Two Grades of One Alloy Family
Hastelloy C22 and Hastelloy C276 belong to the same nickel-chromium-molybdenum family of corrosion-resistant alloys and are both produced as strip, sheet and plate to ASTM B575/ASME SB575. The strip form is used for bellows, gaskets, springs, heat exchanger fins, expansion joints and other fabricated components that require close tolerance, thin-gauge material. Despite their similarity, the two grades are balanced differently and are not freely interchangeable.
Composition: Where the Balance Differs
Hastelloy C22 (UNS N06022) contains about 20.0-22.5% chromium, 12.5-14.5% molybdenum and 2.5-3.5% tungsten, with the balance nickel. Hastelloy C276 (UNS N10276) contains 14.5-16.5% chromium, 15.0-17.0% molybdenum and 3.0-4.5% tungsten. The key difference is chromium versus molybdenum: C22 is richer in chromium, which favors resistance to oxidizing media, while C276 is richer in molybdenum and tungsten, which favors resistance to reducing media and localized corrosion in chlorides.
Corrosion Resistance Comparison
In oxidizing environments, such as those containing ferric or cupric chlorides, chlorine, nitric acid or other oxidizing acids, the higher chromium content of C22 gives it a measurable advantage. In reducing environments, such as hydrochloric and sulfuric acid at moderate temperatures, C276 performs better because of its higher molybdenum and tungsten content. Both grades resist pitting, crevice corrosion and stress corrosion cracking far better than conventional stainless steels, but the optimum choice always depends on the exact chemical composition of the process stream, its temperature and the presence of oxidizing or reducing species.
Weldability and Fabrication
Both C22 and C276 are designed for easy welding without mandatory post-weld heat treatment. Matching nickel-based filler metals are used, and heat tint should be removed after welding to restore corrosion resistance. C22 is often described as having slightly better weldability because its composition is less sensitive to segregation in the weld metal, which is why it is frequently preferred for welded fabrications in mixed-acid service. In strip form, both grades cold form well, though their work-hardening rate must be considered in rolling and forming operations.
Physical and Mechanical Notes
Both alloys are supplied in the solution-annealed condition with the mechanical properties required by ASTM B575. Density is about 8.69 g/cm³ for C22 and 8.89 g/cm³ for C276. Designers should verify tensile, yield and elongation values from the mill certificate for each heat, since specification minimums govern acceptance rather than typical values. For pressure equipment, allowable stresses must be taken from the governing code, such as ASME Section II Part D.
Selection Guidance
Choose C22 strip when the service is predominantly oxidizing, when mixed acids and strong oxidizers are present, or when the fabrication route involves extensive welding and you want maximum weld-metal integrity. Choose C276 strip when the service is predominantly reducing, when hydrochloric or sulfuric acid dominates, or when the project specification already calls for UNS N10276. In borderline cases, corrosion testing under actual service conditions is the most reliable way to decide.
FAQ
Q: What is the difference between C22 and C276?
A: C22 has higher chromium (about 20.0-22.5%) for better resistance to oxidizing media, while C276 has higher molybdenum (15.0-17.0%) and tungsten for better resistance to reducing media and pitting.
Q: Is Alloy C276 a Hastelloy?
A: Yes. Alloy C276 is one of the most widely used grades in the Hastelloy family of nickel-based superalloys, designated UNS N10276.
Q: Is Hastelloy the same as stainless steel?
A: No. Hastelloy grades are nickel-based alloys containing roughly 50% or more nickel plus molybdenum and chromium, whereas stainless steel is iron-based. Hastelloy provides much higher corrosion and heat resistance at a much higher cost.
Q: Which standard covers C22 and C276 strip?
A: Both are covered by ASTM B575/ASME SB575 for nickel alloy plate, sheet and strip; UNS N06022 for C22 and UNS N10276 for C276.
Q: Can C22 replace C276 in existing equipment?
A: Not automatically. The two grades are balanced for different media; substitution requires evaluation of the service environment and may require re-qualification under the governing code or project specification.
Q: Which alloy has better weldability?
A: Both weld well, but C22 is generally regarded as slightly more tolerant during welding because its higher chromium content reduces the risk of weld-metal segregation in aggressive service.
