Inconel vs Hastelloy: How to Choose by UNS Number Instead of Brand Name

Inconel vs Hastelloy is a choice between two trademarks, not between two alloy families. Inconel belongs to Special Metals, Hastelloy to Haynes International, and both mills sell some of the same UNS numbers. Choose by specification, not by brand: UNS N06625 where strength and heat matter, N10276 for reducing acids, and N06022 for oxidizing and mixed acids and for hard chloride crevice duty, with N06686 the grade a peer-reviewed, government-hosted ranking puts above all of them for crevice duty. Quote the ASME section, product form and condition too: the alloy 625 mill bulletin reports a 400 °F spread in the same Division 1 vessel between its Grade 1 and Grade 2 material, the annealed and the solution-annealed conditions.

Inconel vs Hastelloy: the short answer, in four inputs and one table

Four inputs determine the grade: your maximum metal temperature and the code that governs it, the service (reducing or oxidizing), your chloride level and crevice geometry, and the product form you need certified. Answer those four and the UNS number drops out of the table below. Ignore them and you’re guessing.

Most published comparisons ask readers to supply the first three inputs and then never resolve them. The fourth is the one the code section later in this article turns on, because the allowable depends on the condition you actually bought. A brand name can’t carry any of the four, because both trade names span several alloys with different behaviour.

The same four inputs decide the grade in oil and gas, petrochemical, power generation and chemical processing. Sector on its own is a weak predictor of grade. A heat exchanger in a refinery and one in a desalination train can need different alloys, while two plants in different industries running one acid at one temperature usually need the same grade.

Below is the selection table those four inputs produce. Each row is a UNS number, because that’s what a mill can quote and a second supplier can bid against. TiAlloy supplies 625, C-276, C-22, 718, 600 and Monel 400 from the nickel alloy plate, tube and bar range.

Composition ranges from ASTM B443, B575, B333, B435, B670 and B168 as published in the Special Metals and Haynes International bulletins listed under References. Service guidance is our own reading of the data shown later. That data resolves three of these grades, N06625, N10276 and N06022; the other rows are placed from their published composition and specification, so treat those as a starting shortlist rather than as a tested ranking.

Both names are trademarks, and both mills sell some of the same alloys

Inconel C-276 is a real product. Special Metals publishes technical bulletins for INCONEL alloy C-276, INCONEL alloy 22, INCONEL alloy HX and INCONEL alloy G-3. Every one of those is an alloy Haynes International also sells, under the Hastelloy mark and the same UNS number. Same alloy, two trademarks, two supply chains.

Confirmation comes from the mill itself. Special Metals’ own alloy 625 bulletin carries a footnote stating that Hastelloy is a trademark of Haynes International. The mill that sells the alloy does not pretend the trademark is the metal; only the market does. When somebody insists that Hastelloy and Inconel are different metals, they’re describing a purchase order, not the metal.

One phrase to retire: “C276 stainless steel”. UNS N10276 is a nickel-based alloy with a nickel balance and 4–7 % iron. Calling it stainless steel inverts the base metal, and any mill reading that enquiry will either quote the wrong thing or stop to ask.

Real differences between the two alloys begin with molybdenum content and end with a code table, never with the trademark. Every one of the differences that matters is visible in a specification and invisible on a brand name.

What chromium, molybdenum and tungsten actually buy you

Chromium buys oxidation resistance and passivity in oxidizing environments. Molybdenum and tungsten buy reducing-acid and crevice resistance. Niobium buys precipitation hardening, which is why alloy 625 can age and gain strength in service and C-276 doesn’t. Read those three numbers, chromium, molybdenum and tungsten, off a composition table and you can predict any nickel alloy’s bias before reading its datasheet.

Chemical composition: nickel, chromium and the molybdenum gap

Nickel and chromium set the baseline for both families, and two numbers do most of the work. Chromium content in Inconel 625 runs 20.0–23.0 % against 14.5–16.5 % in C-276. Molybdenum runs the other way and by more, 8.0–10.0 % against 15.0–17.0 %. It earns its place twice over in 625, because the same additions that resist acid also deliver solid solution strengthening in the nickel matrix. Those two elements pull in opposite directions, and that opposition is the source of the key differences in chemical composition between the grades — and the reason the acid crossover two sections below exists at all.

Three grades carry most of this comparison. Their alloying elements sit as follows, all limiting values from the mill bulletins.

Cobalt matters more than its position on the list suggests: these three bulletins allow between 1.0 % and 2.5 % of it, so a buyer who needs a tighter ceiling has to write one onto the enquiry as an extra line. No source read for this article records what limit any particular industry sets, so ask the project rather than the mill. Source: Special Metals INCONEL alloy 625 and alloy C-276 bulletins; Haynes International HASTELLOY C-22 datasheet. Ranges are specification limits from the ASTM standards named above, not a heat analysis.

The pitting index, worked once and then qualified

PREN(W) = %Cr + 3.3 × (%Mo + 0.5 × %W). For N06022 at nominal Cr 22, Mo 13, W 3, that’s 22 + 3.3 × (13 + 1.5) = 22 + 47.85 = 69.9. Running the arithmetic on midpoint compositions instead gives N10276 about 74.5, N06625 about 51.2, and N06022 about 70.8 rather than 69.9. Those are our calculations from published limits, not mill-quoted figures.

The Crevice Gap: a 15 °C interval that holds across three publishers

Most Hastelloy vs Inconel arguments are pitting arguments, and in the acidified ferric chloride immersion Special Metals reports, the four strongest grades all return the same greater-than value rather than a number. Which mechanism discriminates depends on the bath, so name the bath before you rank the grades. Compare Inconel 625 against Hastelloy C-276 on crevice temperature and the two separate consistently across three publishers.

The Special Metals datasheet gives N06625, N10276, N06022 and N06686 the same critical pitting temperature, above 85 °C, so that bath can’t separate them. Haynes, testing in acidified 6 % ferric chloride to the ASTM G48 method, publishes crevice temperatures of 40 °C for alloy 625 against 55 °C for C-276. Its pitting column resolves both grades, 100 °C for alloy 625 against 150 °C for C-276, so in that bath pitting separates them by 50 °C, more than three times the crevice separation. The two mills’ columns point at different mechanisms: same two UNS numbers, the same ASTM G48 family of ferric-chloride immersion tests, the same ranking, and a different place where the measurement stops. Both mills put C-276 above alloy 625. Special Metals censors its pitting column above 85 °C, where Haynes resolves the same two grades to 100 °C and 150 °C. What the two columns disagree about is resolution, not which grade wins, which is the argument the next section makes. What holds across that disagreement is the crevice interval, about 15 °C between those two grades, on which the published record agrees more closely than it agrees on any absolute value.

Why four alloys appear to tie in corrosive environments

A shared “> 85 °C” isn’t four alloys tying. It’s a censored value. The bulletin stops at 85 °C and reports everything above it the same way, and it does not say why it stops there. Two distributors publish a ferric chloride stability limit, one near 60 °C and one at 85 °C for a 10 % solution, which is a plausible reason and not the bulletin’s own.

The censoring in that column is one-sided, exactly as a solution-breakdown ceiling predicts: every pitting value that resolves sits below the bound, and no row is bounded from below. The crevice column censors in the other direction for a single row, where the weakest grade runs off the bottom at less than 0 °C. Read a bound in either direction as a missing measurement, not as a property.

Ferric chloride immersion of the ASTM G48 type, as published in the Special Metals C-276 bulletin. Haynes’ figures for the same two grades, quoted in the section above, run 5 °C higher and are read at 72 h. Three further rows of the published table are excluded here: two because their Celsius and Fahrenheit values disagree with each other, and one INCOLOY alloy 825 row because it is neither a candidate in this comparison nor the floor reference for it. The rows above come from Special Metals publication SMC-019, copyright 2004, so they are more than twenty years old. Independently published crevice temperatures for the same grades differ by several degrees because bath chemistry, specimen finish and crevice former are not standardized between laboratories. Use the interval to rank the grades; don’t use it as a design temperature.

Three publishers land on the same 15 °C interval between 625 and C-276 while their absolute values span only 10 °C: 35/50, 40/55 and 45/60. That convergence is useful, but it’s a convergence of the published record rather than three independent experiments, because all three are mill or stockist documents. Two other publishers put the ferric chloride stability limit at different places, 60 °C and 85 °C, and neither figure comes from the three tables above. When the record disagrees with itself, say so.

Crevices aren’t laboratory abstractions. They arrive as gasket faces, tube-to-tubesheet joints, weld spatter, lap joints and settled deposits in a dead leg. Any of those can hold a chloride concentration an order of magnitude above the bulk fluid, which is why crevice temperature and not pitting temperature is the number worth arguing about.

Stress corrosion cracking follows temperature, not chloride concentration

One more chloride failure mode gets specified backwards. Buyers screen for chloride concentration, and concentration is close to useless as a predictor on its own. At the boiling point, both alloy 625 and C-276 survive 1,000 h of U-bend exposure in 42 % magnesium chloride, 33 % lithium chloride and 26 % sodium chloride, while type 316 stainless cracks in 24 h to 300 h in the same baths.

Raise the temperature and the picture inverts. At 232 °C, C-276 and its low-carbon sibling C-4 still resist 25 % sodium chloride yet crack in magnesium chloride of equivalent chloride content. Cation chemistry and temperature decide the outcome; the chloride number by itself doesn’t. Write your maximum metal temperature and the actual salt into the enquiry, not a parts-per-million chloride limit.

The Acid-Type Crossover: where 625 beats C-276 and where it does not

Hastelloy isn’t simply better at corrosion. Which alloy wins flips with acid type. C-276 fares better in acidic environments when the acid is reducing, alloy 625 and alloy 22 win where the acid oxidizes, and the gaps run to orders of magnitude rather than percentage points: one row of the mill’s own table spans close to three of them.

Alloy 625 and alloy 22 are the grades suitable for oxidizing environments; C-276 is not. Every grade here has good corrosion resistance in some duty and loses in another. None has superior corrosion resistance across all seven of the chemical environments below, which is why the family name settles nothing.

Before the numbers, the method caveat, because it changes how the numbers may be used:

Corrosion testing by its very nature precludes complete standardization… accelerated corrosion tests give indicative results only, or may even be entirely misleading.

Read that as a licence to act on order-of-magnitude gaps and to ignore small ones.

Inconel alloy 625 vs Hastelloy C-276 in acid service

Every rate above is one row of Table 5 of the Special Metals INCONEL alloy C-276 bulletin, 168-hour immersion, mils per year. One publisher, one laboratory, one test series, and worth noticing who the publisher is: Special Metals lists all three of these grades under its own INCONEL trademark, including the two the rest of the market buys as Hastelloy. So this is a mill ranking its own products, not one mill scoring a rival. The bulletin is Special Metals publication SMC-019, copyright 2004, so these rates are more than twenty years old; independently published rates for nominally the same medium differ, and aeration, velocity, specimen preparation and impurity level are not held constant between laboratories. Read the order of magnitude rather than the digit. Rates are laboratory values under the ASTM G31 caveat above.

The cleanest single illustration of the crossover is the last row. In boiling 10 % nitric plus 3 % hydrofluoric acid, the one oxidizing medium in the series, C-276 corrodes at 95 mpy against 28 mpy for alloy 625: roughly 3.4 times faster than the Inconel it is usually assumed to beat, in the Inconel mill’s own table.

Three different rank orders appear in seven rows of one table. In boiling 10 % sulfuric all three land between 17 and 22 mpy, a spread that settles nothing. Move to 95 % sulfuric at 50 °C and the published pair is 48 mpy against 0.1 mpy. Read that as two to three orders of magnitude rather than as a literal 480×: the divisor carries one significant figure and sits at the resolution these tables are printed to, so the ratio is a magnitude and not a measurement. That magnitude decides a vessel. The bulletin publishes no alloy 22 value for that medium, so the row ranks two grades and not three. Through the four reducing rows that follow, C-276 leads every time and 625 trails by anywhere from 2.5× to more than 18×. Those four rows are a selection. Table 5 runs to seventeen, and in three of the rows left out — 20 % and 40 % sulfuric at 80 °C, and 10 % sulfuric plus 2 % hydrochloric at 50 °C — alloy 625 ties or beats C-276, at rates of 5 mpy and below. The C-276 lead is real in hot and concentrated reducing acid. It is not the whole table. Then the single oxidizing row inverts the order and C-276 is the worst of the three, 95 against 23 for alloy 22, which is the opposite of what C-276’s reputation predicts. Your variable is reducing versus oxidizing service plus chloride contamination; it is never the brand on the mill certificate.

Choosing inside each family: selecting the right alloy, not the right brand

Picking a family settles almost nothing. Inconel spans a corrosion alloy and a precipitation-hardened strength alloy that behave nothing alike, and the Hastelloy corrosion-resistant range alone offers five grades that a single service can separate by an order of magnitude. Choose the grade, then the mill.

Inconel alloy grades: 600, 601, 625 and 718

Use alloy 625 plate, pipe and bar when you need corrosion resistance and load capacity in one part. Inconel 600 is a nickel-chromium alloy without molybdenum: excellent in dry chlorine and caustic, poor in chloride-bearing water. Alloy 601 adds aluminium for oxidation resistance in furnace hardware. Inconel 718 is a high-strength alloy and it’s where people go wrong most often. The strength of Inconel 718 comes from gamma double prime precipitation, and that’s a mechanical property, not a corrosion property. Specify 718 for a bolt or a disc, never as a substitute for 625 in acid.

Hastelloy alloys: C-276, C-22, C-2000 and B-3, and where alloy 686 sits

Compare Hastelloy alloys on chromium first and molybdenum second, because that pair predicts which end of the acid range each Hastelloy grade targets. Hastelloy C-276 plate and tube remains the family standard and the safe default in mixed reducing chemistry. Hastelloy C-22 earns its place through chromium. Haynes International writes that C-22’s higher chromium content gives it “much higher resistance to oxidizing media than the family standard, C-276 alloy”. C-2000 adds copper for sulfuric across concentration. Hastelloy B-3 handles hydrochloric acid at any concentration and fails fast on any oxidizing contaminant. Do not read an older stock listing for Hastelloy B-2, sometimes written “Hastelloy B2,” as B-3: B-3 is the successor grade, not a spelling variant. Alloy 686 is not a Hastelloy grade at all: Special Metals sells N06686 as INCONEL alloy 686, and it belongs in this list because it is where a buyer goes when C-22 runs out of crevice margin. Datasheets credit it with excellent corrosion resistance in crevice duty. The mill crevice table earlier only bounds it rather than resolving it, but the peer-reviewed electrochemical study behind that section ranks protection temperature 625 < C-4 < C-276 < alloy 22 ≈ alloy 59 ≈ C-2000 < alloy 686, which is an independent measurement rather than a bound. It reappears later as a weld filler for exactly that reason.

Two adjacent grades come up in the same conversations. Monel 400 is nickel-copper, strong in hydrofluoric acid and seawater, and irrelevant in oxidizing acid, while Monel K-500 is its age-hardened version for shafting and fasteners. Neither belongs in a chloride crevice argument, though both show up in the same enquiries.

The Four-Ceiling Problem: high-temperature performance is not one number

High-temperature applications are where this goes wrong most expensively. One alloy doesn’t have a maximum temperature. It has four of them, and they disagree by hundreds of degrees. Metallurgical capability, the code-listed allowable for the grade you actually bought, the applicable code case and the unlisted-material route each produce a different ceiling for alloy 625, and quoting the wrong one is how piping specifications get over-constrained.

The layers are structural. We quote the figures behind rows 2, 2b and 3 verbatim below instead of restating them here, because they’re a mill bulletin’s summary of ASME Section II, Part D and not the code table itself. Distributor summaries of the same paragraph circulate with the grades transposed, or with a Section XII line and a bolting line the bulletin doesn’t contain, which is the argument for reading the mill’s own document rather than a reseller’s table. One further scope point: these layers aren’t alternatives for one component. The Boiler and Pressure Vessel Code governs vessels and boilers, B31.3 governs process piping, and what you’re building decides which one applies. A project built to the EU Pressure Equipment Directive runs on a different set of harmonized standards with its own material-approval route, so confirm which regime the project has adopted before reading any ceiling off an ASME table.

Allowable design stresses for Grade 1 material for Section VIII, Division 1 construction up to 1200°F, for Section III, Class 2 and 3 construction up to 800 °F, and for Grade 2 material for Section VIII, Division 1 construction up to 1600°F are reported in Table 1B of ASME Section II, Part D… Allowable stresses and rules for Section 1 construction with Grade 1 material up to 1100°F are found in ASME Code Case 1935.

Those are the bulletin’s words, not the code’s. Table 1B sits behind the ASME paywall and wasn’t opened for this article, and the current edition is BPVC-2025, so treat the quotation as the question to take to your code engineer rather than as the answer.

Read row 2 against row 2b once more: confusing the two is the easiest mistake in this table to avoid, and the most expensive to make. Same alloy, same code, same Division 1 vessel, and on the bulletin’s own figures a 400 °F spread between its Grade 1 and Grade 2 material, Grade 1 being the annealed condition and Grade 2 the solution-annealed one — a mapping this article takes from a distributor’s summary of ASTM B443 rather than from the specification, which was not opened. That difference turns on a thermal-processing state a purchase order can omit in silence, and a mill will ship whichever one the order actually specified.

The code commentary says an unlisted material may be used only when it conforms to a published specification covering chemistry, physical and mechanical properties, manufacturing method, thermal processing and quality control, and when it otherwise meets the Code. Its allowable stress must then be determined from the applicable Code basis or a more conservative one.

The Trademark-to-UNS Translation Table

Most differences between Inconel and Hastelloy paperwork disappear at this level, because both trademarks resolve to the same UNS numbers and the same ASTM specifications.

Here is what to write on the purchase order. Each row gives a trade name, its UNS number, the Werkstoff number European mills use, and the ASTM specification by product form. Quote the row, not the brand, and any qualified mill in aerospace, energy or chemical process supply can bid the same scope.

Specification assignments compiled from the Special Metals and Haynes International bulletins listed under References, cross-checked against ASTM B575-17(2023). Confirm the current edition before issuing an enquiry.

Add two lines to any enquiry built from this table. First, the condition, because annealed and solution annealed aren’t interchangeable and the code-listed allowable stress in the previous section depends on which one you buy. Second, an EN 10204 3.1 material test certificate, which is the practical acceptance document on the receiving dock and the one that lets you trace a heat number back through the mill. Our supplier verification checklist covers what to ask for before an order ships, and our nickel alloy range shows which grades we supply in plate, tube and bar. If a row of this table is already your answer, send it to us as a specification and we will quote against it.

What actually drives the price difference

Chemistry sets a floor on price. Process route sets the bill. Mill datasheets for C-276 and C-22, both grades covered by ASTM B575-17(2023), give N10276 at 15.0–17.0 % molybdenum with 3.0–4.5 % tungsten against N06022 at 12.5–14.5 % and 2.5–3.5 %, a real input difference. B575 is the controlling document and the one to name on the purchase order. Manufacturing route moves the price much further than that gap does.

Melt route is the first multiplier: vacuum induction melting, vacuum arc remelting and electroslag remelting are three different cost bases before the metal is ever rolled, and a specification that demands double melting has priced itself before anyone quotes chemistry. Product form is the second: tube drawn with no weld seam, cold-drawn bar and clad plate are separate manufacturing processes, not shapes of one. Quantity is the third, because a non-standard size means a dedicated mill run rather than a cut from stock.

The published cost literature on nickel superalloys, including the peer-reviewed IN625 route comparison and a manufacturing cost analysis, is worth reading with one caution attached. Those studies compare process routes for a single alloy, wrought against additive, and their percentages describe that comparison and nothing else. None of them tells you what 625 costs against C-276, and quoting their numbers in a brand comparison is how a false price gap gets into a specification.

A quieter driver is density. N10276 runs 8.89 g/cm³ against 8.44 g/cm³ for N06625, so it carries 5.3 % more mass through the same part geometry. That’s our own arithmetic on published densities, and it means a per-kilogram price comparison understates the delta on a finished component.

Fabrication: the weld is usually the weak link

Corrosion data describes base metal. Your equipment fails at the weld. Pitting tests published in the Special Metals INCONEL alloy C-276 bulletin show a C-276 weldment made with matching C-276 filler suffering 6.2 mm of attack in the weld under gas tungsten arc welding and 3.4 mm under pulsed gas metal arc welding, while the base metal beside the weld shows zero. Both figures come from one bath, the boiling quadruple-acid mixture the bulletin calls Green Death. The same table carries ASTM G48 Method C rows for the identical pairing, and there the matching filler suffers no attack at all, zero under both welding processes. Read the 6.2 mm as what one bath did to one filler pairing rather than as a property of matching filler. Qualify the joint for your own service rather than treating matching filler as safe or unsafe in general.

Overmatching fixed it in that published bath. Substituting a 686CPT filler on the same C-276 base metal returned zero attack in both base and weld. Solid-solution alloying carries a solidification penalty: these alloys segregate molybdenum as they freeze, leaving the dendrite cores lean, and a filler one step richer than the base compensates. Defaulting to a matching filler because the label matches is a habit, not a specification.

Machinability and formability pull in opposite directions across this group, which is worth knowing before a grade is locked. The alloy 625 bulletin gives single-point turning at 4.0–10.7 m/min with high-speed steel tooling and 14–34 m/min with coated carbide, and these alloys work-harden aggressively, so the cheaper bar can become the more expensive finished part once you count tool life and cycle time. The same bulletin’s annealed-condition data explains why alloy 625 forms more readily than it machines; do not transfer one grade’s values to another.

Can Hastelloy and Inconel be joined to each other? Published qualification data on 625-to-Hastelloy-X joints reports tensile values of 835.6 MPa for gas metal arc welding, 825.3 MPa for gas tungsten arc welding and 817.0 MPa for shielded metal arc welding, with all specimens fracturing on the alloy X side and all passing dye penetrant, radiographic and guided-bend testing. Those figures come from a single published test programme reported by Special Metals, so treat them as evidence that the joint is qualifiable rather than as a design allowable.

What has actually changed, and what has not

The claim you will see repeated is that buying is shifting from brand level to specification level, with trademarks giving way to UNS numbers on enquiries, purchase orders and material test certificates. At the specification level the machinery has been in place for decades, and the evidence contradicts the trend from two directions.

ASTM E527, the standard that creates UNS numbers, was already carrying a 1983 revision when most of the specifications in use today were written, and the edition current at the time of writing is E527-23. ASTM’s catalogue still lists E527-83 and its reapprovals, so 1983 is a revision date and not an origin: the practice is older than the number on the current edition suggests. Specification-level identification is older than most of the engineers using it, so there is no movement to report. Search demand cannot serve as evidence either, because it measures information-seeking and not purchasing behaviour.

Sharper still, ASTM E527-23 states that a UNS number is not itself a specification. It provides uniform identification and cross-referencing and establishes no requirements for form, condition or quality. Advice to “just specify UNS” is therefore incomplete on the standards body’s own authority, and the previous sections show why: UNS N06625 covers both the annealed and solution-annealed conditions of alloy 625, so the number alone cannot separate the two Division 1 ceilings the mill bulletin reports for the same alloy.

What genuinely changed is metallurgy rather than paperwork. The corrosion-resistant family has grown well past C-276 and C-22 to alloy 59, C-2000, 686 and the newer HYBRID-BC1 (N10362). A 2025 peer-reviewed review treats wet flue-gas desulfurization corrosion as still open; a distributor active in that market reports alloy 22 use growing in the duty, and no peer-reviewed source publishes a growth figure. ASME BPVC now sits on its 2025 Edition with earlier code cases absorbed into Section II, which is exactly why every code-derived temperature here names the section and the document it was read from, and why the bulletin figures are labelled as bulletin figures.

Frequently asked questions

Q: Is Hastelloy C276 the same as Inconel C276?

Yes. Both are UNS N10276, Werkstoff 2.4819, supplied to ASTM B575 for plate. Hastelloy is a Haynes International trademark and Inconel a Special Metals trademark, and both mills publish bulletins for this same alloy.

Specifying the trade name restricts your bidder list to one mill’s distribution chain without changing the metal you receive. Write UNS N10276 with the ASTM specification, product form and condition, and any qualified supplier can quote the identical scope. Special Metals’ own alloy 625 bulletin carries a footnote acknowledging Hastelloy as a Haynes trademark, so the two mills agree on the point even where the market does not.

Q: Is Hastelloy better than Inconel?

Neither is better as a family, because both names cover several alloys. C-276 wins in reducing acid by two to three orders of magnitude in 95 % sulfuric at 50 °C, while alloy 625 and alloy 22 win in oxidizing acid, where C-276 is the worst of the three.

Rank order changes twice across the seven published media in the corrosion table above, which is what produces three different orders. Decide on reducing versus oxidizing conditions, chloride level and crevice geometry, then read the grade off the specification rather than the brand. Every grade in this comparison is good somewhere and not one of them is superior across all seven chemical environments in that table, so the family name settles nothing.

Q: What is the main difference between Inconel 625 and Hastelloy C-276?

Molybdenum and niobium. Chromium and molybdenum are the two largest gaps by percentage, but molybdenum and niobium are the pair that explains the functional split: C-276 carries 15.0–17.0 % molybdenum for reducing acids and crevice resistance; 625 carries 8.0–10.0 % plus 3.15–4.15 % niobium, which adds strength through precipitation hardening but leaves 625 weaker in concentrated reducing acid.

Published critical crevice temperatures capture the practical effect: 35 °C for alloy 625 against 50 °C for C-276 under ferric chloride immersion of the ASTM G48 type. Choose Inconel 625 where you need load capacity with corrosion resistance in one part. Both resolve to UNS numbers a mill can quote against, N06625 and N10276, so the decision is an acid-type and crevice-geometry question rather than a question about which brand you prefer.

Q: Can Inconel and Hastelloy be used together?

Yes, and dissimilar joints between them are qualifiable. Published tests on 625-to-Hastelloy-X welds report 835.6 MPa by gas metal arc welding, all specimens fracturing on the alloy X side and passing dye penetrant, radiographic and guided-bend testing.

Two cautions apply in any specific application. No galvanic series was read for this article, so treat the couple as unassessed rather than as safe: a wetted joint between two different nickel alloys has to be checked against your own electrolyte before it is welded. More importantly, the filler determines corrosion performance at the joint, not the two base metals, and in the one bath published for that pairing, matching filler on C-276 produced 6.2 mm of weld attack where an overmatched 686CPT filler produced none. Qualify the procedure for your service, not for the metals in general.

Q: Why is Inconel expensive?

Alloying load first, processing second. Nickel and molybdenum are exchange-priced, and a double-melt requirement, a product form with no weld seam and a non-standard quantity each add cost before anyone quotes chemistry.

The published cost studies on nickel superalloys compare process routes for one alloy, wrought against additive, so their percentages describe that comparison and not what 625 costs against C-276; no figure from them is quoted here. What moves a real quotation is the melt route, vacuum induction melting through vacuum arc remelting to electroslag remelting, then the product form, then the quantity. Ask your supplier which exchange settlement the alloy adjuster references and over what averaging period, because that mechanism is auditable while a printed price per pound is stale within a week.

Q: Will a magnet stick to Inconel?

Generally no. Alloy 625 and C-276 are austenitic and effectively non-magnetic in the annealed condition, so a magnet test cannot distinguish them and cannot substitute for a material test certificate.

A non-magnetic result also cannot distinguish either nickel alloy from annealed 316L stainless steel. Identification on a receiving dock comes from the EN 10204 3.1 material test certificate traced back to a heat number and checked against the UNS number, ASTM specification, product form and condition on the purchase order. Positive material identification by handheld X-ray fluorescence is the field check when a certificate needs confirming.

Q: Do these alloys need sour-service qualification?

If the service is wet and carries hydrogen sulfide, yes. Sour service is governed by NACE MR0175 / ISO 15156, which is a separate qualification from the ASTM material specification and from the ASME code section, and it is routinely a condition of enquiry on oil-and-gas and petrochemical projects.

Both mills state the listing in their own bulletins: the C-276 bulletin records UNS N10276 as listed in NACE MR0175 for oil and gas, and the alloy 625 bulletin records the same for N06625. A listing is not a blanket clearance. The standard sets limits by material condition and hardness, and the part of it that applies depends on the component and the environment, so ask the project which section governs and get the qualification written on the purchase order beside the UNS number, specification, product form and condition. The limits themselves belong in the standard, not in an article.

References and sources

1. Special Metals Corporation, INCONEL alloy 625 technical bulletin: composition, corrosion rates, code ceilings, gamma double prime ageing, dissimilar weld data, machining speeds.
2. Special Metals Corporation, INCONEL alloy C-276 technical bulletin: composition, crevice and pitting temperatures, corrosion rates, weld pitting data.
3. Special Metals Corporation, INCONEL technical bulletin index: evidence that INCONEL alloy C-276, alloy 22, alloy HX and alloy G-3 are published products.
4. Haynes International, HASTELLOY C-22 alloy: composition, code listings and the oxidizing-media comparison against C-276.
5. Haynes International, HASTELLOY C-276 alloy: crevice and pitting temperatures, ferric chloride test conditions.
6. ASTM International, ASTM G31-21(2025) Standard Guide for Laboratory Immersion Corrosion Testing of Metals: the accelerated-testing caveat quoted before the corrosion table.
7. ASTM International, ASTM E527-23 Standard Practice for Numbering Metals and Alloys in the Unified Numbering System: UNS is identification, not specification.
8. ASTM International, ASTM B443-19 catalog entry: linked for the N06625 plate, sheet and strip designation only. The paywalled standard was not read for this article, and it is not used here as evidence for the Grade 1/Grade 2 mapping or the reported code ceilings.
9. ASTM International, ASTM B575-17(2023): molybdenum and tungsten ranges for N10276 and N06022.
10. ASME, Standards and certification training material: the ASME B31.3 Para. 323.1.2 unlisted-material provision summarized in the four-ceiling section.
11. ASME, Boiler and Pressure Vessel Code standards and Code Cases: section structure, 2025 Edition status.
12. National Institute of Standards and Technology, peer-reviewed study of alloy 625 crevice corrosion by ASTM G48 Method D: 25 °C wrought against 30 °C laser powder-bed fusion.
13. National Institute of Standards and Technology, AM-Bench IN625 benchmark lot description: the measured lot assay quoted in the price section.
14. Springer, manufacturing-economics analysis and ScienceDirect, wrought versus additive IN625 cost comparison: process-route cost studies of a single alloy, cited for the melt-route and product-form argument only.
15. Emerald, 2025 review of material degradation and protection: cited for direction of travel in flue-gas desulfurization research; it publishes no growth figure.
16. United States Department of Commerce, Foreign-Trade Zone production notification guidance, and NASA, SSTD-8070-0122-WELD qualified welding standard: UNS-level identification in official practice.
17. National Library of Medicine (PubMed Central), peer-reviewed review of chloride stress corrosion cracking in nickel alloys: the source behind the 232 °C sodium chloride against magnesium chloride contrast, and the temperature-before-concentration reading of that section.
18. NeoNickel, distributor comparison of C-276, alloy 22 and alloy 59: the two pitting-index formulas printed side by side, the ranking inversion between them, and the statement that the 10 % ferric chloride test solution breaks down above 85 °C. A distributor document, quoted here as a published table rather than as an independent measurement.
19. Jacquet, alloy C-276 stockist datasheet: the third published crevice pair, 45 °C against 60 °C, and the 1,000 h U-bend figures. A stockist document, and labelled as one in the text.
20. NeoNickel, flue-gas desulfurization plant materials: the market signal on alloy 22 use in that duty, which is a supplier statement and not a measured growth figure.

INCONEL is a registered trademark of Special Metals Corporation. HASTELLOY is a registered trademark of Haynes International, Inc. TiAlloy is an independent supplier of the UNS numbers discussed here and is not affiliated with, endorsed by or licensed by either company.