Incoloy vs Inconel vs Hastelloy vs Monel: Nickel Alloys Explained

The four nickel-alloy families at a glance

Each name is an alloy system, and it's defined by its dominant alloying element. Read the element signature and you've already narrowed the grade. The table below gets you from a trade name to the chemistry behind it fastest, and from there, to the service it was built for.

Key takeaways:

The chemistry hides in the family name, and once you decode it, the service follows.

People mix up Incoloy and Inconel all the time, the names rhyme, after all. But Incoloy is iron-bearing, a step up from stainless, while Inconel runs nickel-rich and is built to handle heat.

• Hastelloy carries the most molybdenum, so it owns the reducing-acid and severe-pitting duties the others cannot.
• Monel has no chromium at all. That trade-off costs it resistance to oxidizing acids but buys unmatched performance in hydrofluoric acid and flowing seawater.

Inconel, Incoloy and Monel are registered marks of Special Metals Corporation, and Hastelloy belongs to Haynes International. What you actually order and certify are the generic grades behind those names: UNS N08825, N06625, N10276, N04400.

What are nickel alloys? (and are they stainless steel?)

A nickel alloy is a metal in which nickel is the base, the majority element. That's the line separating it from stainless steel. Stainless is an iron-base alloy that uses nickel as an addition, typically 8–25%, to stabilize an austenitic structure. A nickel alloy flips that ratio: nickel becomes the matrix. So the honest answer to "are nickel alloys stainless steel?" is no. They sit one rung above it on the corrosion ladder.

The boundary is not sharp, and that is the point of this page. Super-austenitic stainless steel grades such as 904L (UNS N08904) carry roughly 25% nickel and ~4.5% molybdenum, enough to behave like a nickel alloy in many acids while remaining iron-base and far cheaper. That is exactly where super-austenitic stainless ends and the true nickel alloys begin. For the mechanism of ordinary austenitic stainless steel and for a map of what each alloying element does, follow the linked explainers.

Why nickel? The metallurgical mechanism

Nickel forms a face-centered-cubic (austenitic) matrix that stays ductile, tough and phase-stable while dissolving large amounts of chromium, molybdenum and copper in solid solution. Three consequences follow from this, and together they explain why an engineer pays for a nickel base instead of more stainless steel:

It raises the chloride stress-corrosion-cracking threshold, the one factor that cracks ordinary austenitic stainless more often than anything else and the single most common reason 316L gets abandoned.

High levels of Cr, Mo and Cu each bring their own kind of corrosion resistance, whether that's oxidizing media, reducing acids, or sulfuric acid, and none of them destabilize the structure.

Solid-solution strengthening gives it high-temperature strength and creep resistance, and in grades like 718 and K-500, precipitation hardening adds even more.

Nickel and chloride stress-corrosion cracking

Standard austenitic stainless steels (304, 316) are susceptible to chloride stress-corrosion cracking. In hot, chloride-bearing service they can crack under tensile stress even when general corrosion looks negligible. Resistance to this failure mode climbs steeply with nickel content. Push nickel past roughly 25-30%, and the SCC threshold moves out of the range most process plants operate in. That's why the step from 316L to 904L, Incoloy 825 or a true nickel alloy so often means a jump in nickel. See chloride stress-corrosion cracking for the full mechanism.

What each alloying element does

The families differ because their elements differ. Each element below carries one job; the family is just a recipe of those jobs. This also anchors the mechanism before it gets scaled up. Molybdenum gives 316 the pitting resistance that plain 304 lacks, and that same molybdenum, pushed to roughly 16%, is what turns Hastelloy C276 into an acid specialist.

For how molybdenum and chromium combine into a single pitting index, see PREN.

Incoloy: the Ni-Fe-Cr bridge alloys

Incoloy grades are nickel-iron-chromium alloys, sitting between super-austenitic stainless and the fully nickel-base families. Iron keeps the price down. Nickel and chromium push corrosion resistance and temperature capability well past what stainless can handle. The flagship grades split two ways: 800 / 800H / 800HT for high-temperature and creep service, and 825 for aqueous corrosion.

Incoloy 825 (UNS N08825, Werkstoff 2.4858) is titanium-stabilized and holds roughly 38–46% nickel, 2.5–3.5% molybdenum, and 1.5–3% copper. That mix lets it stand up to both reducing and oxidizing acids. Molybdenum and copper cover the reducing side; chromium covers the oxidizing side. It's also qualified for sour service under NACE MR0175 / ISO 15156. In mixed-acid and sour-gas duty, where 316L and 904L run out of margin, this alloy is the workhorse. For the full grade detail, read what Incoloy 825 is. Ready to order? Source it as Incoloy 825 coil.

Is "Inconel 825" the same as Incoloy 825?

No, this is the most common mis-specification our CRA buyers make. There is no such grade as "Inconel 825." The 825 alloy is an Incoloy (Alloy 825, UNS N08825), a nickel-iron-chromium alloy, not an Inconel (nickel-chromium). The mix-up traces back to a naming accident: both marks belong to Special Metals, and the names rhyme. Getting the designation right on the purchase order and the mill test certificate matters, because the certificate reports the UNS number, not the trade name, and a mismatch invites receiving-inspection disputes. When you mean N08825, write "Incoloy 825" or "Alloy 825."

Inconel: the Ni-Cr high-performance alloys

Inconel grades fall into the nickel-chromium family, and they carry the highest nickel content of the four groups, roughly 50 to 72%. Some include added molybdenum and niobium. What sets them apart is how they hold up under heat: strong mechanical performance and resistance to oxidation, even where stainless steel starts to soften and scale. 600 and 601 are the classic choices for heat and oxidation resistance. 718, meanwhile, is the precipitation-hardened grade built for high strength, used in fasteners, turbines and springs.

Inconel 625 (UNS N06625, Werkstoff 2.4856) is the versatile one. Its chromium-molybdenum-niobium chemistry adds serious wet-corrosion resistance on top of the heat capability, so it goes head-to-head with Incoloy 825 and Hastelloy in tough aqueous environments. When 825 and 625 overlap, the decision comes down to acid type and temperature. Check the comparison in the service map below.

Hastelloy: the Ni-Mo-Cr acid specialists

Hastelloy grades are nickel-molybdenum-chromium alloys carrying the highest molybdenum of any family, and molybdenum is what defeats reducing acids and severe localized attack. C276 / C22 / C2000 are the corrosion grades for hydrochloric acid, hot sulfuric acid, wet chlorine and the worst pitting and crevice environments in chemical processing; X is the high-temperature grade for combustion and gas-turbine hardware. When a medium combines sulfuric-acid resistance with heavy pitting and crevice corrosion risk, this is the family that answers. Order the grade directly as Hastelloy C276.

Monel: the Ni-Cu alloys

Monel grades are nickel-copper alloys, roughly 65% Ni and 30% Cu, with no chromium. Without chromium, they don't passivate the way the other families do, and that's why they should not go into strongly oxidizing acids like nitric acid or oxidizing chlorides. The copper earns its keep elsewhere, though. It gives standout performance in two niches none of the other families can touch: hydrofluoric acid and flowing seawater. There's also decent resistance to hydrofluoric-acid alkylation and reducing conditions. 400 is the solid-solution grade. K-500 gets age-hardened for extra strength. Nandini supplies Monel 400 and K-500 to order, right alongside the families it keeps in stock.

Which family do I need? A service-to-alloy map

Service: acid type, temperature, chloride, H₂S, halide. Map that to the family first, then narrow to the grade. The table includes the super-austenitic rungs, 904L and Alloy 20, since they often handle the duty at a fraction of what a nickel base costs. Don't pay for a nickel base when a copper-bearing stainless already clears the iso-corrosion limit.

For a guided answer, use the grade selector. When a corrosion-resistant alloy needs to be matched to a documented service, though, our engineering team will cross-check it against the iso-corrosion data first. Do that before you commit a heat.

Where super-austenitic stainless (904L) and Alloy 20 fit

904L and Alloy 20 sit directly below the nickel alloys on the ladder. Both are iron-base, copper-bearing grades, and both handle a surprising amount of sulfuric-acid and chloride duty. 904L (super-austenitic, PREN ~34) resists sulfuric acid and outperforms 316L in chloride SCC. It's our third-party-certified specialty.

The classic error is reaching for Inconel 625 or titanium for sulfuric-acid service that 904L super-austenitic already covers at lower cost. Check the acid concentration and temperature against the 904L limits first, before paying for an upgrade you don't need. On relative cost the ladder runs 316L → 904L → Incoloy 825 → Inconel 625 → Hastelloy C276 (ascending). Specify the lowest rung that clears your service, and no higher.

The PREN & corrosion ladder

PREN, or pitting resistance equivalent number, is calculated as PREN = Cr + 3.3 × Mo + 16 × N, and it ranks the grades by how well they resist pitting. The index works well for austenitic and duplex stainless steels. For high-molybdenum nickel alloys like Hastelloy C276, though, it sells the real performance short. PREN measures pitting resistance, nothing more. For Ni-Mo alloys such as Hastelloy, that's only part of the corrosion picture.

The nickel alloys push the ladder a step past super-austenitic stainless; the full method lives in PREN, the pitting resistance equivalent.

Equivalents & designations

The trade name is just marketing. What ends up on the certificate and the purchase order are the UNS and Werkstoff (WNr) numbers. Specify by designation, not brand.

For the wider table across UNS, Werkstoff and JIS systems, see the UNS / Werkstoff cross-reference.

Standards, forms & how to order

Nickel-alloy flat products follow the B-series ASTM specifications, not the A-series used for stainless, a routing fact worth remembering. Incoloy 825 sheet and plate are supplied to ASTM B424 / ASME SB-424, in contrast to 304/316/904L, which follow ASTM A240 / ASME SA-240. For sour-gas duty the heat must be supplied in the qualified condition to NACE MR0175 / ISO 15156, call it out on the order. Every order ships with an EN 10204 3.1 mill test certificate (3.1 or 3.2 as required), and the certificate reports the UNS number, heat chemistry and mechanical results so the grade is fully traceable, see the mill test certificate page.

Nandini Steel is an ISO 9001:2015-certified stockist and supplier, founded in 2003. The certificate (No. 1283438, issued by Unitas Certification Services UCS-UK) runs through 11 January 2028. Our certified specialty, backed by third-party audit, spans 904L, Alloy 28, Incoloy 800 and Hastelloy plate. Within the nickel-alloy space, we keep the following ex-stock for immediate dispatch:

• Incoloy, 800, 800H, 800HT, 825
• Inconel, 600, 601, 625, 718
• Hastelloy, C22, C276, C2000, X
• Alloy 20 and Alloy 28 (the super-austenitic / high-alloy bridge grades)

Monel 400 and K-500 come supplied to order. Every piece of material is genuine mill plate, sourced from names like Aperam, Outokumpu, Jindal, POSCO, Nippon Steel and JFE. Pricing isn't pulled from a list; it's quoted live against your heat, size and quantity, so send an RFQ. Want to know the section weight first? Use the weight calculator.

Applications by industry

• Oil & gas (sour service), Incoloy 825 and Inconel 625 for NACE-qualified sour-gas wellhead, downhole and separator duty.
• Petrochemical, nickel alloys for reactors, exchangers and transfer lines running mixed hydrocarbons and acids at temperature.
• Chemical / acid processing, Hastelloy C276/C22 for hydrochloric and hot sulfuric acid; 904L and Incoloy 825 where the service is milder.
• Marine, Monel 400 and high-Mo grades for seawater pumps, valves and heat-exchanger service.
• Fertilizer / acid-regeneration service, copper-bearing grades (904L, Incoloy 825, Alloy 20) for phosphoric- and sulfuric-acid plant.

Welding nickel alloys (note)

Nickel alloys need matching or deliberately over-alloyed fillers to keep the weld metal at least as corrosion-resistant as the base. Never under-alloy the joint. Incoloy 825 gets welded with ERNiFeCr-1 (FM 65); for 904L, reach for an over-alloyed filler such as ER385, or a nickel-base rod. Heat input, interpass temperature and cleanliness all carry more weight here than they do for stainless. See how to weld Incoloy 825 and the broader note on welding nickel alloys, and check how to select a grade before you fix the filler.

FAQ

Specify the right CRA: request a quote

Give us the medium, the temperature, the chloride and H₂S levels, and the form you're after, and the Nandini Steel Engineering Team will check the family and grade against documented iso-corrosion data before you commit to a heat. Then we quote live against current stock. We keep Incoloy, Inconel, Hastelloy and Alloy 20/28 ex-stock, ready for immediate dispatch, and every order ships with EN 10204 3.1/3.2 certification.

• RFQ / contact: get in touch · Email reachus@nandinisteel.com · WhatsApp +91 99202 09184
• Why Nandini: about us · certifications · quality assurance

Nandini Steel · 25, Dreamland Building, 601, 6th Floor, Mama Parmanand Marg, Charni Road, Opera House, Girgaon, Mumbai, Maharashtra 400004, India · GSTIN 27ADJPA7240C1Z0 · over 307 projects completed · 260+ clients served · exports reaching 36+ countries · more than 20 years in the business.