Hastelloy C276 vs Inconel 625: Selection Guide

• Hastelloy C276 (UNS N10276) and Inconel 625 (UNS N06625) solve different problems. C276 is a corrosion alloy: 15-17 % Mo plus 3-4.5 % W gives it a PREN of roughly 65-76 and makes it the default choice for reducing acids, wet chlorine and mixed oxidising/reducing process streams. Inconel 625 is a corrosion-plus-strength alloy: 8-10 % Mo gives a PREN above 50, and niobium raises minimum yield strength to 414 MPa versus 283 MPa for C276 — about 45 % more.

• For reducing acids the gap is decisive. C276 is one of the few wrought alloys usable in hydrochloric acid and in sulfuric acid contaminated with chlorides or oxidising species; Inconel 625 is not normally specified for HCl service.

• For load-bearing service above roughly 600 °C, the gap reverses. Inconel 625 retains over 60 % of its room-temperature strength at 650 °C and has a 700 °C / 1000 h creep-rupture strength near 140 MPa, against roughly 80 MPa for C276. ASME BPVC Section II Part D gives SB-575 C276 plate a maximum design temperature of 1250 °F (675 °C) for Section VIII Division 1.

• Both alloys are listed in NACE MR0175 / ISO 15156-3 for sour service at any H2S partial pressure up to 232 °C (450 °F). Inconel 625 covers the majority of sour wells; you upgrade to C276 when elemental sulfur is present, or when high chlorides combine with temperatures above roughly 190 °C.

• Neither alloy is precipitation-hardenable by heat treatment in its standard grade, and neither requires post-weld heat treatment. C276 is welded with ERNiCrMo-4; Inconel 625 with ERNiCrMo-3. C276 cannot be heat treated between 600 and 1150 °C in sour service — doing so embrittles it.

• C276 is supplied to ASTM B575 (plate), B622 (seamless pipe), B619 (welded pipe), B626 (tube), B574 (bar), B564 (forgings) and B366 (fittings). Inconel 625 is supplied to B443, B444, B446, B564, B366 and B705, with ASME SB- equivalents for coded construction.

Hastelloy C276 (UNS N10276) and Inconel 625 (UNS N06625) are the two nickel alloys most often quoted against each other, and the comparison is frequently answered with a shrug: "both are good, take whatever is cheaper." That answer costs money.

The two alloys were designed around different problems, and in the wrong duty either one can fail early. C276 is a corrosion alloy built on molybdenum and tungsten. Inconel 625 is a corrosion-plus-strength alloy built on molybdenum and niobium. This guide puts numbers on both and gives you the decision rules that separate them.

Hastelloy C276 vs Inconel 625: Which Alloy Should You Specify?

Choose Hastelloy C276 when chemistry is the limiting factor, and Inconel 625 when mechanical load, pressure rating or temperature is. These are not two grades of the same product — they are two different design philosophies that happen to overlap in the middle of the corrosion spectrum.

Hastelloy C276 carries 15.0–17.0 % molybdenum and 3.0–4.5 % tungsten against Inconel 625's 8–10 % molybdenum and none. That single difference is why C276 survives hydrochloric acid, wet chlorine and sulfuric acid carrying chlorides — duties where Inconel 625 is simply not specified. It also gives C276 a pitting resistance equivalent number in the range of roughly 65–76, versus above 50 for 625.

Inconel 625 carries 3.15–4.15 % niobium, and that is the other half of the story. Niobium raises the minimum yield strength to 414 MPa against C276's 283 MPa — about 45 % more — and it stabilises the matrix at temperature. At 650 °C, 625 still holds over 60 % of its room-temperature strength; C276's strength falls away much faster above about 425 °C. For a high-pressure pipeline or a loaded high-temperature component, that strength converts directly into thinner wall, less weight and fewer supports.

The practical consequence: most engineers who "upgrade" from 625 to C276 are buying corrosion margin they may not need, and most who "downgrade" from C276 to 625 are buying a failure. The rest of this article is about telling those two situations apart. If you need the background on C276 first, start with the Hastelloy C276 hub guide or What Is Hastelloy C276?.

What Are the Chemical Compositions of C276 and Inconel 625?

C276 is a nickel–molybdenum–chromium alloy with tungsten; Inconel 625 is a nickel–chromium–molybdenum alloy with niobium. Both keep carbon low, but C276 goes much further — 0.01 % maximum versus 0.10 % for 625 — and that is what lets C276 be used in the as-welded condition without losing corrosion resistance in the heat-affected zone.

Nominal chemical composition, weight percent

Two details deserve a second look when you write a specification. First, C276's 0.01 % carbon ceiling is not cosmetic — it is the reason the alloy can go into service as-welded, and it is the reason earlier C-type alloys with 0.08 % carbon were replaced. Second, do not read C276's lower chromium as a weakness by default; it is balanced by molybdenum and tungsten. But it does have one real consequence, covered below: C276 does not have enough chromium for hot concentrated nitric acid.

How Do PREN and Chloride Pitting Resistance Compare?

Hastelloy C276 has the higher PREN — roughly 65–76 against above 50 for Inconel 625 — and in practice that margin matters only once your chloride level, temperature and flow condition cross a threshold. Below that threshold, 625 is the better buy.

PREN = %Cr + 3.3 × %Mo + 16 × %N is a screening index, not a design value. It ranks alloys' resistance to chloride pitting, and it is useful precisely because it is simple. Using it: Inconel 625 reaches about 51, C276 about 68 on chromium and molybdenum alone, or about 76 if you count tungsten via %Cr + 3.3 × (%Mo + 0.5 × %W). Both figures are quoted in the literature, which is why you see ranges.

PREN ladder — where C276 and 625 sit against common alternatives

The decision rule that follows from the ladder: if your duty is flowing, aerated seawater at ambient temperature, Inconel 625 is normally sufficient and cheaper, and the extra PREN of C276 buys you nothing measurable. If your duty involves stagnation, low flow, biofouling, concentrating crevices, hot brines, or chlorination that raises the local oxidising potential, C276's margin is worth paying for. See which is better for seawater for a dedicated treatment.

Which Alloy Resists Reducing Acids — HCl, H2SO4 and HF — Better?

Hastelloy C276, clearly and by a wide margin. This is the single largest technical difference between the two alloys, and it is the reason C276 exists. Inconel 625 is not normally specified for hydrochloric acid service at all.

Molybdenum is the element that carries reducing-acid resistance, and C276 has roughly double the molybdenum of Inconel 625 plus tungsten on top. Hydrochloric acid is the clearest example: it is a reducing, non-oxidising acid that attacks the passive film of most stainless and nickel-chromium alloys, and C276 is one of the very few wrought materials with useful resistance across a meaningful range of concentration and temperature.

Sulfuric acid is more subtle, because the ranking depends on concentration and on contaminants. In clean sulfuric acid both alloys can be usable in parts of the concentration range. The moment the acid carries chlorides, or oxidising ions such as Fe³⁺ or Cu²⁺, or is aerated, the duties diverge and C276 becomes the safe specification.

Relative resistance in reducing and mixed acid media (screening guide)

This table is a screening guide, not a design basis. Corrosion rate in these media depends on concentration, temperature, velocity, aeration and trace contaminants, and it can move by an order of magnitude between adjacent conditions. Before specifying, check the manufacturer's iso-corrosion chart for your exact stream, or commission a coupon test.

Which Alloy Handles Oxidising Media and Wet Chlorine Better?

Hastelloy C276 still wins in most mixed oxidising–reducing process streams, but there is one important exception: C276 does not have enough chromium for hot concentrated nitric acid. Inconel 625, with 20–23 % chromium against C276's 14.5–16.5 %, is the better choice there.

The reason C276 dominates the mixed-chemistry cases is that it was designed for exactly that combination. Most corrosion alloys are good at either oxidising or reducing environments. C276's chromium handles oxidising species while its molybdenum and tungsten handle reducing ones, which is why it is the standard material for wet chlorine, hypochlorite, chlorine dioxide and chlorate service — streams that swing between the two regimes.

Oxidising and halogen service — relative performance

That nitric-acid row is worth memorising because it is the most common way C276 gets mis-specified. Teams that adopt C276 as their "universal" alloy and carry it into a hot concentrated nitric duty will get a surprise. If your stream is strongly oxidising and nitric-based, look at Hastelloy C22 instead — see Hastelloy C22 vs C276 and the C22 vs C276 selection guide.

How Do the Two Alloys Compare in Seawater and Marine Service?

Both are fully suitable, and Inconel 625 is usually the more economical choice. It has a PREN above 50, is essentially immune to chloride stress-corrosion cracking because of its nickel content, and is the established material for seawater piping, pump shafts, marine exhaust systems and offshore riser sheathing.

C276 earns its premium in the harder corners of marine service: stagnant or low-flow seawater where crevices concentrate, hot brine duties, chlorinated cooling water where the oxidising potential is raised deliberately, and biofouling-prone systems where under-deposit chemistry develops. If none of those apply, the extra PREN is unlikely to show up in your inspection reports.

Where marine service also involves cyclic wave or vortex-induced vibration loading, Inconel 625's higher fatigue and tensile strength adds a second argument in its favour. That combination — corrosion resistance plus fatigue strength — is why 625 dominates offshore riser and flowline duty; see Inconel 625 for offshore oil and gas and Inconel 625 for marine engineering. For a broader view across all candidate materials, what alloy should I use for seawater piping walks the full ladder.

How Do Tensile and Yield Strength Compare for Pressure-Boundary Design?

Inconel 625 is roughly 45 % stronger in yield and it stays stronger as temperature rises. Minimum yield strength is 414 MPa for solution-annealed 625 against 283 MPa for solution-annealed C276, and minimum tensile is 827 MPa against 690 MPa.

That strength gap has a direct commercial consequence. On a high-pressure piping run or a thick-walled vessel, allowable stress drives wall thickness, so 625 can often be specified thinner. Less wall thickness means less weight, less weld metal, smaller supports and lower freight. On large projects this recovers a meaningful share of the material price difference, which is why a simple price-per-kilogram comparison between the two grades is misleading.

Mechanical properties in the solution-annealed condition (specification minimums unless noted)

Note the density difference too. C276 at 8.89 g/cm³ is about 5 % heavier than 625 at 8.44 g/cm³, so on a tonnage basis you buy slightly more kilograms of 625 for the same volume. Combined with the strength advantage, this reinforces the point that volumetric and lifecycle comparisons beat per-kilogram comparisons. For the full dataset on 625, see Inconel 625 mechanical properties.

Which Alloy Retains Strength Better at Elevated Temperature?

Inconel 625, decisively, and the gap widens with temperature. Below roughly 425 °C both alloys are viable and corrosion governs the choice. Above that, strength retention starts to separate them, and above about 600 °C under sustained load 625 is the clear choice unless the corrosion environment is severe enough to override it.

The mechanism is niobium. When Inconel 625 is held in the 550–750 °C range, niobium precipitates as gamma-double-prime (Ni₃Nb), which contributes substantial strengthening. C276 is not precipitation-hardenable at all — it relies purely on solid-solution strengthening from molybdenum and tungsten, which gives it lower strength but excellent ductility and formability.

Typical tensile and yield strength at temperature

Creep tells the same story more sharply. At 700 °C, the stress to rupture in 1000 hours is approximately 140 MPa for Inconel 625 against roughly 80 MPa for C276 — nearly a factor of two. For any component that carries sustained load above 600 °C, that is the number that governs. A useful rule of thumb from marine and furnace practice: at 600 °C, 625 tolerates roughly four to five times the stress of 316L for the same creep life.

What Are the Maximum Service Temperatures and ASME Code Limits?

In air, C276 resists oxidation to about 1040 °C (1900 °F) and Inconel 625 to about 980 °C (1800 °F). But those are oxidation limits, not design limits — for coded pressure equipment the ASME numbers are far lower, and ignoring the difference is a common specification error.

The distinction matters because the two figures answer different questions. The oxidation limit tells you when the alloy starts scaling badly in air with no load. The ASME allowable stress tells you what you may legally and safely use in a pressure boundary. Designing to the oxidation limit would be dangerous for either alloy.

Temperature limits — oxidation versus ASME code

Two cautions. First, always take allowable stresses from the current edition of ASME BPVC Section II Part D for your specific product form and thickness — the values vary. Second, C276's allowable stress falls steadily with temperature: roughly 23.3 ksi at 38 °C, 20.7 ksi at 200 °C and 18.5 ksi at 316 °C for SB-575 plate. Design against those, not against the 690 MPa room-temperature tensile minimum. For more detail on the C276 side, see Hastelloy C276 temperature limits and Inconel 625 temperature range.

How Do C276 and 625 Perform at Cryogenic and LNG Temperatures?

Both are excellent, and this is one area where there is genuinely no reason to prefer one over the other on performance grounds. Neither alloy exhibits a ductile-to-brittle transition, and both retain high impact toughness at LNG temperature (−196 °C) and below.

Inconel 625 actually gets stronger as it gets colder — typical tensile strength rises to roughly 1100 MPa at liquid nitrogen temperature, with yield near 690 MPa and elongation still above 30 %. C276 shows the same pattern; typical tensile is around 965 MPa at −196 °C with full ductility retained.

Because of this, cryogenic selection is normally decided by corrosion, availability and cost rather than by mechanical properties. In LNG service Inconel 625 tends to dominate on availability and fabrication familiarity — see Inconel 625 for LNG processing and the Inconel 625 LNG pipe spool case study. C276 is chosen where the process stream carries acid or chlorine contaminants into the cold end.

Which Alloy Is Easier to Weld?

Both are among the most forgiving nickel alloys to weld, and weldability should not drive your selection. The important difference is consumable: C276 is welded with ERNiCrMo-4 and Inconel 625 with ERNiCrMo-3, and using the wrong one quietly creates the weakest point in the fabrication.

The reason both weld so well is composition control. C276's 0.01 % maximum carbon and 0.08 % maximum silicon prevent the grain-boundary precipitation that historically ruined welded C-type alloys, which is why as-welded C276 has essentially the same corrosion resistance as the base metal. Inconel 625's niobium is balanced against carbon so that NbC forms rather than chromium carbide, preserving the matrix.

Welding consumables and key parameters

Two field failures are worth naming because they are entirely preventable. The first is cross-using consumables: an ERNiCrMo-3 deposit on C276 does not carry the parent metal's molybdenum, so in an acid stream the weld becomes the failure site. The second is skipping the argon backing purge on pipe roots — an un-purged root oxidises into a rough, chromium-depleted surface that pits immediately. For full procedures, see Hastelloy C276 welding guide and Inconel 625 welding guide.

Do Either of These Alloys Need Post-Weld Heat Treatment?

No. Both Hastelloy C276 and Inconel 625 are used in the as-welded condition, and for C276 in sour service, post-weld heat treatment is actively prohibited in the 600–1150 °C range because it embrittles the alloy.

This surprises fabricators who are used to stress-relieving carbon steel for sour service, and it is worth stating explicitly on the welding procedure specification. The prohibition is not a precautionary footnote: heating C276 into that band precipitates second phases that destroy both toughness and corrosion resistance, and the damage cannot be undone except by a full solution anneal.

For Inconel 625 the situation is milder but the conclusion is the same — stress relief is unnecessary and generally avoided. One genuine caution does apply to 625: the alloy can suffer strain-age cracking if it is heated into the intermediate range while under restraint, so heavy restrained fabrications should be sequenced and supported accordingly.

How Do Machinability and Cold Forming Compare?

Both are difficult, and C276 is generally the harder of the two to machine. Neither should be compared with stainless steel — plan tooling, speeds and cycle times on the assumption of a work-hardening nickel alloy, because that is what both are.

C276 has a higher work-hardening rate than the austenitic stainless steels, and its molybdenum and tungsten content push it above Inconel 625 in this respect. Practical rules for both: carbide or ceramic tooling, positive rake geometry, heavy constant feeds so the tool cuts rather than rubs, rigid setups, flood coolant, and never allow the tool to dwell. For Inconel 625, carbide at roughly 14–34 m/min is standard shop practice.

On forming, the advantage reverses. C276's 40 % minimum elongation against 30 % for 625, combined with its lower yield strength, makes it somewhat more formable for complex shapes. Both work-harden rapidly, so multi-stage forming will need intermediate solution annealing, and once you exceed the cold-work limits discussed in the sour-service section you must re-anneal to restore compliance. See Hastelloy C276 machining tips.

Which Alloy Qualifies Better for Sour Service Under NACE MR0175?

Neither — both are fully qualified. NACE MR0175 / ISO 15156-3 lists C276 (Table A.26) and Inconel 625 (Table A.29) as acceptable corrosion-resistant alloys for sour service at any H₂S partial pressure up to 232 °C (450 °F), in the solution-annealed condition. The real question is which one matches the severity of your well.

Because both are intrinsically resistant, neither carries the H₂S partial-pressure or chloride restrictions that bind carbon and low-alloy steels. That is a genuine simplification: you can specify either without the elaborate environmental cracking calculations required for steel. The controls that do apply are metallurgical condition, hardness and cold work — and those are where projects actually fail.

NACE MR0175 / ISO 15156-3 requirements — verify against the current edition

Data conflict to resolve before you specify: published hardness limits for these two alloys disagree. Inconel 625 is widely quoted at 22 HRC maximum in the annealed condition under NACE MR0175, while other tabulations of ISO 15156-3 Table A.29 give 35 HRC. C276 is commonly quoted at 35 HRC, with some product-form references at 40 HRC. Do not average them — specify the tighter limit explicitly on your purchase order and confirm against the edition of the standard your client has invoked.

The audit point that catches the most shipments is cold work, not chemistry. A component that passes its hardness test can still be non-compliant if the forming strain exceeded the listed limit, because cold work raises both hardness and tensile residual stress — the two conditions that drive sulfide stress cracking. Any forming past the limit must be followed by a full solution anneal. Cold-worked Inconel 625 beyond roughly 15 % reduction is frequently rejected for sour service even when hardness passes, which is worth knowing before you substitute cold-finished bar for annealed stock. More background in sour gas alloy selection.

When Must You Upgrade from Inconel 625 to Hastelloy C276?

Three conditions force the upgrade: elemental sulfur in the produced fluid, high chlorides combined with temperatures above roughly 190 °C, and reducing-acid or chlorine chemistry in the process stream. Outside those, Inconel 625 is usually sufficient and easier to source.

Elemental sulfur is the most under-appreciated of the three. Inconel 625 has documented stress-corrosion-cracking susceptibility above roughly 190 °C in testing, particularly in HIP-clad material, and the presence of free sulfur sharpens that risk considerably. C276's higher molybdenum and tungsten push its envelope into the most aggressive deep sour gas environments, which is why it is the upgrade path for sulfur-laden high-pressure wells.

Sour-service decision matrix

Note the last row: the decision is not one-directional. If your sour service also involves significant mechanical loading — high pressure, fatigue, vibration — Inconel 625's strength advantage is a real argument, and the right answer may be 625 even at some cost in corrosion margin. This is the case for offshore pipeline alloy selection, where 625 is the dominant choice.

How Do Material Cost and Lifecycle Economics Compare?

Hastelloy C276 is typically the more expensive grade per kilogram, because it carries roughly twice the molybdenum plus tungsten, and molybdenum is among the most volatile and costly alloying elements. But per-kilogram price is the wrong basis for this decision, and it will often point you the wrong way.

Three effects work against a naive price comparison. First, C276 at 8.89 g/cm³ is about 5 % denser than 625 at 8.44 g/cm³, so you buy more kilograms of 625 for the same volume. Second, 625's higher allowable stress can mean a thinner wall, which reduces weight, weld metal and supports. Third, and largest of all, the two grades have different service lives in a given duty, and one avoided unplanned shutdown usually dwarfs the material delta.

Cost drivers — what actually moves the number

Nickel and molybdenum surcharges move independently on the LME, so the price gap between these two grades widens and narrows with the market. Treat any published per-kilogram figure as indicative only and request a live quotation for both grades simultaneously. See current C276 pipe pricing for orientation, then ask us to quote both.

Which Applications Favour C276 and Which Favour Inconel 625?

C276 dominates chemical process, pollution control and acid service. Inconel 625 dominates offshore, marine, aerospace, nuclear and high-temperature mechanical duty. The overlap is real but narrower than most people assume.

Application split by industry and mechanism

If you are working on the C276 side of this table, the cluster goes deeper: C276 for chemical plants, C276 for acid service, C276 for FGD systems, C276 for pulp and paper and C276 for hydrochloric acid service.

What Product Forms and ASTM/ASME Standards Cover Each Alloy?

Both alloys are produced in all standard mill forms and both have a full set of ASME SB- counterparts for coded construction. C276 uses Werkstoff 2.4819 (EN NiMo16Cr15W); Inconel 625 uses Werkstoff 2.4856 (DIN 17744, JIS NCF 625). Getting the specification right on the purchase order is what converts an alloy choice into a compliant delivery.

Product-form standards

Source C276 plate, C276 pipe, C276 bar, C276 flanges and C276 fittings for the C276 side; and 625 pipe, 625 flanges and 625 plate for the other. Specification cross-checks: ASTM standards for Inconel 625 and Inconel 625 equivalent grades.

Can You Substitute One Alloy for the Other? Three Swaps to Avoid

Only rarely, and never without re-running the governing calculation. Three substitution errors account for most of the field failures we see, and each is preventable with one check.

• Substituting 625 into a C276 acid duty: Inconel 625 lacks the molybdenum for hydrochloric acid and for sulfuric acid carrying chlorides or oxidising ions. This substitution can fail within months.

• Substituting C276 into a 625 load-bearing duty: 283 MPa yield against 414 MPa, and roughly 80 MPa against 140 MPa creep-rupture at 700 °C. Re-run the ASME allowable stress calculation before swapping, or you may put a pressure boundary outside its code margin.

• Substituting on chemistry alone while ignoring condition: a grade name on a certificate is not a guarantee. Solution-annealed condition, hardness cap and cold-work limit are what actually deliver compliance, and a cold-finished substitute can fail all three.

The safe way to substitute is in the direction of more margin and with a documented re-rating. Substituting C276 for 625 in a corrosion duty is usually safe mechanically provided you re-check allowable stress. Substituting 625 for C276 anywhere is the move that needs real justification and corrosion data for the specific stream.

How Do C276 and 625 Compare to C22, 825, 904L and Super Duplex 2507?

C276 and 625 sit in the upper-middle of the corrosion-resistance ladder. Below them sit the superaustenitic and super duplex stainless grades; alongside C276 sits Hastelloy C22, which trades slightly different oxidising performance for similar overall capability.

Where the two alloys sit in the wider selection ladder

Useful framing: 625 is the alloy you reach for when you need corrosion resistance and strength in the same component. C276 is what you reach for when corrosion is the whole problem. 825 and Alloy 20 are the cost-led options when the duty is genuinely milder; C22 is the step up when the stream is strongly oxidising. Comparisons worth reading: 625 vs Incoloy 825, 625 vs 316L, 625 vs Inconel 718, 625 vs Monel 400 and C22 vs C276.

What Should Go on Your Purchase Order for Each Alloy?

The alloy name alone is not a specification. A purchase order that names the grade but omits condition, hardness cap, cold-work limit and certification is how non-compliant material reaches site — and it happens with both grades.

Purchase-order checklist

Two lines do the most work. The hardness cap and the cold-work limit are where non-compliance hides, because a shipment can pass chemistry and tensile testing and still be outside the sour-service envelope. State both, and ask for the survey results rather than a pass/fail statement.

What Inspection and Certification Should You Require?

Require an EN 10204 3.1 mill test certificate as the minimum for both alloys, upgraded to 3.2 with third-party witness for critical service, plus positive material identification on every pressure-retaining component. Beyond that, the inspection regime is driven by the code and the duty rather than by the alloy.

• Mill test certificate to EN 10204 3.1 (3.2 for critical service), documenting heat chemistry, mechanical properties and heat treatment against the cited ASTM or ASME specification.

• Positive material identification (PMI) on all pressure-retaining parts, and on weld deposits where an over-alloyed filler has been specified — this is what catches a swapped consumable.

• Hardness survey of weld cap and heat-affected zone where sour service applies, reported as values rather than pass/fail.

• Non-destructive examination per the governing code: typically PT on all weld surfaces, with RT or UT on butt welds per ASME B31.3 or Section VIII requirements.

• For weld overlay, a dilution and chemistry check on the final layer, typically by PMI or spot chemical analysis, confirming molybdenum meets the composition envelope.

• For offshore and subsea work, third-party inspection agency witness (DNV, Bureau Veritas, Lloyd's Register or ABS), and NORSOK M-630 supply with M-650 manufacturer qualification where invoked.

One clause is worth adding to every order: a statement that no post-weld heat treatment has been applied and none is permitted. It is a one-line control that prevents an expensive and irreversible mistake on C276. For fabrication strategy, see pipe spool prefabrication versus field welding.

How to Choose Between Hastelloy C276 and Inconel 625

The procedure below is the short form of everything above. Work through it in order — the first step that gives a clear answer usually settles the decision, and you rarely need all seven.

1. Write down the dominant corrosion mechanism first, not the alloy. Before comparing grades, name the mechanism you are actually fighting: general corrosion, chloride pitting, crevice attack, chloride stress-corrosion cracking, sulfide stress cracking, or high-temperature oxidation. If the mechanism is general or localised attack by a reducing acid, start from C276. If it is chloride pitting plus mechanical load, start from 625.

2. List concentration, temperature and contaminants for every stream. Record acid type and concentration, maximum metal temperature, chloride level, oxidising species (Fe3+, Cu2+, dissolved O2, chlorate, hypochlorite), H2S partial pressure and whether free elemental sulfur can form. Averages hide the failure case — capture startup, shutdown and upset conditions.

3. Check the temperature against both envelopes. Below roughly 425 °C both alloys are viable and corrosion governs the choice. Between 425 °C and 650 °C strength retention starts to separate them. Above roughly 600 °C under sustained load, move to Inconel 625 unless the corrosion environment is severe enough to override the strength argument.

4. Run the PREN screen against chloride risk. Calculate PREN = %Cr + 3.3 x %Mo + 16 x %N. C276 lands around 65-76, Inconel 625 above 50. If your chloride level and temperature put you above the 625 threshold — stagnant or low-flow seawater, concentrating crevices, hot brines — C276 buys margin.

5. Confirm the sour-service position against NACE MR0175 / ISO 15156-3. Both alloys are qualified at any H2S partial pressure to 232 °C in the solution-annealed condition. Specify solution annealed, cap hardness, limit cold work, and confirm the maximum temperature in the current edition of the standard for your product form before you commit.

6. Size the wall with the correct allowable stress. Use ASME BPVC Section II Part D allowable stresses, not room-temperature tensile minima. Inconel 625's higher yield strength can cut wall thickness and weight on high-pressure piping, which often recovers part of its material cost.

7. Price the lifecycle, not the kilogram. Compare installed cost against inspection interval and replacement cycle. A C276 component that eliminates one unplanned shutdown usually beats a cheaper 625 part that needs replacement mid-campaign. Get a live quote for both before finalising — nickel and molybdenum surcharges move independently.

Frequently Asked Questions

Q1. Is Hastelloy C276 better than Inconel 625?

Neither alloy is universally better; each wins in a different envelope. Hastelloy C276 is better for corrosion: its 15-17 % molybdenum and 3-4.5 % tungsten give a PREN of roughly 65-76, so it resists hydrochloric acid, sulfuric acid with chlorides, wet chlorine and mixed oxidising-reducing streams that attack Inconel 625. Inconel 625 is better for strength and temperature: niobium raises its minimum yield strength to 414 MPa versus 283 MPa for C276, it retains over 60 % of room-temperature strength at 650 °C, and its 700 °C / 1000 hour creep-rupture strength is near 140 MPa versus roughly 80 MPa for C276. Choose C276 when chemistry is the limiting factor and 625 when load, pressure rating or temperature is.

Q2. What is the main difference between Hastelloy C276 and Inconel 625?

The main difference is the alloying strategy and what it buys. C276 is a nickel-molybdenum-chromium alloy with tungsten and only 0.01 % carbon, optimised for chemical resistance in both oxidising and reducing media. Inconel 625 is a nickel-chromium-molybdenum alloy with 3.15-4.15 % niobium, optimised for strength plus good corrosion resistance. Practically: C276 has roughly twice the molybdenum, Inconel 625 has roughly 45 % more yield strength, and Inconel 625 can be used as a structural high-temperature material while C276 is primarily an aqueous-corrosion material.

Q3. Which is more corrosion resistant, C276 or Inconel 625?

Hastelloy C276 is more corrosion resistant in reducing acids and in aggressive mixed chemistry. Its PREN of roughly 65-76 compares with above 50 for Inconel 625, and it is one of the few wrought alloys specified for hydrochloric acid service and for sulfuric acid contaminated with chlorides or oxidising ions. Inconel 625 still performs very well in seawater, brines, oxidising atmospheres and many process streams. The exception worth knowing: C276 has only 14.5-16.5 % chromium and lacks sufficient chromium for hot concentrated nitric acid, so do not assume it wins every oxidising environment.

Q4. Is Inconel 625 stronger than Hastelloy C276?

Yes. Solution-annealed Inconel 625 has a minimum yield strength of 414 MPa and a minimum tensile strength of 827 MPa under ASTM B443 / B446, while solution-annealed C276 has minimums of 283 MPa yield and 690 MPa tensile under ASTM B575. That is roughly 45 % more yield strength. The advantage persists at temperature: at 650 °C typical yield strength is around 300-400 MPa for 625 versus 180-240 MPa for C276. In pressure-boundary design this means 625 can often use a thinner wall for the same rating.

Q5. What is the maximum temperature for Hastelloy C276 and Inconel 625?

In air, C276 resists oxidation up to about 1040 °C and Inconel 625 up to about 980 °C. For coded pressure equipment the numbers are much lower: ASME BPVC Section II Part D gives SB-575 C276 plate a maximum design temperature of 1250 °F (675 °C) for Section VIII Division 1, 800 °F for Section III and for Section VIII Division 2. Inconel 625 is commonly limited to about 870 °C for continuous loaded service to control creep. Both alloys retain good toughness down to cryogenic temperatures.

Q6. Can Hastelloy C276 and Inconel 625 be used in hydrochloric acid?

Hastelloy C276 can be used in hydrochloric acid and is one of the standard choices for it; Inconel 625 is not normally specified for HCl service. C276's high molybdenum content gives it useful resistance across a wide range of HCl concentrations at moderate temperature, though rates rise steeply with both concentration and temperature, and aeration or oxidising contaminants make it worse. Always check the manufacturer's iso-corrosion chart for your exact concentration and temperature rather than relying on a general rating.

Q7. Which alloy is better for seawater, C276 or Inconel 625?

Both are excellent, and Inconel 625 is usually the more economical choice. Inconel 625 has a PREN above 50, is essentially immune to chloride stress-corrosion cracking because of its high nickel content, and is the standard material for seawater piping, pump shafts, marine exhaust and offshore sheathing. Hastelloy C276 gives more margin in hot, stagnant, low-flow or crevice-prone seawater and where biofouling or chlorination raises the local oxidising potential. For flowing aerated seawater at ambient temperature, 625 is normally sufficient and cheaper.

Q8. Is C276 or Inconel 625 better for sour gas service?

Both are listed in NACE MR0175 / ISO 15156-3 as qualified corrosion-resistant alloys for sour service at any H2S partial pressure up to 232 °C (450 °F), in the solution-annealed condition. Inconel 625 covers the majority of sour wells and is easier to source. You upgrade to C276 when elemental sulfur is present or can form, or when high chloride concentrations combine with temperatures above roughly 190 °C, where 625 has documented stress-corrosion-cracking susceptibility. Cold-worked 625 beyond about 15 % reduction is also frequently rejected for sour service even when hardness passes.

Q9. What hardness limit applies to Inconel 625 and C276 under NACE MR0175?

Sources differ, so specify the tighter limit and confirm against the current edition. For Inconel 625, NACE MR0175 / ISO 15156-3 is widely cited as a maximum of 22 HRC (237 HBW) in the annealed condition, while some tabulations of ISO 15156-3 Table A.29 give 35 HRC and a cold-worked limit of 40 HRC up to 35 % cold work. For C276, ISO 15156-3 Table A.26 is commonly cited as 35 HRC maximum with 20 % maximum cold work, though some product-form references quote 40 HRC. Put the hardness cap, the annealed condition and the cold-work limit explicitly on your purchase order.

Q10. Do C276 and Inconel 625 need post-weld heat treatment?

No. Neither alloy requires post-weld heat treatment, and both are normally used in the as-welded condition. C276 achieves this because its 0.01 % maximum carbon and 0.08 % maximum silicon prevent grain-boundary precipitation in the heat-affected zone. For C276 in sour service, post-weld heat treatment is specifically not permitted in the 600-1150 °C range because it embrittles the alloy. Stress relief is also unnecessary and generally avoided for both grades.

Q11. What welding filler metal is used for C276 and for Inconel 625?

Hastelloy C276 is welded with ERNiCrMo-4 filler metal (AWS A5.14) or ENiCrMo-4 covered electrode (AWS A5.11); ERNiCrMo-10 is often preferred for sour service and for weld overlay because its lower iron content improves localised corrosion resistance of the deposit. Inconel 625 is welded with ERNiCrMo-3 filler or ENiCrMo-3 electrode. Never swap the two: an ERNiCrMo-3 deposit on C276 will not carry the parent metal's molybdenum level and will become the weak link in an acid stream.

Q12. Which alloy is more expensive, C276 or Inconel 625?

Hastelloy C276 is typically more expensive per kilogram than Inconel 625, because it carries roughly twice the molybdenum plus tungsten, and molybdenum is one of the most volatile and costly alloying elements. Treat any published figure as indicative only: nickel and molybdenum surcharges move independently on the LME and the gap between the two grades widens or narrows with the market. Always request a live quotation for both grades at the same time, and compare on installed and lifecycle cost rather than price per kilogram.

Q13. Can I substitute Inconel 625 for Hastelloy C276 in an acid plant?

Usually no, and this is one of the costliest substitution errors in materials selection. Inconel 625 does not have the molybdenum content to handle reducing acids such as hydrochloric acid, or sulfuric acid carrying chlorides and oxidising contaminants. A 625 replacement in a C276 hydrochloric acid or chlorine dioxide duty can fail in months. Substitution is only defensible when the real duty is milder than the original specification — for example, flowing ambient seawater or a dilute oxidising stream.

Q14. Can I substitute Hastelloy C276 for Inconel 625 in a high-temperature application?

Not without re-rating the component. C276's minimum yield strength is 283 MPa against 414 MPa for Inconel 625, and its strength falls off faster above roughly 425 °C, with 700 °C / 1000 h creep-rupture strength near 80 MPa versus roughly 140 MPa for 625. Swapping in C276 without recalculating wall thickness or allowable stress can push a pressure boundary or a loaded structural part outside its code margin. Re-run the ASME BPVC Section II Part D calculation before substituting.

Q15. What are the ASTM standards for C276 and Inconel 625?

Hastelloy C276: ASTM B575 plate, sheet and strip; B622 seamless pipe and tube; B619 welded pipe; B626 welded tube; B574 rod, bar and wire; B564 forgings; B366 fittings; B462 rod, bar and forging stock. Inconel 625: ASTM B443 plate, sheet and strip; B444 seamless pipe and tube; B446 rod, bar and wire; B564 forgings; B366 fittings; B705 welded pipe. Each has an ASME SB- counterpart for coded vessels and piping. C276 is Werkstoff 2.4819; Inconel 625 is Werkstoff 2.4856.

Q16. Is Hastelloy C276 magnetic?

No. Both Hastelloy C276 and Inconel 625 are austenitic nickel-base alloys and are essentially non-magnetic in the solution-annealed condition, with magnetic permeability close to 1.0. This makes both suitable for instrumentation, MWD and LWD tooling and other applications where magnetic interference matters. Cold work can produce a very slight magnetic response, but it is negligible for practical purposes.

Q17. Which alloy is easier to machine and weld?

Both are demanding, and C276 is generally the harder of the two to machine because its higher molybdenum and tungsten content raise the work-hardening rate. For Inconel 625, carbide tooling at roughly 14-34 m/min with heavy constant feeds and flood coolant is standard practice. For both alloys: use rigid setups, positive-rake carbide, avoid dwelling, and never let the tool rub. Weldability is excellent for both — they are among the most forgiving nickel alloys to weld, which is a large part of why both are so widely specified.

Q18. Can C276 and Inconel 625 be welded to each other?

Yes, and it is done routinely, but the filler choice matters. When a C276 component is welded to an Inconel 625 component, use an over-alloyed filler — ERNiCrMo-4 or, preferably for corrosive service, ERNiCrMo-10 — so the weld metal matches the more demanding side of the joint. Using ERNiCrMo-3 under-alloys the deposit relative to C276. Qualify the procedure to ASME Section IX, control heat input and interpass temperature, and use argon backing purge on the root.

Q19. What is the PREN of Hastelloy C276 and Inconel 625?

Using PREN = %Cr + 3.3 x %Mo + 16 x %N, Inconel 625 comes out at approximately 51 (about 21.5 % chromium plus 8-10 % molybdenum), and the value is commonly quoted as above 50. Hastelloy C276 falls in the range of roughly 65 to 76 depending on whether tungsten is counted and how. On chromium and molybdenum alone it is about 68; including tungsten using the form %Cr + 3.3 x (%Mo + 0.5 x %W) it reaches about 76. Both are far above super duplex 2507 at around 43 and 316L at around 25.

Q20. How do I choose between C276 and Inconel 625 for a new project?

Work through four questions in order. First, what is the chemistry? If the stream contains reducing acids, wet chlorine, hypochlorite, chlorine dioxide or sulfuric acid with chlorides, C276 is the starting point. Second, what is the temperature under load? Above roughly 600 °C sustained, Inconel 625 wins on creep and strength retention. Third, is it sour service with elemental sulfur or chlorides above about 190 °C? That also pushes you to C276. Fourth, is the duty a flowing ambient seawater or marine mechanical application? Inconel 625 is usually sufficient and cheaper. If the answers point both ways, get the corrosion data for your exact stream and price both grades on lifecycle cost.