Hastelloy C276 vs C22: Oxidizing vs Reducing Acid Performance

Hastelloy C276 and Hastelloy C22 are both nickel-chromium-molybdenum alloys built to survive chemical environments that destroy stainless steel in weeks or days. They look similar on a mill certificate and cost roughly the same, so it is tempting to treat them as interchangeable. They are not. The two alloys were engineered for opposite chemistry: one for oxidizing acids, the other for reducing acids. Specifying the wrong one does not just shorten service life - it can cause unplanned shutdowns, leaks, and safety incidents in pressure equipment.

This guide answers the questions engineers ask most often when choosing between C276 and C22, starting each answer with a direct conclusion before the supporting explanation.

What Is the Core Difference Between Hastelloy C276 and C22?

Hastelloy C22 is formulated with higher chromium content to resist oxidizing acids, while Hastelloy C276 is formulated with higher molybdenum and tungsten content to resist reducing acids. Chromium and molybdenum protect metal through two different chemical mechanisms, and that single design choice explains almost every performance difference between the two alloys.

Both alloys rely on a thin, invisible oxide layer on the metal surface - a passive film - to keep the acid from reaching the base metal. Chromium builds a passive film that is stable when the environment is oxidizing, meaning it has plenty of free electron-accepting species (dissolved oxygen, nitric acid, ferric or cupric ions) to keep rebuilding the film if it is scratched or dissolved. Molybdenum and tungsten do the opposite job: they stabilize the passive film and resist localized breakdown in reducing acids, where there is little oxidizing power available to repair the film once it is damaged.

C22 was developed after C276, specifically to close a performance gap: C276 performed superbly in reducing acids and chloride pitting, but corroded faster than desired in strongly oxidizing acids such as nitric acid. Haynes International raised the chromium content and added controlled tungsten to create C22, trading some reducing-acid margin for a substantial gain in oxidizing-acid resistance.

Which Alloy Performs Better in Oxidizing Acids?

Hastelloy C22 outperforms C276 in oxidizing acids such as nitric acid, mixed acids containing nitric acid, hot contaminated phosphoric acid, and oxidizing chloride salts like ferric chloride and cupric chloride.

Oxidizing acids supply the electrochemical conditions chromium needs to keep repairing the passive oxide film. With roughly 20-22.5% chromium - about five to seven percentage points more than C276 - C22 forms a thicker, more chemically stable protective layer under these conditions. This makes C22 the standard recommendation for:

• Nitric acid and nitric-hydrofluoric acid pickling baths
• Flue-gas desulfurization (FGD) systems with oxidizing scrubbing chemistry
• Pulp and paper bleaching lines using chlorine dioxide or hypochlorite
• Ferric chloride and cupric chloride etching or waste-treatment processes
• Mixed-acid environments where nitric acid is present alongside other acids

In these services, C276 can still perform acceptably at low temperatures and dilute concentrations, but its corrosion rate rises faster than C22's as temperature, concentration, or oxidizing strength increases.

Which Alloy Performs Better in Reducing Acids?

Hastelloy C276 outperforms C22 in reducing acids such as hydrochloric acid, dilute-to-moderate sulfuric acid, and sour, chloride-bearing process streams containing hydrogen sulfide.

Reducing acids lack the free oxidizing species that chromium's passive film depends on, so protection shifts to molybdenum and tungsten, which resist the film's breakdown directly rather than relying on rapid re-passivation. C276 carries roughly 15-17% molybdenum plus 3-4.5% tungsten, noticeably more of both elements than C22. That composition makes C276 the preferred choice for:

• Hydrochloric acid duty at low-to-moderate concentrations and temperatures
• Sulfuric acid service, particularly dilute and moderately concentrated grades
• Sour gas and oilfield environments containing H₂S and chlorides
• Wet-process phosphoric acid with reducing contaminants
• General chemical processing equipment exposed to mixed non-oxidizing acids

C22 still resists these media reasonably well, which is why both alloys are broadly described as "universal" corrosion-resistant alloys. But where the process chemistry is clearly reducing, C276 provides a wider safety margin and is the more common specification worldwide.

How Do Their Chemical Compositions Compare?

C22 (UNS N06022) contains roughly 20-22.5% chromium and 12.5-14.5% molybdenum, while C276 (UNS N10276) contains roughly 14.5-16.5% chromium and 15-17% molybdenum. This chromium-versus-molybdenum trade-off is the single number that predicts which alloy wins in a given acid.

Both alloys hold carbon below 0.015% by design ("low-carbon" or "-C" grades). Low carbon limits the precipitation of chromium-carbides at grain boundaries during welding, which is what caused the original 1930s-era Hastelloy C alloy to fail from intergranular corrosion in heat-affected zones. This is also why both C276 and C22 are considered readily weldable without mandatory post-weld heat treatment for corrosion resistance, unlike their predecessor.

How Does Pitting and Crevice Corrosion Resistance Compare in Chloride Environments?

Both alloys offer outstanding pitting and crevice corrosion resistance in chloride solutions, among the best of any commercial alloy family. C22 has an edge in oxidizing chloride salts like ferric chloride, while C276 has an edge in reducing, chloride-rich acids. In most general seawater or brine service, the two alloys perform comparably.

Pitting and crevice corrosion resistance in chloride media is driven by chromium, molybdenum, and tungsten acting together, often summarized with the Pitting Resistance Equivalent Number (PREN = %Cr + 3.3×%Mo + 16×%N). Both C276 and C22 post PREN values well above 40, far exceeding super-duplex stainless steels and even most other nickel alloys. In practice, the deciding factor is rarely "pitting resistance" in isolation - it is whether the surrounding chloride solution is oxidizing (favor C22) or reducing (favor C276).

How Do They Perform at High Temperatures?

Hastelloy C276 generally offers greater high-temperature stability and resistance to grain-boundary precipitation during prolonged heat exposure, making it the more common choice for elevated-temperature corrosive service, while C22 is typically favored for moderate-temperature, strongly oxidizing duty.

Both alloys are used from cryogenic temperatures up to several hundred degrees Celsius in corrosive service, and both retain useful strength at temperatures where austenitic stainless steels would need to be derated. Published continuous-service temperature limits vary by source and by which property is being measured (oxidation resistance in air versus aqueous corrosion resistance versus mechanical strength), so design engineers should always confirm the applicable limit against the current Haynes International datasheet and the specific process conditions rather than relying on a single quoted number.

How Do Mechanical Properties Compare?

C22 has a modest edge in tensile and yield strength in the annealed condition, while C276 has a slightly lower density difference that is negligible for most design purposes. For most projects, mechanical properties are a secondary factor behind corrosion performance when choosing between these two alloys.

Exact mechanical properties vary by product form (plate, bar, tube, fittings), heat treatment, and mill. Always verify against the mill test certificate for the specific heat and product form being purchased.

Which Alloy Should You Choose for Your Application?

Choose C22 when the dominant acid or contaminant is oxidizing; choose C276 when it is reducing; and when the process chemistry is mixed, uncertain, or swings between the two, consult a metallurgist and consider third-generation alloys such as C-2000 or Alloy 59 that were designed to bridge both regimes.

Frequently Asked Questions

Is Hastelloy C276 or C22 more expensive?

Pricing fluctuates with nickel and molybdenum markets, but C22 is commonly reported as somewhat more expensive per pound than C276, largely offset by C22's slightly lower density when purchased by volume. Current pricing should always be confirmed with a mill or distributor, since both alloys track volatile nickel-alloy commodity markets.

Can Hastelloy C276 and C22 be welded to each other?

Yes. Both alloys are readily weldable to themselves and to each other using matching or compatible nickel-alloy filler metal, and neither requires mandatory post-weld heat treatment to restore corrosion resistance, thanks to their low carbon content. Weld procedure qualification and filler-metal selection should still follow the applicable code and the fabricator's engineering review.

Is C22 or C276 better for sulfuric acid service?

C276 is generally preferred for sulfuric acid because sulfuric acid behaves as a reducing acid across much of its concentration and temperature range, which favors C276's higher molybdenum and tungsten content. At certain high concentrations and temperatures sulfuric acid becomes more oxidizing, so the specific concentration and temperature envelope should be checked against a corrosion-rate isocorrosion chart before finalizing the material.

What are the UNS designations for Hastelloy C276 and C22?

Hastelloy C276 is UNS N10276, covered by specifications including ASTM B575, B574, and B619. Hastelloy C22 is UNS N06022, covered by the same family of ASTM specifications. Both are also recognized under equivalent ASME specifications for pressure equipment.

Which alloy has better overall, all-around corrosion resistance?

Neither alloy is universally "better" - they are complementary. C22 has the broader resistance profile across oxidizing and moderately reducing media, which is why some engineers default to it for unknown or mixed chemistries, while C276 retains an edge specifically in strongly reducing acids and remains the more widely stocked of the two. The right choice always depends on the specific acid, concentration, temperature, and chloride content of the service.

Are there newer alloys that outperform both C276 and C22?

Alloy C2000 and Alloy 59 are later-generation Ni-Cr-Mo alloys designed to combine strong oxidizing- and reducing-acid resistance in a single composition. They are worth evaluating for applications where process chemistry swings between oxidizing and reducing conditions, though they typically carry a cost and availability trade-off compared with the well-established C276 and C22.