Hastelloy is the trademarked family of nickel-molybdenum and nickel-chromium-molybdenum corrosion-resistant alloys produced by Haynes International, used generically to describe high-nickel alloys engineered for the most aggressive chemical environments encountered in process-industry valves. Within the valve materials classification system, Hastelloy occupies the severe-corrosion tier - specified when a fluid is too aggressive for stainless steel, duplex, or even Inconel, particularly in reducing acids such as hydrochloric and sulfuric acid where chromium-bearing alloys alone offer limited protection. The defining feature of the family is its very high molybdenum content, which delivers resistance to reducing media that no stainless steel can match.
Key Takeaways
- Hastelloy is a nickel-based alloy family (typically 55-65% nickel) whose high molybdenum content - from about 16% in the C-grades to roughly 28% in the B-grades - provides outstanding resistance to reducing acids such as hydrochloric and sulfuric acid.
- The two main branches are the nickel-molybdenum B-series (best for hydrochloric acid at all concentrations) and the nickel-chromium-molybdenum C-series (resistant to both oxidizing and reducing media, wet chlorine, and chloride pitting).
- Hastelloy C-276 (UNS N10276) is the workhorse valve grade, resisting wet chlorine, hypochlorite, ferric and cupric chlorides, and mixed-acid streams while remaining immune to chloride stress corrosion cracking.
- In valves it appears as body, trim, and weld-overlay material for chemical processing, flue-gas desulfurization, pulp bleaching, and sour oil-and-gas service where the corrosion environment exceeds the limits of stainless and nickel-chromium alloys.
How It Works
The Nickel-Molybdenum-Chromium System
Hastelloy alloys share a nickel-rich face-centred-cubic (FCC) austenitic matrix - the same intrinsically tough, ductile structure that gives all high-nickel alloys their immunity to chloride stress corrosion cracking - to which large quantities of molybdenum (and, in the C-grades, chromium and tungsten) are added as the primary corrosion-resisting elements. Molybdenum is the key: dissolved in the nickel matrix it suppresses the active corrosion of metal in reducing (non-oxidizing) acids, the regime where the protective chromium oxide film that stainless steels rely on cannot form or remain stable. Chromium, present at 15-22% in the C-series, adds a stable passive film for oxidizing environments, so the C-grades uniquely resist both reducing and oxidizing media in the same component - a combination critical for process streams whose oxidizing potential fluctuates during operation.
Resistance to Reducing vs Oxidizing Acids
The central engineering distinction within the Hastelloy family is the trade-off between reducing-acid and oxidizing-acid resistance. The nickel-molybdenum B-series (no deliberate chromium) provides the best available resistance to pure hydrochloric acid across all concentrations and temperatures, but its lack of chromium makes it vulnerable to oxidizing contaminants such as ferric ions or dissolved oxygen. The nickel-chromium-molybdenum C-series sacrifices a small amount of pure reducing-acid performance in exchange for a chromium passive film that confers resistance to oxidizing acids, wet chlorine, and oxidizing salt solutions - making the C-grades the more versatile choice for the mixed and variable chemistries typical of real process plant. Selecting between the families therefore depends on whether the service is consistently reducing or contains oxidizing species, a determination addressed in the material for acid service reference.
Pitting, Crevice and Stress Corrosion Cracking Resistance
The combination of high molybdenum (15-17% in C-276) and chromium produces pitting resistance equivalent numbers (PREN) well above those of super duplex stainless steel, giving the C-grades exceptional resistance to localized pitting and crevice attack in chloride-bearing media - the failure modes that limit stainless steel in seawater and acidic chloride service. Equally important, the high nickel content places Hastelloy far above the nickel threshold for immunity to chloride stress corrosion cracking, the cracking mechanism that makes 304 and 316 stainless unsuitable for hot chloride environments. This makes Hastelloy a primary candidate alongside duplex stainless steel wherever chloride SCC is the governing risk, with Hastelloy selected when the additional reducing-acid resistance is also required.
Main Hastelloy Grades
Hastelloy C-276 (UNS N10276)
Hastelloy C-276 is the most widely specified valve grade, with a nominal composition of approximately 57% nickel (balance), 15-17% molybdenum, 14.5-16.5% chromium, 3-4.5% tungsten, and 4-7% iron (Werkstoff 2.4819). It is one of very few materials that resists wet chlorine gas, hypochlorite and chlorine-dioxide bleaching solutions, and strongly oxidizing ferric and cupric chlorides, while also handling reducing acids - the breadth of resistance that makes it the default choice when a single alloy must survive an uncertain or variable chemical environment. C-276 is fully resistant to chloride stress corrosion cracking and is readily welded without post-weld heat treatment, supporting its use in fabricated valve bodies and weld overlays.
Hastelloy C-22 (UNS N06022)
Hastelloy C-22 raises chromium to roughly 20-22.5% and carries about 12.5-14.5% molybdenum and 2.5-3.5% tungsten, with nickel near 56%. The higher chromium gives it superior performance in oxidizing and mixed-acid environments compared with C-276, and it offers a marked improvement over stainless steels and alloy 625 in moderate-concentration hydrochloric acid. C-22 is frequently selected for valves in flue-gas desulfurization scrubbers and in chemical service where the stream alternates between oxidizing and reducing conditions, a duty that demands the broadest possible passive-film stability.
Hastelloy B-3 (UNS N10675)
Hastelloy B-3 is the modern nickel-molybdenum grade, with approximately 65% nickel and 27-32% molybdenum (nominally 28.5%) and only 1-3% chromium. It provides excellent resistance to hydrochloric acid at all concentrations and temperatures, as well as to sulfuric, acetic, formic, and phosphoric acids and other non-oxidizing media - making it the specialist choice for valves dedicated to hydrochloric acid and reducing-acid service. Because B-3 lacks chromium it must not be used where oxidizing contaminants are present, so it is reserved for well-characterized, consistently reducing streams rather than general-purpose duty.
Advantages in Valve Service
Universal Corrosion Resistance
The C-series advantage is breadth: a single C-276 valve can be installed in a line whose chemistry is not fully predictable, or which sees both oxidizing and reducing conditions during start-up, normal operation, and cleaning cycles, without the corrosion failures that would occur if a more specialized material were chosen. This eliminates the risk of selecting an alloy that is excellent for the design case but fails during an off-design excursion, which is a common cause of premature valve failure in chemical plants and the reason C-276 is often specified as a conservative default for severe and uncertain service.
Wet Chlorine and Chloride Resistance
Hastelloy C-grades retain integrity in wet chlorine, chlorine dioxide, and concentrated chloride salt solutions that rapidly destroy stainless steels through pitting, crevice corrosion, and chloride stress corrosion cracking. This makes Hastelloy the standard material for valves in chlor-alkali production, pulp and paper bleach plants, and seawater-derived process streams, where the combination of chloride content and oxidizing potential exceeds the capability of duplex and super-austenitic stainless steels. For broader media-based selection guidance, see the corrosive media valve selection reference.
Weldability and Fabrication
Unlike many high-performance alloys, the Hastelloy C-grades can be welded in the as-fabricated condition without mandatory post-weld heat treatment while retaining their corrosion resistance in the weld and heat-affected zone, because their chemistry resists the sensitization and intermetallic precipitation that degrade weld corrosion performance in lesser alloys. This supports their use as weld-overlay cladding on lower-cost carbon or stainless steel valve bodies - a cost-control strategy that places the expensive alloy only on the wetted surfaces while a cheaper backing material carries the pressure load.
Typical Applications in Valves
Chemical and Petrochemical Processing
Hastelloy C-276 and C-22 valves handle the most aggressive streams in chemical plants - hydrochloric acid, sulfuric acid, mixed-acid pickling liquors, chlorinated organics, and oxidizing salt solutions - where stainless steel and Inconel would suffer rapid pitting or general corrosion. Control valves, isolation valves, and sampling valves in reactor and distillation service are commonly specified in C-276 to guarantee both containment integrity and freedom from product contamination by corrosion debris.
Flue-Gas Desulfurization (FGD)
Wet FGD scrubbers in coal-fired power and waste-incineration plants produce a hot, acidic, chloride-laden slurry that is one of the most corrosive industrial environments, and Hastelloy C-276 or C-22 is the standard material for damper, isolation, and drain valves in the scrubber and absorber circuits. The combination of low pH, high chloride, and oxidizing conditions demands the C-series breadth of resistance, and the alloy's chloride SCC immunity prevents the cracking failures that limit stainless steel in this duty.
Pharmaceutical and Pulp Bleaching
In pharmaceutical and fine-chemical manufacturing, Hastelloy valves prevent metallic contamination of high-value products by resisting corrosion in aggressive synthesis and cleaning chemicals, supporting the purity standards required by GMP manufacturing. In pulp and paper bleach plants, C-276 valves withstand chlorine-dioxide and hypochlorite bleaching liquors that aggressively attack stainless steel, maintaining reliable shutoff over long service intervals.
Sour Oil and Gas Service
Hastelloy grades are used for valve trim and components in sour oil-and-gas production where high partial pressures of hydrogen sulfide combine with chlorides and carbon dioxide, conditions that require both sulfide stress cracking resistance and chloride pitting resistance. The detailed material qualification requirements for these environments, including NACE MR0175 limits, are covered in the material for H2S service reference, and the comparison with competing nickel alloys is addressed in the Inconel vs Monel reference.
Frequently Asked Questions
What is the difference between Hastelloy and Inconel?
Both are nickel superalloys, but they are optimized for different duties. Inconel (nickel-chromium) is selected primarily for high-temperature strength and oxidation resistance, while Hastelloy (nickel-molybdenum and nickel-chromium-molybdenum) is selected primarily for wet corrosion resistance in reducing acids such as hydrochloric and sulfuric acid. For severe chemical service a Hastelloy C-grade usually outperforms Inconel; for hot-gas and high-pressure-high-temperature mechanical service Inconel is usually preferred.
Is Hastelloy a stainless steel?
No. Hastelloy is a nickel-based alloy in which nickel is the majority element (typically 55-65%), whereas stainless steel is iron-based with nickel as a minor addition. The high nickel and molybdenum content is what gives Hastelloy its resistance to reducing acids and chloride stress corrosion cracking that ordinary 304 and 316 stainless steels cannot withstand.
Which ASTM specifications cover Hastelloy valve components?
Hastelloy C-276 (UNS N10276) is covered by ASTM B575 for plate and sheet, B574 for rod, and B622 for seamless pipe and tube; Hastelloy C-22 (UNS N06022) and B-3 (UNS N10675) are covered by the corresponding B-series specifications and ASTM B564 for forgings. Valve castings are typically specified to ASTM A494 equivalent grades (CW-12MW / CW-2M for C-type chemistry).
Why is Hastelloy so expensive?
Hastelloy contains 55-65% nickel plus 13-28% molybdenum and, in the C-grades, tungsten - all costly raw materials. It also work-hardens rapidly and is difficult to machine and weld, raising fabrication cost. As a result Hastelloy valves typically cost several times more than stainless steel equivalents, so they are specified only where the chemical environment would cause unacceptable corrosion of less capable alloys.
Conclusion
Hastelloy defines the upper boundary of wet-corrosion valve material performance: where stainless steel, duplex, and even Inconel are attacked by reducing acids, wet chlorine, or aggressive chloride solutions, the nickel-molybdenum and nickel-chromium-molybdenum Hastelloy grades provide reliable service. C-276 offers the broadest all-round resistance, C-22 adds oxidizing-acid capability, and B-3 specializes in hydrochloric acid. Matching the specific grade to the actual stream chemistry, rather than defaulting to the most expensive option, is the core of cost-effective severe-service valve selection - a process set out in full in the valve material selection guide.