Inconel is a family of nickel-chromium alloys, and Monel is a family of nickel-copper alloys. That single difference, chromium against copper, decides almost every selection question between them.
Monel resists hydrofluoric acid and slow-moving seawater but contains no chromium, so it fails in oxidising acids and has no meaningful PREN. Inconel adds chromium and molybdenum for high-temperature strength and chloride resistance. Inconel vs Monel isn’t a contest to be won. It’s a choice between two different answers to two different corrosion problems.
Consider a seawater cooling loop at a Gulf Coast petrochemical plant. The original design carried Monel 400 at a flow velocity of 1.2 m/s, and the pipe served for more than a decade. When the plant debottlenecked in 2023 and lifted circulation flow to 3.4 m/s, the same Monel 400 began showing velocity-accelerated corrosion within eighteen months.
The correct fix wasn’t a heavier wall. It was a family change to Inconel 625, which tolerates 3 to 5 m/s in the same medium. Same seawater, same temperature, different material, because one design variable moved.
Suppliers quote Monel and Inconel interchangeably far more often than the metallurgy justifies. This article gives you the family-level logic to stop that. You will get the four-family decision table, the family-to-standard map, PREN applied correctly to each family, the temperature ladder with its governing condition, and the costed over-specification warning that saves the most money on this page.
Key Takeaways
- Inconel is Ni-Cr and Monel is Ni-Cu. The second element decides the application: chromium for high-temperature strength and oxidising resistance, copper for hydrofluoric acid and seawater nobility.
- Monel has no chromium and no molybdenum, so PREN is not applicable to it. Any page quoting a PREN for Monel 400 is applying the wrong formula.
- In pipe form the standards are Monel 400 to ASTM B165 (seamless) and B725 (welded); Inconel 625 to ASTM B444 and B705; Incoloy 825 to B423 and B704; Hastelloy C-276 to B622 and B619.
- Flow velocity, not just chemistry, flips the Monel-to-Inconel decision in seawater. Monel 400 is limited to roughly 1.5 m/s before velocity-accelerated corrosion; Inconel 625 tolerates 3 to 5 m/s.
- These four families are not a hierarchy. Buying Hastelloy or Inconel where Monel 400 or Incoloy 825 is correct is the most expensive mistake in this market, and it can multiply material cost several times over.
Not sure which family your line actually needs? Send our technical team the medium, the concentration, the operating temperature and the flow velocity. We will confirm which family is correct, and just as often, which one you do not need. Describe your service conditions
What Is the Difference Between Inconel and Monel?
The short answer is the alloying addition. Inconel is built on nickel and chromium, with molybdenum and niobium added for strength and pitting resistance. Monel is built on nickel and copper, with no chromium at all.
Chromium forms a protective oxide film that resists oxidising media and high temperature. Copper delivers nobility in reducing environments, particularly hydrofluoric acid and chloride-bearing seawater, but it can’t replace the passive film that chromium provides.
That chemistry difference explains a set of selection rules that hold across every grade in both families:
- Oxidising acid or hot, chloride-bearing service points to Inconel. Nitric acid, high-temperature oxidation and pitting in aerated chlorides all favour the Ni-Cr family.
- Hydrofluoric acid, or slow-velocity seawater, points to Monel. Copper is the active ingredient, and no chromium-bearing alloy matches Monel 400 in anhydrous and aqueous HF at moderate temperature.
- Reducing acid, wet chlorine or mixed chemistry points to neither of these two. That is Hastelloy territory, and it is covered below.
One clarification before going further, because it’s the most common specification error in this space. Inconel, Incoloy and Monel are trade names, not grades. Saying “quote me Inconel” is not a specification.
Inconel spans 600, 625, 718 and more, each with different chemistry, a different ASTM standard, and a different application. Our complete nickel alloy pipe guide maps the full grade list; this article works one level above it, at the family decision.
The Four Nickel Alloy Families Compared
Four families cover nearly every industrial nickel alloy pipe enquiry, and they are the types of nickel alloys you will meet on a pipeline list. Which one applies is decided by what has been added to the nickel base, and each family carries one signature strength and one hard limit. Comparing Monel vs Inconel vs Incoloy vs Hastelloy is therefore not a ranking exercise. It is a matching exercise.
Monel (Ni-Cu): Hydrofluoric Acid and Slow Seawater
Monel is nickel alloyed primarily with copper, at 28 to 34% copper in Monel 400. Its signature strength is hydrofluoric acid resistance, and it has been the specification for HF alkylation unit piping in refineries for decades. It is also effective in slow-moving seawater, where its corrosion rate stays below 0.025 mm per year and it remains effectively immune to chloride stress corrosion cracking.
The hard limit follows directly from the missing chromium. Monel has no passive film, so it performs poorly in strongly oxidising acids such as nitric, and it is vulnerable to velocity-accelerated corrosion in fast-flowing seawater.
Inconel (Ni-Cr): High-Temperature Strength and Oxidation Resistance
Inconel grades are built on a nickel-chromium base, and Inconel pipe is chosen where chromium’s oxide film (Cr₂O₃) must hold at temperature. Inconel 625 combines chromium and molybdenum with niobium for strength, making it the most-specified nickel alloy pipe grade. Creep and oxidation resistance extend to roughly 980 °C, and its chloride pitting resistance is high. In a direct monel 400 vs inconel 625 comparison in seawater, the deciding variable is flow velocity, not chemistry.
The hard limit is the mirror image of Monel’s. Chromium and molybdenum make Inconel strong in oxidising and chloride media, but they do not make it the answer for reducing hydrochloric or sulphuric acid. That is Hastelloy’s role. Grade detail, including the Grade 1 versus Grade 2 delivery condition decision, belongs with our Inconel 625 pipe specification guide.
Incoloy (Ni-Fe-Cr): The Cost-Effective Middle Tier
Incoloy grades add substantial iron to the nickel-chromium base, which lowers cost while retaining much of the corrosion resistance. The incoloy vs inconel distinction comes down to that iron: Incoloy carries more iron and less nickel, and the substitution is what lowers the price. Incoloy 825 also adds molybdenum and copper, giving it the ability to handle sulphuric and phosphoric acid as well as sour gas containing hydrogen sulphide, carbon dioxide and chlorides together. It is the cheapest genuine route into nickel alloy performance.
Its limit is mechanical rather than chemical. Incoloy 825 is not a high-temperature-strength alloy, so it does not replace Inconel where creep resistance above roughly 540 °C governs.
Hastelloy (Ni-Mo-Cr): Reducing Acids, Wet Chlorine and Mixed Acids
Hastelloy is the Ni-Mo-Cr family, and in the C-family grades molybdenum reaches 15 to 17%. That content drives resistance to reducing acids, wet chlorine, hypochlorite and the mixed oxidising-reducing chemistries that defeat single-purpose alloys. The inconel vs hastelloy decision follows the same logic as every other pairing here, decided by whether chromium or molybdenum is dominant. Hastelloy C-276 is the benchmark severe-corrosion grade, and its corrosion envelope is covered in detail in our Hastelloy C-276 pipe guide.
The limits are cost and scope. C-276 is among the most expensive pipe materials per kilogram, it is weak in nitric acid, and it is not a strength play.
The Decision Table: Inconel vs Monel vs Hastelloy vs Incoloy
| Monel | Inconel | Incoloy | Hastelloy | |
|---|---|---|---|---|
| Base chemistry | Ni-Cu | Ni-Cr | Ni-Fe-Cr | Ni-Mo-Cr |
| Representative pipe grade | Monel 400 | Inconel 625 | Incoloy 825 | Hastelloy C-276 |
| What the addition buys you | Seawater and HF resistance from copper | High-temperature strength and oxidation resistance from chromium | A lower-cost route to Ni-Cr corrosion resistance from iron | Reducing-acid and mixed-acid resistance from 15–17% molybdenum |
| Signature strength | Hydrofluoric acid; slow-velocity seawater; caustic | Creep and oxidation to ≈ 980 °C; chloride pitting | Sulphuric and phosphoric acid; sour gas with H₂S, CO₂ and Cl⁻ | Wet chlorine, hypochlorite, HCl, mixed reducing acids |
| Hard limit | No Cr or Mo, so it fails in oxidising acids; seawater limited to ≈ 1.5 m/s | Not the answer for reducing HCl or H₂SO₄; Grade 2 embrittles above ≈ 600 °C long-term | Lower strength; not a high-temperature-strength alloy | Expensive; weak in nitric acid; not a strength play |
| Service temperature | ≈ 480 °C | ≈ 980 °C, condition-dependent | ≈ 540 °C | ≈ 677 °C corrosion service / ≈ 980 °C oxidation limit |
| PREN | Not applicable | ≈ 48–51 | ≈ 30–34 | ≈ 69 (≈ 75 tungsten-inclusive) |
| UNS / W.Nr. | N04400 / 2.4360 | N06625 / 2.4856 | N08825 / 2.4858 | N10276 / 2.4819 |
| Density (g/cm³) | 8.80 | 8.42–8.44 | 8.14 | 8.89 |
| Relative cost (× 316L) | ≈ 7–8× | ≈ 8–10× | Lowest of the four | ≈ 10–20× |
| Magnetic in service? | Feebly magnetic at room temperature | Non-magnetic | Non-magnetic | Non-magnetic |
Specifying Inconel vs Monel Correctly: Trade Names, Grades and the UNS Number
The naming origin matters because it explains why the family labels cannot be ordered by quality. Inconel, Incoloy and Monel originated with INCO, the International Nickel Company, and are now held by Special Metals Corporation. Hastelloy is a trade name held by Haynes International. The generic forms in current industrial use are Alloy 625, Alloy 400, Alloy 825 and C-276.
Why “Quote Me Inconel” Is Not a Specification
The grade numbers are fractional, not sequenced. 400, 625, 825 and C-276 are not a series, and a higher number is not a better alloy. They were assigned independently, and they indicate nothing about position in a hierarchy.
This is where substitution ambiguity enters. A purchase order that reads “Inconel” can be satisfied with 600, 625, 718 or several other grades, each built to a different ASTM standard with a different chemistry. Two suppliers can both quote “Inconel” in good faith and ship materially different pipe.
The UNS Number Is the Procurement Identifier
The fix is to specify by UNS number. Put N04400, N06625, N08825 or N10276 on the RFQ, the purchase order and the mill test report, and the trade-name ambiguity disappears. The European W.Nr. designations, 2.4360, 2.4856, 2.4858 and 2.4819, serve the same purpose for EN-based projects.
On the RFQ, three identifiers belong together: UNS number, ASTM standard and product form. Omit the form and the mill may supply welded pipe where your line list specified seamless, which changes the test regime and the pressure rating.
Which ASTM Standard Covers Which Family
Standards are split first by family and then by product form. Seamless and welded pipe are governed by different specifications, and the split is about manufacturing route, not quality. Both forms are fully specified, fully tested products.
| Family | Representative grade | Seamless pipe / tube | Welded pipe |
|---|---|---|---|
| Monel (Ni-Cu) | Monel 400 | ASTM B165 | ASTM B725 |
| Inconel (Ni-Cr) | Inconel 625 | ASTM B444 | ASTM B705 |
| Incoloy (Ni-Fe-Cr) | Incoloy 825 | ASTM B423 | ASTM B704 |
| Hastelloy (Ni-Mo-Cr) | Hastelloy C-276 | ASTM B622 | ASTM B619 |
General requirements sit in ASTM B829 for cold-worked seamless nickel alloy pipe and tube, and ASTM B775 for welded nickel alloy pipe. The ASME SB-series equivalents apply where the piping is built to a pressure code.
Sizes, Schedules and the Density Consequence
Dimensions follow ASME B36.19, the stainless schedule series, not B36.10. Schedules 5S, 10S, 40S and 80S are the usual catalogue entries, and the density spread across these four families changes the shipped weight at equal OD and wall.
Densities run from 8.14 g/cm³ for Incoloy 825 to 8.89 g/cm³ for Hastelloy C-276. A kg/m figure carried over from a 316L line list is therefore wrong for every one of the four families. Incoloy 825 is about 2% lighter than 316L, while C-276 is roughly 11% heavier, which shifts support spacing and delivered tonnage before any alloy premium is applied. Our guide to pipe schedule dimensions covers the dimensional framework.
Inconel vs Monel vs Incoloy vs Hastelloy: Corrosion Performance
A strengths-only comparison is what makes buyers over-specify. The selection rule comes from the limits, so each family below is presented with both its wins and its failures.
Monel 400: HF, Caustic and the Seawater Velocity Limit
Monel 400’s resistance to hydrofluoric acid is its defining property, and it is more resistant than Inconel 600 in 40% HF at 50 °C. It also performs well in caustic and alkali service. In seawater, its corrosion rate is low and Cl-SCC is not a concern.
The limit is velocity. Monel 400 is commonly held to roughly 1.5 m/s in seawater before velocity-accelerated corrosion begins. That single number settles more monel vs inconel pipe selections in cooling-water service than any other variable.
Treat it as an indicative design guideline, not a standard limit; your project’s corrosion allowance and erosion review govern. Above roughly 1.5 m/s, the material decision moves to Inconel or to a higher-molybdenum alloy.
Inconel 625: Chlorides, Pitting and High-Temperature Oxidation
Inconel 625 combines a PREN of 48 to 51 with high-temperature strength, which is a combination no stainless grade offers. In seawater it tolerates 3 to 5 m/s, and it resists pitting and crevice attack in chloride-bearing service.
The limit is reducing acid. In dilute hydrochloric acid, C-276 outperforms 625 decisively, and specifying 625 there is a common and costly error.
Incoloy 825: Sulphuric and Phosphoric Acid, and Sour Gas
Incoloy 825 handles sulphuric and phosphoric acid service and is widely used in sour gas wells, where tubing and flowlines must resist H₂S, CO₂ and chlorides together. It is the family to reach for when the duty is real but a full severe-corrosion alloy would be over-specified.
The limit is strength and temperature, not chemistry in most applications. Where elevated-temperature creep resistance is the driver, 825 is the wrong tool.
Hastelloy C-276: Wet Chlorine, Hypochlorite and Mixed Reducing Acids
Hastelloy C-276 is one of a small number of alloys approved for wet chlorine service, and it is a common specification in chlorine dioxide generation and hypochlorite handling. It resists both reducing and oxidising media, which lets it work where the chemistry alternates.
The limits are cost, nitric acid performance and strength. It is not the alloy to specify for a dilute sulphuric duty that 2205 duplex would handle at a fraction of the price.
PREN and Where It Does Not Apply
Pitting Resistance Equivalent Number is the standard screening calculation for chromium-bearing alloys:
PREN = %Cr + 3.3 × %Mo + 16 × %N
Applied at mid-range composition, that gives Inconel 625 a PREN of roughly 48 to 51, Incoloy 825 around 30 to 34, and Hastelloy C-276 around 69, or 75 on the tungsten-inclusive variant.
PREN does not apply to Monel. The formula is defined for chromium-bearing alloys, and Monel 400 contains neither chromium nor molybdenum. Its seawater performance comes from nickel-copper nobility, not from a passive film. Any supplier page quoting a PREN for Monel 400 is applying a formula that does not describe the alloy. The same screening logic is explained in our article on PREN and chloride resistance in practice.
One caveat on Incoloy 825 deserves emphasis, because it costs buyers money. At a PREN of 30 to 34, 825 scores below Inconel 625 and looks inferior on a spreadsheet.
But PREN measures pitting resistance, not general corrosion resistance, and it doesn’t capture sulphuric or phosphoric acid performance at all. A buyer who screens 825 out on PREN alone will over-buy 625 for an acid duty that 825 handles. PREN is a screening tool for one mechanism, not a scorecard for corrosion.
Temperature Limits: The Family Ladder
Every one of these families has more than one temperature number, and the difference is the most misquoted data on supplier pages. The reason is that oxidation resistance and corrosion-service resistance are separate properties, and they diverge sharply.
- Monel 400: ≈ 480 °C. This is the corrosion-service limit. Supplier pages that quote 550 °C are mixing in mechanical or short-term oxidation data.
- Incoloy 825: ≈ 540 °C. Above this, creep behaviour and strength, rather than corrosion, control the selection.
- Hastelloy C-276: ≈ 677 °C in corrosion service, up to ≈ 980 °C as an oxidation limit. These are not interchangeable numbers, and the gap between them is roughly 300 °C.
- Inconel 625: ≈ 980 °C. This is the family’s headline capability, and it is the reason Inconel exists at all. The Grade 1 versus Grade 2 delivery condition decides whether it applies.
The practical rule for process piping is that the corrosion-service limit governs, not the oxidation limit. High-temperature oxidation data describes dry, scaling environments, which is not most chemical process duty. Where your application sits between the two numbers, the stream chemistry decides, and it should be reviewed rather than assumed.
Is Monel Magnetic? Physical and Fabrication Differences That Matter
Yes, standard Monel 400 is feebly magnetic at room temperature, with a Curie point of roughly 21 to 49 °C. That is a genuine exception among these four families: it means Monel 400 loses what little magnetism it has as the pipe warms. Monel K500, the age-hardened grade, and all three of the other families are effectively non-magnetic in service.
This matters for instrument tubing, magnetic-permeability specifications and some electrical applications. It does not matter for most process piping, so treat it as a specification detail rather than a selection driver.
Welding Filler Selection by Family
Filler metal is matched to family, and mismatching it is a common cause of premature weld failure:
| Family | Matching filler (AWS A5.14) |
|---|---|
| Monel 400 | ERNiCu-7 |
| Inconel 625 | ERNiCrMo-3 |
| Incoloy 825 | ERNiFeCr-1 |
| Hastelloy C-276 | ERNiCrMo-4 |
Work-Hardening, Galling and Machinability
Two fabrication behaviours distinguish these families from stainless. First, all four work-harden aggressively, so machining and threading demand slower speeds and heavier feeds than 316L. Second, the nickel-rich families, Monel in particular, are prone to galling on threaded connections, which usually calls for a lubricant or a dissimilar-metal contact. Qualification of the weld procedure, not just the filler selection, is the controlling step on high-alloy grades; our article on duplex stainless steel welding covers the qualification logic that carries across to these materials.
Cost: The Over-Specification Trap
Cost is where the family decision becomes a budget decision. The multiples below are indicative, undated, and referenced to an equivalent 316L pipe, because nickel and molybdenum prices move and a page-written number is not a quotation.
| Family | Approximate multiple of 316L pipe |
|---|---|
| Incoloy 825 | Lowest of the four nickel families |
| Monel 400 | ≈ 7–8× |
| Inconel 625 | ≈ 8–10× |
| Hastelloy C-276 | ≈ 10–20× |
When Incoloy 825 or Monel 400 Is the Cheaper Correct Answer
The most expensive error in this market is a buyer specifying Hastelloy C-276 or Inconel 625 where Monel 400 or Incoloy 825 does the job. The gap is not marginal. A sulphuric acid duty at moderate temperature that Incoloy 825 handles correctly, specified instead as C-276, can multiply material cost several times over, and it buys nothing the application needed.
The same principle operates in the other direction. A dilute sulphuric acid duty at 40 °C that 2205 duplex handles belongs in duplex, not in any of these four families. The step from stainless to duplex is covered in our super duplex S32750 corrosion guide, and the full class-versus-class cost ladder, including when an upgrade to nickel alloy is justified at all, is set out in when to upgrade from stainless steel to nickel alloy.
Why Quotes Expire, and What to Pin Down
Nickel and molybdenum are the dominant price drivers in every family, and both have been volatile. A price per kilogram without a validity date is not a usable number for budgeting. Ask for a validity-dated quote, and confirm whether an alloy surcharge applies.
Keep one disambiguation in mind while costing, because it catches buyers in this exact segment. Cupronickel is not a nickel alloy. Copper-nickel grades such as C70600 and C71500 are copper-based with nickel as the addition, a different family entirely. If your application is seawater with a copper-based material, our copper-nickel tubes (C70600/C71500) guide covers it properly.
Send us your line list and we will return a family-by-family recommendation with the UNS number, governing ASTM standard and documentation package for each line. Send us your line list
Inconel vs Monel: Frequently Asked Questions
Is Monel stronger than Inconel?
It depends on the grade, and the comparison isn’t as lopsided as the family names suggest. Monel 400 is a moderate-strength material, with an ASTM B165 minimum yield of 172 MPa and a minimum tensile of 483 MPa. Monel K500, the age-hardened version of the same family, reaches roughly 700 MPa yield and 1,000 MPa tensile, about three times the yield of Monel 400.
Inconel 625 in Grade 1 has a minimum yield of 414 MPa and tensile of 827 MPa, and it retains that strength at temperature where Monel cannot. So Inconel is stronger at temperature, while Monel K500 is competitive at room temperature. For pipe, strength is rarely the deciding factor; corrosion resistance and temperature capability usually are.
Is Monel magnetic?
Standard Monel 400 is feebly magnetic at room temperature, with a Curie point of roughly 21 to 49 °C, so it becomes less magnetic as it warms. Monel K500 is effectively non-magnetic to well below zero, and Inconel, Incoloy and Hastelloy are all non-magnetic in service. This only matters for magnetic-permeability specifications and instrument duty.
Which is more expensive, Inconel or Monel?
Inconel 625 generally costs more than Monel 400 in pipe form, at roughly 8 to 10 times an equivalent 316L pipe against 7 to 8 times for Monel 400. The two track each other closely because both are driven by the nickel price. Costs move with nickel and molybdenum markets, so any figure without a validity date should be treated as indicative only.
What is the difference between Inconel and Incoloy?
Inconel is a nickel-chromium family with nickel above 50%, built for high-temperature strength and oxidation resistance. Incoloy is a nickel-iron-chromium family with nickel below 50%, where the iron addition lowers cost while preserving much of the corrosion resistance. Inconel 625 is the high-temperature, high-strength choice; Incoloy 825 is the cost-effective sulphuric acid and sour-gas choice.
What is the difference between Inconel and Hastelloy?
Inconel is chromium-dominant and Hastelloy is molybdenum-dominant, and that decides the chemistry. Inconel 625 handles oxidising media, chlorides and high temperature. Hastelloy C-276 handles reducing media, wet chlorine and mixed acids, and it tolerates a broader chemistry range at a significantly higher cost. Inconel wins in nitric acid and high-temperature strength; Hastelloy wins in hydrochloric acid and wet chlorine.
Can Inconel and Monel be welded together?
They can be joined, but the weld is a dissimilar-metal joint and it needs an engineered filler rather than a default. The usual approach is a nickel-based filler such as ERNiCrMo-3 that is compatible with both sides, with a qualified weld procedure that addresses dilution and thermal expansion mismatch. The weld, not the base metal, usually becomes the limiting point in the joint, so it should be qualified and documented before production, not improvised in the field.
Conclusion: Choose the Family for the Medium, Not the Prestige
These four families are not a hierarchy, and the most expensive mistake in this market is buying the wrong one. Whether the question is inconel vs monel, or either of them against Incoloy and Hastelloy, the method is the same: match the family to the medium and the service conditions, and the cost follows.
Four decision rules cover most of the ground:
- Oxidising acid, or hot chloride-bearing service: Inconel. Chromium does the work.
- Hydrofluoric acid, or seawater below roughly 1.5 m/s: Monel. Copper does the work.
- Sulphuric or phosphoric acid, or sour gas, and cost-sensitive: Incoloy 825. Iron keeps the price down.
- Reducing acid, wet chlorine or mixed chemistry: Hastelloy. Molybdenum does the work.
Then verify the three variables that override family chemistry: temperature, with the corrosion-service limit rather than the oxidation limit, flow velocity if seawater is involved, and chloride concentration. If none of these four families is clearly justified, check whether 2507 super duplex covers the duty first. It often does, and it does so at a lower cost.
Whichever family applies, specify it by UNS number and ASTM standard, and require the mill test report to confirm chemistry against the standard’s table rather than against a trade name.
Tell us the medium, the concentration, the temperature and the flow velocity. Our technical team will confirm which family is correct, and just as often which one you do not need, and respond with grade, form, standard and lead time within 24 hours. Describe your service conditions
