The All-Rounder Superalloy — Balanced Oxidizing & Reducing Resistance Across Wet Chlorine, Nitric Acid & Mixed Acids  |  ASTM B575  |  Special Alloys

C-22: The Versatile All-Rounder of the C-Family

Hastelloy C-22 is the most widely specified corrosion-resistant nickel alloy for service where both oxidizing and reducing species are present. Introduced by Haynes International in 1985, C-22 was engineered as a metallurgically more stable successor to C-276 and a more chloride-resistant alternative to C-4. By carefully balancing chromium (oxidizing protection), molybdenum (reducing protection), and tungsten (synergistic reducing/crevice protection), C-22 achieves genuine versatility — the ability to handle alternating oxidizing and reducing process streams in a single material.

The 20–22.5% chromium — substantially higher than C-276 (14.5–16.5%) and C-4 — is the key to C-22’s signature performance. Higher Cr delivers superior resistance to oxidizing acids (nitric acid, wet chlorine, ferric and cupric salts) and, critically, the highest critical crevice corrosion temperature (CCT) of any C-family alloy. The molybdenum-plus-tungsten combination provides reducing-acid and chloride protection, while the ultra-low carbon (0.015% max) and silicon (0.08% max) prevent harmful grain-boundary precipitates in the weld heat-affected zone.

For flue gas desulfurization (FGD) systems, pharmaceutical API reactors, fine chemical plants, and any facility handling complex or variable chemistries, C-22 is the proven, cost-effective specification. It is the material of choice whenever oxidizing contaminants (dissolved oxygen, Fe³⁺, Cu²⁺, wet Cl&sub2;) are present alongside halides — environments where C-276 and C-4 fall short and where C-2000 would be over-specified and over-priced.

Five Defining Advantages of Hastelloy C-22

Chemical Composition — ASTM B575 UNS N06022

The balanced Cr + Mo + W chemistry defines C-22’s versatility. Every element is precisely controlled to maximize broad-spectrum corrosion resistance while maintaining weldability and thermal stability:

★ Starred elements define C-22’s unique corrosion performance — Cr, Mo, and W are the three pillars of its oxidizing/reducing versatility

Mechanical Properties — Room Temperature (Solution-Annealed)

C-22 is a solid-solution strengthened alloy. There is no precipitation hardening mechanism. Properties are stable after solution annealing and do not degrade with moderate temperature exposure. The combination of high Cr, Mo, and W provides solid-solution strengthening above a pure nickel matrix, yielding a tensile strength of ~690 MPa minimum with excellent ductility (≥45% elongation).

Elastic Constants & Physical Constants (Solution-Annealed at 20°C)

Physical Properties — UNS N06022

C-22’s physical properties are comparable to other C-family alloys. The density (8.69 g/cm³) reflects the high chromium and balanced tungsten content. Thermal conductivity is moderate for a nickel alloy, and the alloy is non-magnetic in all conditions.

Heat Treatment Procedures

C-22 requires a solution anneal to dissolve any intermetallic phases formed during hot working and restore uniform corrosion resistance. The anneal window is well established and less sensitive than C-276’s, but rapid quenching remains essential to prevent precipitation during slow cooling:

Corrosion Resistance — The Versatile All-Rounder

C-22’s defining advantage is balanced versatility — it handles oxidizing and reducing media in a single material better than any other widely available C-family alloy. The high Cr (oxidizing protection) combined with Mo + W (reducing/chloride protection) creates a corrosion resistance envelope that is both broad and commercially proven across decades of FGD, pharmaceutical, and chemical-processing service.

Oxidizing Acids ★ — DEFINING STRENGTH

This is C-22’s signature capability. The 20–22.5% chromium — the highest among commercial C-family alloys except C-2000 — provides superior resistance to nitric acid at all concentrations up to boiling, wet chlorine gas, and mixed acids containing oxidizing chloride salts (Fe³⁺, Cu²⁺, hypochlorite). In these environments C-22 outperforms C-276 substantially and is the material of choice for oxidizing-dominated process streams. For pure HNO&sub3; service, C-22 offers best-in-class C-family performance second only to C-2000.

Hydrochloric Acid (HCl)

Good resistance to HCl at low-to-moderate concentrations and temperatures. The Mo + W content provides reducing-acid protection comparable to C-276. For pure HCl service at higher temperatures, Hastelloy B-3 is preferred, but C-22 provides adequate resistance with the added benefit of oxidizing acid protection that B-3 lacks entirely.

Sulfuric Acid (H&sub2;SO&sub4;)

Good resistance across a wide concentration and temperature range, with strong performance in reducing (air-free) conditions. However, in intermediate-concentration sulfuric acid (especially 20–60% at boiling), C-2000’s copper addition provides dramatically better passivation. For pure or intermediate H&sub2;SO&sub4; service, C-2000 is the superior choice; for oxidizing-lean sulfuric streams, C-22 remains a strong, cost-effective option.

Mixed Acids ★ — BEST-IN-CLASS FOR OXIDIZING MIXTURES

C-22 excels in mixed oxidizing-chloride environments — HNO&sub3;/HCl, HNO&sub3;/HF pickling, and aqua regia-type mixtures — that attack C-276 (insufficient oxidizing resistance). In chemical plants running diverse batch chemistries with oxidizing contaminants, C-22 is the safest single-material specification for mixed-acid reactors, heat exchangers, and piping.

Pitting & Crevice Corrosion

Excellent localized corrosion resistance. The critical crevice temperature (CCT) is the highest of any C-family alloy — a direct result of the balanced Cr + Mo + W chemistry. PREN ≈ 65 and CPT in ASTM G48 Method C exceeds 95°C. The high Cr and Mo + W combination provides exceptional resistance in seawater, brine, FGD scrubber, and bleach-plant environments.

Stress-Corrosion Cracking (SCC)

Immune to chloride SCC at all temperatures — a fundamental advantage over austenitic stainless steels (which crack above ~60°C in chloride service). In boiling 45% MgCl&sub2; (ASTM G36) testing, C-22 shows no cracking after 1008 hours. The austenitic Ni matrix with high alloy content provides complete chloride SCC immunity.

Intergranular Corrosion (IGC)

Passes ASTM G28 Method A and B in the solution-annealed condition. As-welded: passes for most common environments thanks to ultra-low C (0.015% max) and Si (0.08% max). For the most aggressive environments requiring guaranteed as-welded IGC resistance, a full post-weld solution anneal is recommended.

Corrosion Rate Comparison — C-22 vs C-2000 vs C-276 (mm/yr)

Green = excellent; Red = unacceptable. Data from Haynes International published corrosion tables and independent lab reports. Values are typical — actual performance depends on specific process chemistry, aeration, velocity, and contaminants.

Hastelloy Grade Selection Guide — C-Family Comparison

C-22 is the versatile all-around choice when oxidizing resistance is paramount and cost matters. It earns 5-star ratings in two of five selection criteria and 4-star ratings in the remaining three, at a moderate premium ($$$$-level) below C-2000:

Key Applications

C-22 is the premium, proven choice for environments where oxidizing species coexist with halides — flue gas desulfurization, pharmaceutical synthesis, and complex chemical processing. Its combination of oxidizing resistance, localized-corrosion resistance, and excellent as-welded performance makes it irreplaceable in these sectors:

Applicable Standards & Specifications

Available Product Forms from HT PIPE

We supply C-22 in solution-annealed condition with EN 10204 3.1 certification on every shipment. ASTM G28 IGC testing per heat is available on specification. C-22 is a premium-priced alloy and lead times vary by mill — contact us for current availability and project quotations.

HT PIPE Project Experience — C-22

For additional project references matching your specific process chemistry, contact our technical sales team. We can provide anonymized case studies for FGD, pharmaceutical, fine chemical, pickling, and nuclear reprocessing applications.

Frequently Asked Questions — Hastelloy C-22