Hastelloy C276 vs Alloy 20: Complete Selection Guide

Hastelloy C276 (UNS N10276) and Alloy 20 (UNS N08020, 20Cb-3) are both specified for acid service, but they are not interchangeable. C276 is a nickel-chromium-molybdenum-tungsten alloy built for mixed, chloride-bearing and hydrochloric acid duty up to 677 C (1250 F) under ASME Section VIII Division 1.

Alloy 20 is a niobium-stabilised iron-nickel-chromium alloy with 3-4 percent copper, designed for sulfuric acid, and it stops at 427 C (800 F) under the same Code. This guide compares them acid by acid, gives the ASME allowable stresses side by side, and shows where the 2.5-4x price gap actually buys you something.

· Hastelloy C276 (UNS N10276) is a Ni-Cr-Mo-W alloy: ~57% Ni, 15-17% Mo, 14.5-16.5% Cr, 3-4.5% W.

· Alloy 20 (UNS N08020, 20Cb-3) is an Fe-Ni-Cr-Cu-Mo alloy: 32-38% Ni, 19-21% Cr, 3-4% Cu, 2-3% Mo, Nb-stabilised.

· PREN is roughly 68-76 for C276 and about 28 for Alloy 20 — a 2.5x gap in chloride pitting resistance.

· ASME Section VIII Division 1 lists C276 to 677 C (1250 F) but Alloy 20 only to 427 C (800 F).

· Alloy 20 was designed for sulfuric acid and remains the economical default there; C276 wins in hydrochloric acid, wet chlorine and seawater.

· C276 corrodes at 0.03-0.06 mm/y in 10-30% sulfuric acid at 79 C; Alloy 20 runs about 0.13-0.25 mm/y at 80 C.

· Alloy 20 has a corrosion 'hump' in 60-75% sulfuric acid where rates reach ~1.3 mm/y — C276 stays near 0.16 mm/y.

· Alloy 20 is not recommended for hydrochloric acid at any concentration; C276 is the standard choice there.

· At 800 F the ASME allowable stress is 23.1 ksi for C276 plate versus 16.8 ksi for Alloy 20 — about 38% higher.

· C276 typically costs 2.5-4x more per kilogram than Alloy 20, and is 10% denser, so the gap widens on a per-metre pipe basis.

· Alloy 20 welds with ER320LR / E320LR (ASME P-No. 45); C276 welds with ERNiCrMo-4 / ENiCrMo-4 (P-No. 43).

· Both alloys resist chloride stress-corrosion cracking far better than 316L; only C276 resists crevice corrosion in seawater.

Bottom line first: if your stream is sulfuric acid below roughly 80 C, essentially chloride-free, and the design temperature stays under 427 C (800 F), Alloy 20 (UNS N08020) will almost always give you the lowest installed cost and it has half a century of sulfuric-plant history behind it. If the stream contains hydrochloric acid, hydrofluoric acid, wet chlorine, hypochlorite, seawater, hydrogen sulfide, or if the design temperature exceeds 427 C, Hastelloy C276 (UNS N10276) is the correct specification and its 2.5-4x material premium is not negotiable.

What Are Hastelloy C276 and Alloy 20, and Why Are They Compared So Often?

They are compared constantly because both are sold as 'acid alloys' and both appear on the same chemical-plant piping line lists, yet they sit at opposite ends of the corrosion-resistance and price spectrum. C276 is a nickel-chromium-molybdenum-tungsten alloy with roughly 57 percent nickel and 15-17 percent molybdenum. Alloy 20 is an iron-nickel-chromium alloy with about 34 percent nickel, 3-4 percent copper and only 2-3 percent molybdenum, stabilised with niobium. The comparison is really a question about how much corrosion resistance you are actually buying, and whether your specific acid pays for it.

The confusion is compounded by the fact that Alloy 20 is frequently, and correctly, described as 'the sulfuric acid alloy'. That label is historically earned: Alloy 20 was developed specifically for sulfuric acid duty and it remains the default specification for sulfuric acid piping, acid coolers and absorber circuits worldwide. See our overview of Hastelloy C276 for acid service for the C276 side of that comparison, and nickel alloy pipe selection by acid type for the broader acid-by-acid framework.

What makes the pair genuinely interesting is that the simple story — Alloy 20 for sulfuric, C276 for everything harder — is broadly right for commercial reasons but not always right for corrosion reasons. In pure reagent-grade sulfuric acid at 80 C, published laboratory data actually puts C276 ahead of Alloy 20 across most of the concentration range. Alloy 20 still wins the commercial argument, but on cost and field history rather than on the laboratory corrosion rate. Understanding that distinction is what separates a defensible specification from a habit.

Table 1. At-a-glance scorecard: Hastelloy C276 vs Alloy 20

How Do the Chemical Compositions of C276 and Alloy 20 Differ?

They differ in three ways that drive everything else: molybdenum content, copper content, and the stabilising element. C276 carries 15-17 percent molybdenum plus 3-4.5 percent tungsten, which is what gives it resistance to reducing acids such as hydrochloric and to chloride pitting. Alloy 20 carries only 2-3 percent molybdenum but adds 3-4 percent copper, which is what gives it its sulfuric acid performance. C276 is stabilised by keeping carbon and silicon extremely low (0.01 and 0.08 percent maximum respectively); Alloy 20 is stabilised by adding niobium.

Table 2. Chemical composition limits, weight percent

The practical consequence of the copper difference is worth stating plainly: copper is the reason Alloy 20 performs in sulfuric acid, but copper does nothing for hydrochloric acid and nothing for chloride pitting. That single line in the composition table explains most of the selection outcomes in this guide. For the C276 composition in more depth, see Hastelloy C276 chemical composition.

Why Does PREN Fail as a Selection Tool for This Pair?

PREN ranks resistance to chloride pitting and nothing else. By that measure C276 (PREN roughly 68-76) is more than twice as resistant as Alloy 20 (PREN roughly 28), and for seawater, brackish cooling water, hypochlorite and any creviced chloride service that ranking is decisive. But PREN does not contain a term for copper or nickel, and both of those elements dominate behaviour in sulfuric acid, so PREN systematically understates Alloy 20 in the one environment it was designed for.

Table 3. Pitting resistance indices and what they actually predict

Do not specify on PREN alone. PREN would tell you C276 is 2.5x better than Alloy 20 in every medium, which is false in sulfuric acid and dangerously optimistic in hot concentrated nitric acid, where both alloys fail. Use PREN to compare chloride resistance between two candidate alloys that have already passed the acid screen.

Critical pitting and crevice temperatures are laboratory numbers from ASTM G48 ferric chloride and vary with surface finish, heat and test lab; treat the bands above as screening values and ask for mill or third-party G48 data when crevice corrosion is the governing failure mode.

Which Alloy Resists Sulfuric Acid Better?

On published laboratory corrosion rates in pure acid, C276 is generally better than Alloy 20 across the concentration range, and dramatically better in the 60-75 percent band where Alloy 20 has a well-documented corrosion hump. That is the honest reading of the data. Alloy 20 nevertheless remains the industry default for sulfuric acid plant piping, and the reason is cost plus the behaviour of commercial acid rather than laboratory acid.

Table 4. Corrosion rate in sulfuric acid at approximately 80 C, mm per year

Two caveats keep that table from being a blanket recommendation for C276. First, Carpenter's own guidance notes that corrosion rates in sulfuric acid are usually much lower in the presence of iron, copper and chromium ions, which are almost always present in commercial acid and in pickling and plating solutions — so plant rates for Alloy 20 are frequently better than the reagent-grade curve.

Second, Carpenter explicitly states that 20Cb-3 would not usually be recommended for service in 60 to 75 percent sulfuric acid. If your process can drift into that band during startup, dilution or upset, that is a specification risk whether or not the steady-state numbers look acceptable.

Table 5. Corrosion rate in hot and boiling sulfuric acid, mm per year

Decision rule for sulfuric acid: use Alloy 20 when the acid is essentially chloride-free, the temperature stays below about 80 C, the concentration stays outside the 60-75 percent hump, and cost drives the project. Move to C276 when chlorides or fluorides are present, when the concentration can drift through 60-75 percent, or when the duty includes acid plus oxidising contaminants that attack copper-bearing alloys.

Where the sulfuric duty is only part of a wider acid portfolio on the same plant, the acid-by-acid nickel alloy pipe selection guide is the faster route; for a single sulfuric circuit, the 98 percent case study in 316H pipe for 98% sulfuric acid service shows how the concentration band changes the answer.

Which Alloy Handles Hydrochloric Acid, and Why Is This Decisive?

Hydrochloric acid is the single most decisive discriminator in this comparison, because Alloy 20 is not recommended for it at any concentration while C276 is the industry standard. Chloride ions attack the passive film directly, and molybdenum plus tungsten is the only effective defence. With 2-3 percent molybdenum and no tungsten, Alloy 20 has essentially no defence at all.

Table 6. C276 iso-corrosion envelope in hydrochloric acid (Haynes data), temperature in C

Read that table as follows: for hydrochloric acid between 8 and 32 percent, C276 stays below 0.1 mm/y only at roughly 30-36 C, and stays below 0.5 mm/y up to roughly 62-68 C. Above about 65 C the rate climbs steeply and you should be looking at a nickel-molybdenum alloy such as Hastelloy B-3 instead. Alloy 20 has no usable envelope in hydrochloric acid at all.

This is also where C276 earns its reputation in mixed streams. Industrial hydrochloric acid almost always carries ferric or cupric ions, dissolved chlorine or sulfuric acid contamination, and C276 holds a stable passive film under those conditions. For a dedicated treatment of that service, see Hastelloy C276 for hydrochloric acid service.

How Do C276 and Alloy 20 Compare in Phosphoric, Nitric and Organic Acids?

In clean phosphoric acid both alloys perform well and Alloy 20 is usually specified because it is cheaper. The moment the acid is wet-process phosphoric acid containing fluorides, chlorides and silica, C276 becomes the safer choice. In nitric acid neither alloy is a good answer, and this is one of the few places where the comparison resolves to 'specify something else'.

Table 7. Media-by-media comparison beyond sulfuric and hydrochloric acid

Nitric acid is the shared blind spot. C276 corrodes at roughly 18 mm/y in boiling 60 percent nitric acid and roughly 22 mm/y in boiling 65 percent nitric acid — orders of magnitude above any usable design rate. Alloy 20 adds a second failure mode in oxidising acid: niobium-stabilised grades can suffer knifeline attack immediately beside the weld. For hot oxidising acid, specify a high-chromium alloy such as Hastelloy G-30, G-35 or C-22.

For phosphoric and mixed fertilizer acid duty, cross-check against Hastelloy C276 for chemical plants and the hub page ultimate guide to Hastelloy C276.

Which Alloy Resists Chloride Pitting, Crevice Corrosion and Stress Corrosion Cracking?

C276 wins this category decisively and it is the most common reason engineers upgrade from Alloy 20. Both alloys are far more resistant to chloride stress-corrosion cracking than 316L, because both have enough nickel to suppress the mechanism, but only C276 has the molybdenum and tungsten needed to resist pitting and crevice attack under deposits, gaskets and lap joints.

This matters more in practice than any acid number, because most field failures are localised rather than general. A sulfuric acid line that is fine on general corrosion will still fail at a flange face, a weld backing ring, a thermowell crevice or under a deposit if the alloy lacks crevice resistance — and cooling water, utility water, sea spray and process upsets all introduce chlorides into nominally chloride-free systems. If your equipment has crevices and any possible chloride source, C276 is the conservative answer.

The chloride question is covered in more depth in Hastelloy C276 corrosion resistance, and the seawater end of the spectrum in Inconel 625 vs Hastelloy C276 for seawater.

What Are the Temperature Limits for C276 and Alloy 20 Under ASME Code?

This is the cleanest, least arguable difference in the whole comparison. Under ASME BPVC Section VIII Division 1, C276 is listed to 677 C (1250 F) while Alloy 20 stops at 427 C (800 F). If your design metal temperature exceeds 427 C, Alloy 20 is not a candidate, and no amount of corrosion testing changes that.

Table 8. ASME BPVC maximum temperature limits by Code section

Two traps catch specifiers here. The first is quoting the oxidation limit instead of the Code limit: C276 resists scaling to about 1040 C in still air, but that number has no load attached and must never be used for a pressure boundary. The second is assuming the 800 F Alloy 20 figure applies to every product form; coverage varies between ASTM B463 plate, B473 bar, B462 forgings, B729 seamless pipe, B464 welded pipe, B468 welded tube and B366 fittings, so read the row for your form in Section II Part D.

For the C276 figures in context, see Hastelloy C276 temperature limits, and for the same treatment on a neighbouring alloy, Inconel 625 temperature range.

How Do the Mechanical Properties and Allowable Stresses Compare?

C276 is stronger at every temperature, both on room-temperature minimums and — far more importantly for a pressure design — on ASME allowable stress at temperature. At 800 F, C276 plate carries 23.1 ksi against 16.8 ksi for Alloy 20, roughly 38 percent more, which is often enough to reduce wall thickness and recover part of the material price difference.

Table 9. Room-temperature minimum mechanical properties, solution annealed

Table 10. ASME maximum allowable stress versus metal temperature, ksi

C276 values are for ASME SB-575 plate in Section VIII Division 1 service; Alloy 20 values are Division 1 design stresses for plate, bar, forgings and seamless tube, with welded pipe and tube carrying a separate lower column. Always pull the current edition of Section II Part D for your product form, thickness and Division — the tabulated values change between editions.

Note also that Alloy 20's higher coefficient of thermal expansion matters when it is welded to carbon steel or to austenitic stainless: about 14.7 um/m.K against 11.2 for C276 and roughly 12 for carbon steel. Dissimilar-metal welds and thermal cycling behave differently as a result. Mechanical detail for the C276 side is in Hastelloy C276 mechanical properties.

Which Alloy Is Easier to Weld and Fabricate?

Both weld well and both are normally used in the as-welded condition, but they get there by different routes and they are qualified under different ASME Section IX P-Numbers. C276 relies on ultra-low carbon and silicon to avoid grain-boundary precipitation; Alloy 20 relies on niobium stabilisation. Both approaches work, and both have a specific failure mode to avoid.

Table 11. Welding and fabrication comparison

The Alloy 20 caution that surprises people is the stress-relief limit. Because it is stabilised with niobium, Alloy 20 should be stress relieved below about 538 C (1000 F) followed by a water quench; taking it into the 538-870 C range risks reheat cracking and defeats the stabilisation. Annealing is normally 940-1010 C (1725-1850 F) for 30 minutes per inch of section, water quenched. Carpenter also warns that annealing at higher temperature, around 2100 F, lowers hardness but can harm the stabilisation of the alloy.

For the C276 side, full procedure detail is in Hastelloy C276 welding guide, and the make-versus-buy economics of welding are covered in pipe spool prefabrication cost versus field welding.

How Do Machining and Forming Characteristics Compare?

Alloy 20 is noticeably easier and cheaper to machine than C276. Both are austenitic, both work harden rapidly, and both need rigid setups, positive rake, sharp tools and constant feed — but C276's higher nickel content and higher strength at temperature make it gummier, raise cutting forces and shorten tool life.

· Expect roughly 30-50 percent more tool wear machining C276 than Alloy 20 for the same operation.

· Never dwell: both alloys work harden, and a rubbing tool burnishes the surface and destroys the next cut.

· Use positive-rake carbide or high-speed steel with a heavy, constant feed and a generous depth of cut.

· Alloy 20's lower work-hardening rate makes it more forgiving in threading, grooving and drilling.

· C276 requires more power and a stiffer setup; light finishing passes on a work-hardened skin are a common cause of premature insert failure.

· Use chlorinated or sulfurised cutting fluids only where subsequent service permits — residues can initiate pitting on nickel alloys.

Machining parameters for C276 are set out in Hastelloy C276 machining tips.

Which Alloy Performs Better in FGD and Flue-Gas Service?

C276, without qualification. Flue gas desulfurization is the canonical C276 application and Alloy 20 is essentially absent from FGD absorber specifications. The environment combines low pH, high chloride, fluoride, wet-dry cycling at the gas-liquid interface, deposits and widely varying chemistry between units and even between campaigns at the same unit.

Table 12. FGD and air-pollution-control duty

If you are specifying an FGD project, start with Hastelloy C276 for FGD systems and compare against the higher-chromium options in Hastelloy C22 vs C276, since C22 outperforms C276 where the FGD chemistry runs oxidising.

Where Does Each Alloy Fit in a Chemical, Fertilizer or Pickling Plant?

The split is clean once you separate sulfuric-acid circuits from everything else. Alloy 20 owns the sulfuric and phosphoric circuits and the general corrosive-service piping; C276 owns the hydrochloric, mixed-acid, chlorinated and upset-prone circuits, and anything that must survive a process excursion it was not designed for.

Table 13. Application fit by plant circuit

For circuit-level selection across the whole plant, use how to choose nickel alloy pipe for chemical plants and chemical equipment material selection; for the pulp and paper end, see Hastelloy C276 for the pulp and paper industry.

How Do C276 and Alloy 20 Compare in Seawater and Marine Service?

C276 is a genuine seawater alloy and Alloy 20 is not. This is not a marginal difference: Alloy 20 has a PREN near 28, which sits in the same band as 316L and well below what is needed to resist crevice corrosion in warm, aerated seawater. Use it in seawater only for fully immersed, cathodically protected or continuously flowing clean duty, and even then expect pitting at deposits and gaskets.

· Seawater cooling and heat exchange: C276, or consider Inconel 625 and super duplex S32750 depending on temperature and velocity.

· Ballast, firemain and seawater lift: C276 where crevices exist; super duplex is usually the economic alternative.

· Marine exhaust and scrubber overboard: C276, because the stream is acidic and chlorinated.

· Offshore topsides process piping: C276 where acid gas or chloride condenses; verify against NACE MR0175.

· Alloy 20 in marine service: restrict to internal, dry, chloride-free chemical circuits on a marine asset.

Compare the seawater options in super duplex S32750 for seawater desalination and duplex 2205 vs super duplex 2507.

What Do C276 and Alloy 20 Cost, and Where Does the Break-Even Sit?

C276 typically costs 2.5 to 4 times as much as Alloy 20 per kilogram, and because it is also about 10 percent denser the gap widens slightly on a per-metre pipe basis. The premium is driven by composition: roughly 57 percent nickel, 16 percent molybdenum and 4 percent tungsten, against roughly 34 percent nickel and 2.5 percent molybdenum for Alloy 20, with iron making up the balance.

Table 14. Cost and availability comparison

Price the installed system, not the kilogram. C276's higher allowable stress can reduce wall thickness by roughly a third at temperature, which recovers a meaningful share of the material premium. Run the wall-thickness calculation before concluding that C276 is unaffordable — for high-pressure, high-temperature acid lines it is sometimes the cheaper option on a fabricated weight basis.

For current C276 pricing, see Hastelloy C276 pipe price per kg and request a live quotation through the JN Alloy contact page.

Which ASTM and ASME Product Specifications Cover Each Alloy?

Specifying the right ASTM number per product form matters as much as choosing the grade, because the ASME allowable stress and the Code temperature cap are tied to the specification, not to the UNS number alone.

Table 15. Product specification map

Full product coverage for both grades is listed under Hastelloy C276 products and Alloy 20 products, including Hastelloy pipe, Hastelloy sheet and plate and Hastelloy round bars.

How Do You Choose Between Hastelloy C276 and Alloy 20?

Work the eight steps below in order. The first three are screening steps that will eliminate one alloy in most projects; the rest are the optimisation work that follows.

1. Define the process chemistry completely. Write down every constituent: acid type and concentration range, temperature range, chloride and fluoride content, oxidising contaminants such as Fe3+ or Cu2+, aeration state, velocity and whether the stream ever dries out or is shut down wet.

2. Screen out Alloy 20 on the hard exclusions first. Rule Alloy 20 out immediately if hydrochloric acid, hydrofluoric acid, wet chlorine, hypochlorite, seawater, sour (H2S) service, or a design metal temperature above 427 C (800 F) is present. Any one of these forces C276 or a higher alloy.

3. Check the sulfuric acid concentration against the 60-75% hump. If your sulfuric acid spends meaningful time in the 60-75 wt% band at temperature, Carpenter's own data shows Alloy 20 corrosion rising to about 1.3 mm/y. Either hold concentration outside that band or specify C276.

4. Confirm the ASME temperature cap for your product form. Pull ASME BPVC Section II Part D for the exact specification and Division. Alloy 20 stops at 427 C (800 F) in Section VIII Division 1 and 343 C (650 F) in Section XII; C276 runs to 677 C (1250 F) in Division 1 and 538 C (1000 F) in Section I.

5. Compare allowable stress, not room-temperature tensile. Design against the tabulated allowable stress at your design temperature. C276 carries roughly 38-44% more allowable stress than Alloy 20 between 400 F and 800 F, which often thins the wall and claws back part of the price gap.

6. Price the installed system, not the kilogram. Multiply material price by density and wall thickness, then add welding cost (C276 needs tighter heat-input control), NDE, and the cost of a field failure. For long runs of sulfuric acid piping below 80 C, Alloy 20 usually still wins.

7. Validate with coupons before you commit. Laboratory rates in reagent-grade acid differ from plant acid. Run ASTM G31 coupons in the real stream, or at minimum ASTM G48 for pitting and ASTM G28 for intergranular attack on the welded condition you will actually ship.

8. Lock the documentation into the purchase order. Require EN 10204 3.1 or 3.2 certification, full chemistry including Cu and Nb for Alloy 20 and Mo and W for C276, solution-anneal records, PMI on every heat, and ASTM A262 Practice E if intergranular corrosion is a risk.

When Should You Specify Neither Alloy?

There are four situations where the C276-versus-Alloy-20 debate is the wrong debate, and pushing either alloy into them produces failures that look mysterious until you recognise the pattern.

Table 16. Services where you should look elsewhere

Cross-check the neighbouring grades before you commit: Hastelloy C22 vs C276, Hastelloy C276 vs Inconel 625, Incoloy 825 vs Hastelloy C276 for sour gas and the full Inconel 625 vs C276 selection guide.

What Documentation and Testing Should You Require From the Mill?

Require EN 10204 3.1 minimum, 3.2 for critical service, and insist that the certificate lists the elements that actually differentiate these two grades. A certificate that omits copper and niobium on Alloy 20, or molybdenum and tungsten on C276, has not proven anything.

Table 17. Mill documentation and verification checklist

JN Alloy ships every order with full EN 10204 3.1 / 3.2 certification and supports project engineering from material selection through weld procedure qualification. Start at the Hastelloy C276 hub page or go straight to contact JN Alloy with your process data.

Where Should You Go Next for More Detail?

This article is one spoke in a hub-and-spoke cluster built around the ultimate guide to Hastelloy C276. The pages below extend each topic covered here.

· What is Hastelloy C276? — grade fundamentals and history.

· Hastelloy C276 chemical composition — full elemental limits and the role of each element.

· Hastelloy C276 corrosion resistance — acid-by-acid behaviour and test methods.

· Hastelloy C276 temperature limits — oxidation limits versus ASME design limits.

· Hastelloy C276 welding guide — fillers, heat input and post-weld treatment.

· Hastelloy C276 for acid service — the acid-service spoke for this cluster.

· Alloy 20 knowledge base — the reference page for the comparison grade.

· Everything you need to know about Alloy 20 pipe — product-form detail.

· JN Alloy FAQ list — answers across all alloy families.

· Case study: Inconel 625 pipe spools for an LNG facility — how a nickel alloy project is executed end to end.

Frequently Asked Questions

Q1. Is Hastelloy C276 better than Alloy 20?

It depends entirely on the medium. C276 is better in hydrochloric acid, hydrofluoric acid, seawater, wet chlorine, hypochlorite, sour gas and any chloride-bearing or creviced service, and it is the only option above 427 C (800 F) under ASME Section VIII Division 1. Alloy 20 is the better commercial choice for clean sulfuric and phosphoric acid below about 80 C, because it costs 2.5 to 4 times less and has decades of sulfuric-plant field history. Neither alloy is universally better.

Q2. Which alloy is better for sulfuric acid, Alloy 20 or Hastelloy C276?

On published laboratory corrosion rates C276 is generally lower, but Alloy 20 is the industry default and usually the right engineering decision. At 80 C, C276 runs roughly 0.03 to 0.48 mm/y across 10 to 90 percent acid while Alloy 20 runs about 0.13 to 1.27 mm/y, with a sharp corrosion hump in the 60 to 75 percent band. Choose Alloy 20 for chloride-free acid below 80 C; move to C276 if chlorides are present, if concentration can drift through 60 to 75 percent, or if temperature rises materially.

Q3. What is the main difference between Hastelloy C276 and Alloy 20?

The main difference is alloying strategy and the corrosion mechanism each one addresses. C276 is a nickel-chromium-molybdenum-tungsten alloy with about 15 to 17 percent molybdenum and 3 to 4.5 percent tungsten, which defends against chloride pitting, crevice corrosion and reducing acids such as hydrochloric. Alloy 20 is an iron-nickel-chromium alloy with 3 to 4 percent copper and only 2 to 3 percent molybdenum, niobium-stabilised, which is optimised for sulfuric acid. That one composition difference explains nearly every selection outcome between them.

Q4. Can Alloy 20 handle hydrochloric acid?

No. Alloy 20 is not recommended for hydrochloric acid at any concentration or temperature. With only 2 to 3 percent molybdenum and no tungsten it has no effective defence against chloride attack on the passive film, and corrosion rates rise steeply even in dilute acid. Hastelloy C276 is the standard nickel alloy for hydrochloric service and holds below 0.5 mm/y up to roughly 62 to 68 C across 8 to 32 percent acid. Above about 65 C, consider Hastelloy B-3.

Q5. What is the maximum service temperature of Alloy 20?

Under ASME BPVC Section VIII Division 1 and Section III Class 2 and 3, Alloy 20 is listed to 427 C (800 F). Section XII transport tanks cap it at 343 C (650 F), and it is not permitted for Section I power boiler construction. Coverage varies by product form, with separate rows for ASTM B463 plate, B473 bar, B462 forgings, B729 seamless pipe, B464 welded pipe, B468 welded tube and B366 fittings, so confirm the row for your form in ASME Section II Part D.

Q6. What is the maximum service temperature of Hastelloy C276?

C276 is listed to 677 C (1250 F) under ASME Section VIII Division 1 and ASME B31.3, 538 C (1000 F) under Section I, 427 C (800 F) under Section III Class 2 and 3 and Section VIII Division 2, and 343 C (650 F) under Section XII. Separately, C276 resists oxidation in air to about 1040 C (1900 F), but that is a scaling limit with no load attached and must never be used as a design temperature for a pressure boundary.

Q7. How much more does Hastelloy C276 cost than Alloy 20?

C276 typically costs 2.5 to 4 times as much as Alloy 20 per kilogram, and because it is about 10 percent denser the gap widens slightly per metre of pipe. The premium comes from composition: roughly 57 percent nickel, 16 percent molybdenum and 4 percent tungsten versus roughly 34 percent nickel and 2.5 percent molybdenum for Alloy 20. However, C276 carries up to 48 percent higher ASME allowable stress at temperature, which can thin the wall and recover part of the premium.

Q8. Can you weld Hastelloy C276 to Alloy 20?

Yes, and it is common in transition pieces and mixed-material plants. The usual practice is to use an over-alloyed nickel filler such as AWS A5.14 ERNiCrMo-3 (Inconel 625 type) for GTAW and GMAW, or ENiCrMo-3 for SMAW and SAW, rather than either matching filler, so the weld metal is noble enough for both base metals. Qualify the procedure under ASME Section IX and control heat input, since C276 sits in P-No. 43 and Alloy 20 in P-No. 45.

Q9. What filler metal do you use for Alloy 20?

The matching filler is AWS A5.14 ER320LR for GTAW and GMAW, and E320LR for SAW and SMAW. The low-carbon LR grade is preferred so the weld metal matches the niobium-stabilised base metal. When welding Alloy 20 to 316 stainless or to higher alloys such as C276 or C22, use ERNiCrMo-3 for GTAW and GMAW or ENiCrMo-3 for SAW. Alloy 20 is ASME Section IX P-No. 45.

Q10. What filler metal do you use for Hastelloy C276?

The matching filler is AWS A5.14 ERNiCrMo-4 for bare rod and wire, and AWS A5.11 ENiCrMo-4 for covered electrodes. Because C276 has a maximum carbon of 0.01 percent and silicon of 0.08 percent, it resists grain-boundary precipitation in the heat-affected zone and is normally used in the as-welded condition. For severe service, a full solution anneal at 1121 C minimum with rapid quench restores optimum corrosion resistance.

Q11. Is Alloy 20 suitable for seawater service?

No, not for normal seawater duty. Alloy 20 has a PREN of roughly 28, which is in the same band as 316L, so it will pit and crevice-corrode in warm aerated seawater, especially under deposits and at gasket faces. Use Hastelloy C276 where seawater combines with acid or chlorine, or consider Inconel 625, super duplex S32750 or copper-nickel for clean seawater where the extra alloy cost is not justified.

Q12. What is the PREN of Alloy 20 compared with Hastelloy C276?

Alloy 20 has a PREN of about 28, calculated as 20 percent chromium plus 3.3 times 2.5 percent molybdenum. Hastelloy C276 has a PREN of about 68 using the same formula, or about 76 using the tungsten-credited version. PREN predicts chloride pitting only, so it correctly shows C276 to be far better in seawater and creviced chloride service, but it says nothing about sulfuric acid, where copper and nickel dominate and Alloy 20 performs far better than its PREN implies.

Q13. Is Alloy 20 resistant to nitric acid?

Alloy 20 has moderate resistance to dilute nitric acid at ambient temperature but is not recommended for hot or concentrated nitric acid, and niobium-stabilized alloys can suffer knife line attack in oxidizing acid immediately beside the weld. Hastelloy C276 is also a poor choice for boiling concentrated nitric acid, corroding at roughly 18 mm/y in boiling 60 percent acid. For hot oxidizing nitric duty, specify a high-chromium alloy such as Hastelloy G-30, G-35 or C-22.

Q14. Can Alloy 20 be used in FGD systems?

No. Alloy 20 is essentially absent from flue gas desulfurization specifications because FGD slurry combines low pH with high chloride, fluoride, wet-dry cycling and deposits. Hastelloy C276 is the industry workhorse for FGD absorbers, inlet and outlet ducts, dampers, stack liners and reheater housings. Where the FGD chemistry runs strongly oxidizing, Hastelloy C22 usually outperforms C276.

Q15. Which alloy should I use for phosphoric acid service?

Use Alloy 20 for clean phosphoric acid, where it performs excellently and costs far less. Use Hastelloy C276 for wet-process phosphoric acid, because that stream carries fluorides, chlorides and silica that attack Alloy 20 and cause localized attack at crevices and under deposits. If the acid also contains sulfuric acid or oxidizing contaminants, C276 is the safer specification.

Q16. Is Alloy 20 the same as 20Cb-3?

Yes. 20Cb-3 is the Carpenter Technology trade name for Alloy 20, which carries UNS N08020 and Worst-off number 2.4660. The Cb stands for columbium, the older name for niobium, which is the stabilizing element. Other trade names include Carpenter 20 and Incoloy alloy 20, and the cast equivalent is ASTM A494 CN7M.

Q17. Is Alloy 20 magnetic?

No. Alloy 20 is a fully austenitic iron-nickel-chromium alloy and is non-magnetic in the annealed condition, with a permeability close to 1.0. Hastelloy C276 is likewise austenitic and non-magnetic. Neither alloy can be hardened by heat treatment; both are strengthened by cold work, and heavy cold work can produce slight magnetic response in the worked zone.

Q18. Does Alloy 20 require post-weld heat treatment?

Usually no, because niobium stabilisation prevents sensitisation and Alloy 20 is normally used as-welded. If stress relief is required, it must be done below about 538 C (1000 F) followed by a water quench; stress relieving in the 538 to 870 C range risks reheat cracking and degrades the stabilisation. Full annealing is 940 to 1010 C for 30 minutes per inch of section, water quenched. Annealing near 2100 F lowers hardness but can harm stabilisation.

Q19. What is the yield strength of Alloy 20 versus Hastelloy C276?

Alloy 20 has a minimum 0.2 percent yield strength of 241 MPa (35 ksi) and a minimum tensile strength of 551 MPa (80 ksi) with 30 percent minimum elongation. Hastelloy C276 has a minimum yield strength of 283 MPa (41 ksi), a minimum tensile strength of 690 MPa (100 ksi) and 40 percent minimum elongation. The more useful comparison for design is ASME allowable stress: at 800 F, C276 carries 23.1 ksi against 16.8 ksi for Alloy 20.

Q20. Is Alloy 20 approved for sour gas service under NACE MR0175?

Alloy 20 is not normally specified for sour service, and it is not the grade engineers turn to when NACE MR0175 / ISO 15156 compliance is required. Hastelloy C276 is qualified for sour service and is widely used for sour gas components in oil and gas production. Where sour service is the governing requirement, specify C276 and confirm the hardness and heat-treatment condition required by the standard for your component and exposure severity.

Q21. Which alloy lasts longer in sulfuric acid piping?

In clean, chloride-free sulfuric acid below about 80 C, Alloy 20 delivers a long service life and is the standard choice; in contaminated, chlorinated, aerated or temperature-cycling sulfuric acid, C276 will last substantially longer. The deciding factors are chloride content, whether the concentration crosses the 60 to 75 percent hump, and whether the line ever sits stagnant or dries out. Where doubt exists, run coupons in the actual stream before committing to a full line specification.