice-corrodes or strips; the nickel-copper film on Monel 400 holds. Monel® is a registered trademark of Special Metals Corporation; TorqBolt is not authorized by, affiliated with or endorsed by Special Metals. Numbers cited on this page are from ASTM B164, ASTM A240, ASTM A276, ASTM F468 and NACE MR0175 / ISO 15156-3.
Where Monel 400 Replaces Stainless Steel
| Service environment | Specified alloy | Reason |
|---|---|---|
| Hydrofluoric acid, all concentrations | Monel 400 (mandatory) | Stainless passivation collapses in HF; the nickel-copper film holds. |
| Flowing seawater, brackish, marine splash | Monel 400 (preferred) | SS 304 and SS 316 pit and crevice-corrode under chloride attack above 30 degrees Celsius. |
| Sour service, H2S above NACE MR0175 thresholds | Monel 400 (qualified to ISO 15156-3) | Nickel-copper qualifies at the 35 HRC ceiling without prequalification; SS 316L is restricted by chloride and temperature clauses. |
| Ambient pharma, food-contact, fresh water | SS 316L | Passivation is sufficient; cost is one-third of Monel 400 by weight. |
| Structural framing, indoor mechanical assemblies | SS 304 | Cheapest austenitic, adequate corrosion margin for non-aggressive ambient. |
| Hot oxidising acids (HNO3, hot H2SO4 dilute) | SS 316L | Monel 400 attacks rapidly in oxidising acids; chromium passivation holds. |
Chemistry Comparison per ASTM B164 and ASTM A240
Monel 400 is a single-phase face-centred-cubic nickel-copper solid solution with no chromium and no molybdenum. SS 304, SS 316 and SS 316L are austenitic (gamma-iron) FCC stainless steels passivated by chromium oxide; SS 316 and 316L add molybdenum for chloride pitting resistance. The corrosion behaviour difference between the families is entirely driven by chemistry: nickel-copper film versus chromium-nickel-molybdenum passivation.
| Element | Monel 400 (N04400) | SS 304 (S30400) | SS 316 (S31600) | SS 316L (S31603) |
|---|---|---|---|---|
| Carbon C, max % | 0.30 | 0.08 | 0.08 | 0.030 |
| Manganese Mn, max % | 2.00 | 2.00 | 2.00 | 2.00 |
| Phosphorus P, max % | 0.024 (S spec) | 0.045 | 0.045 | 0.045 |
| Sulphur S, max % | 0.024 | 0.030 | 0.030 | 0.030 |
| Silicon Si, max % | 0.50 | 0.75 | 0.75 | 0.75 |
| Chromium Cr, % | none | 18.0 to 20.0 | 16.0 to 18.0 | 16.0 to 18.0 |
| Nickel Ni, % | 63.0 to 70.0 | 8.0 to 10.5 | 10.0 to 14.0 | 10.0 to 14.0 |
| Molybdenum Mo, % | none | none | 2.00 to 3.00 | 2.00 to 3.00 |
| Copper Cu, % | 28.0 to 34.0 | residual | residual | residual |
| Iron Fe, % | 2.5 max | balance | balance | balance |
Source: ASTM B164 for Monel 400; ASTM A240 Table 1 for the three stainless grades. Cobalt counting rules on the Monel 400 chemical composition page.
Mechanical Properties Comparison
All four alloys are solution-annealed for service. Strength is similar; Monel 400 cold-worked rod develops 85 ksi (585 MPa) yield per ASTM B164, exceeding any annealed stainless. SS 304 carries the highest annealed yield minimum at 30 ksi.
| Property (annealed) | Monel 400 | SS 304 | SS 316 | SS 316L |
|---|---|---|---|---|
| Tensile strength min, ksi (MPa) | 70 (480) | 75 (515) | 75 (515) | 70 (485) |
| Yield strength 0.2% offset min, ksi (MPa) | 25 (170) | 30 (205) | 30 (205) | 25 (170) |
| Elongation in 2 in. min, % | 35 | 40 | 40 | 40 |
| Hardness max, HBW | 110 to 150 typical | 201 | 217 | 217 |
| Hardness max, HRB | typically 75 to 95 | 92 | 95 | 95 |
| Cold-worked yield (Monel 400) min, ksi (MPa) | 85 (585) | n/a | n/a | n/a |
| Density, g/cm³ | 8.80 | 8.00 | 8.00 | 8.00 |
| Elastic modulus, GPa (Msi) | 179 (26) | 193 (28) | 193 (28) | 193 (28) |
Sources: ASTM B164 Table 1 (Monel 400 hot-worked annealed rod), ASTM A240 Table 2 (stainless). See Monel 400 mechanical properties.
Corrosion Resistance by Service Environment
Corrosion ranking by wetted environment. The cells carry text labels (best, good, poor) only; no colour coding so the table reads cleanly on screen readers and high-contrast modes.
| Environment | Monel 400 | SS 304 | SS 316 / 316L |
|---|---|---|---|
| Hydrofluoric acid, all concentrations | best | poor | poor |
| Seawater, flowing, ambient | best | poor (pitting) | good below 30 deg C |
| Seawater, stagnant, crevice | good | poor | poor (crevice attack) |
| Brackish, estuarine, splash zone | best | poor | good |
| Fresh water, potable, ambient | best | good | best |
| Atmospheric, marine coastal | best | poor (staining) | good |
| Dilute H2SO4, deaerated, ambient | good | poor | good |
| Dilute HCl, deaerated, ambient | good | poor | poor |
| Hot oxidising acid (HNO3, hot) | poor | good | best |
| NaOH caustic, all concentrations | best | good | good |
| Anhydrous ammonia, ambient | good | good | good |
| Food-contact pH 4 to 8 | good | good | best (FDA, EU 1935/2004) |
| Saturated brine | best | poor | good |
| Sour service H2S, NACE MR0175 | best (35 HRC cap) | poor | conditional (annex restrictions) |
Detail on Monel 400 corrosion resistance, HF acid resistance, seawater corrosion and NACE MR0175 sour service.
Cost Ratio: Why Monel 400 is 2.5 to 4 Times SS 316L
Mill price is a weighted average of LME feed. Monel 400: LME nickel (63 to 70 percent) plus LME copper (28 to 34 percent). SS 316L: LME nickel (10 to 14 percent), iron at roughly 65 percent, and LME molybdenum (2 to 3 percent). At mid-cycle nickel with copper near nickel parity, Monel 400 to SS 316L sits at 2.5 to 4 by weight; SS 304 (no Mo) sits at roughly 0.7 of SS 316L. Bolt and stud-bolt conversion costs are alloy-independent. LME-to-quote bridge on the Monel 400 pricing page.
Galling and Seizing on Stainless vs Monel 400 Bolting
Austenitic stainless bolting is notorious for thread galling, especially SS 304 and SS 316 stud-bolts in marine service. The galling initiates from chromium-oxide film welding at asperity peaks under contact stress; lubricant breakdown, salt contamination and tightening above 40 percent yield routinely seize the joint and force a hot-cut on disassembly. Monel 400 fasteners show inherent anti-galling against carbon steel and stainless mating because the nickel-copper film does not weld to chromium-oxide. Monel 400 bolts with stainless studs is a valid mixed-metal joint design. Stud-bolt detail on the Monel 400 stud bolts page.
Machinability and Weldability
Machinability: SS 316L machines easily. Lower nickel and no copper means less work-hardening at the chip interface; CNC turning with carbide at 80 to 120 m/min is routine. Monel 400 needs the free-machining variant Monel R-405 (UNS N04405) at 0.025 to 0.060 percent sulphur to match; standard N04400 work-hardens aggressively and demands rigid setup. Detail on the Monel 400 machinability page.
Weldability: Monel 400 welds without PWHT up to 38 mm thickness. Recommended fillers are ENiCu-7 (SMAW) and ERNiCu-7 (GTAW, GMAW) per AWS A5.11 and A5.14. SS 316L is also weldable with ER316L fillers but needs controlled heat input to avoid chromium-carbide precipitation in the HAZ (the reason 316L exists at 0.030 max C). Dissimilar Monel 400 to SS 316L joints use ERNiCrMo-3 (Inconel 625 filler).
Standards Cross-Reference
| Product form | Monel 400 (UNS N04400) | SS 316L (UNS S31603) | SS 304 (UNS S30400) |
|---|---|---|---|
| Plate, sheet, strip | ASTM B127 | ASTM A240 | ASTM A240 |
| Rod, bar, wire | ASTM B164 | ASTM A276 / A479 | ASTM A276 / A479 |
| Seamless pipe and tube | ASTM B165 | ASTM A312 / A269 | ASTM A312 / A269 |
| Condenser tube | ASTM B163 | ASTM A249 / A688 | ASTM A249 / A688 |
| Forgings and flanges | ASTM B564 | ASTM A182 F316L | ASTM A182 F304 |
| Bolts | ASTM F468 | ASTM F593 Group 2 | ASTM F593 Group 1 |
| Nuts | ASTM F467 | ASTM F594 Group 2 | ASTM F594 Group 1 |
| Sour service | NACE MR0175 / ISO 15156-3, 35 HRC max | ISO 15156-3 Annex A.4 (restricted) | not qualified for sour service |
When SS 316L Suffices
SS 316L is the cheaper correct selection when all of the following hold: ambient temperature below 60 degrees Celsius, chloride below 200 ppm in the wetted phase, no HF, no sour-service H2S above NACE thresholds, no flowing seawater, and no NaOH above 50 percent at temperature. Typical applications: pharma reactors, food and dairy piping, potable water, ambient non-halide acid storage, sheltered architectural marine-adjacent components, and closed-loop demineralised-water tubing. Specifying SS 316L on these services saves the 2.5 to 4 times Monel 400 premium without lifecycle risk.
When Monel 400 is Mandatory
Monel 400 is mandatory when any one is true: HF wetting at any concentration, flowing seawater above 30 degrees Celsius, tidal-zone marine fasteners, sour-service above the ISO 15156-3 Annex A.4 chloride or temperature limits for stainless, anhydrous HF storage, NaOH above 50 percent hot, and brackish heat-exchanger tubing cycling above 200 ppm chloride. Typical landings: HF alkylation internals and bolting per API RP 751; marine engineering for seawater valves, pump and propeller shafts; sour-service flange bolting at the 35 HRC NACE cap; brackish heat-exchanger tubing in coastal refineries. See the nickel-copper alloy family overview.
What About Inconel 625 and Hastelloy C-276?
When the service exceeds Monel 400 but stainless is also inadequate, the next material tier is nickel-chromium-molybdenum. Inconel 625 (UNS N06625) is specified above 540 degrees Celsius and in oxidising acids where Monel 400 fails. Hastelloy C-276 (UNS N10276) is specified for mixed-acid and reducing service across the broadest pH band. Monel 400 remains the specified alloy for HF and seawater at ambient temperatures, because the nickel-copper film holds where chromium-rich passivation fails (chromium is the element attacked in HF). See vs Hastelloy C-276.
Frequently Asked Questions
Is Monel 400 stronger than stainless steel? Not by annealed yield minimum. ASTM B164 Monel 400 rod is 25 ksi (170 MPa) yield, 70 ksi (480 MPa) tensile. ASTM A240 SS 304 is 30 ksi (205 MPa) yield, 75 ksi (515 MPa) tensile. Cold-worked Monel 400 reaches 85 ksi yield, exceeding annealed stainless. Corrosion environment, not strength, drives selection.
Is Monel 400 magnetic versus SS 304? Monel 400 is weakly ferromagnetic below a Curie point near 35 to 50 degrees Celsius and effectively non-magnetic above. SS 304 is austenitic and nominally non-magnetic annealed, but cold work induces strain-induced martensite and raises permeability. For non-magnetic service above 50 degrees Celsius, Monel 400 is cleaner. Detail on Monel 400 magnetic permeability.
Why does not Monel 400 contain chromium? Monel 400 relies on a nickel-copper oxide film, not chromium passivation. Chromium oxide fails in hydrofluoric acid and in chloride pitting environments. The nickel-copper film holds in HF, in deaerated seawater, and in dilute reducing acids.
Can you weld Monel 400 to stainless steel? Yes with a nickel-base filler, typically AWS A5.14 ERNiCrMo-3 (Inconel 625 filler). Use GTAW with argon shielding and moderate heat input. The Monel 400 side does not require post-weld heat treatment.
Does Monel 400 cost more than SS 316L? Yes, approximately 2.5 to 4 times by weight. Monel 400 is driven by LME nickel plus LME copper; SS 316L is driven by LME nickel at 10 to 14 percent plus iron plus LME molybdenum. Where SS 316L is corrosion-adequate it is the cheaper correct answer; where it is not, Monel 400 saves replacement cost.
Related Pages
Continue the Monel 400 reference: chemical composition, mechanical properties, corrosion resistance, HF acid resistance, HF alkylation, seawater corrosion, marine applications, NACE MR0175, nickel-copper alloy, UNS N04400, DIN 17743, vs Monel K-500, vs Hastelloy C-276. Product forms: fasteners, bolts, stud bolts, nuts, round bar, plate, tubing, seamless pipe, forgings, fittings, flanges. Pricing: Monel 400 pricing.
Request a Quote
For a Monel 400 versus stainless steel selection review on a specific service, send the operating temperature, fluid composition (especially chloride ppm and any HF or H2S partial pressure), product form and quantity to info@torqbolt.com or WhatsApp +91-22-66157017. TorqBolt QA-QC returns the alloy-selection memo, EN 10204 certification type, mill source and Ex-Mumbai price within one working day. Domestic India delivery 3 to 7 working days; export shipments through JNPT.
References
ASTM B127, B164, B564 (Monel 400 plate, rod, forgings). ASTM A240/A240M, A276/A276M (stainless plate and bar). NACE MR0175 / ISO 15156-3 (H2S environments, Part 3, CRAs). Nickel Institute publications on Monel 400 in seawater and HF. AWS A5.11 and A5.14 nickel-alloy welding consumables.
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