Alloy 22 (commercially known as Hastelloy C-22 or Inconel 22, corresponding to UNS N06022 / W.Nr. 2.4602) is a fully austenitic nickel-chromium-molybdenum-tungsten alloy, renowned for its unmatched and comprehensive corrosion resistance. Through a precisely balanced content of chromium (providing resistance in oxidizing media) along with molybdenum and tungsten (providing resistance in reducing media), it delivers exceptional resistance to a wide range of aggressive environments, including oxidizing and reducing acids, halides, mixed acids, and seawater. Its outstanding resistance to pitting, crevice corrosion, and chloride-induced stress corrosion cracking (SCC) makes it the material of choice for the most demanding applications in chemical processing, pollution control, marine, and nuclear industries.
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Alloy 22 (Hastelloy C-22 / Inconel 22) – The All-Rounder Nickel-Chromium-Molybdenum-Tungsten Alloy
Alloy 22 (Hastelloy C-22 / Inconel 22) – The All-Rounder Nickel-Chromium-Molybdenum-Tungsten Alloy
Alloy 22 (UNS N06022), commonly known as Hastelloy C-22 or Inconel 22, is a high-quality nickel-chromium-molybdenum-tungsten superalloy. It is hailed as the ultimate "all-rounder" material for extreme environments, exhibiting excellent resistance to pitting corrosion, crevice corrosion, and stress corrosion cracking in a wide range of industrial applications.
What is C-22 material?
It is a nickel-chromium-molybdenum alloy. Hastelloy C-22 alloy (UNS N06022) is a well-known and reliable nickel-chromium-molybdenum alloy material. Its main characteristics are resistance to oxidizing and non-oxidizing chemical corrosion, and it effectively prevents pitting corrosion, crevice corrosion, and stress corrosion cracking.
1. Chemical composition of Hastelloy C-22 alloy (UNS N06022)
The chemical composition (weight percentage) of Alloy 22 is shown in the table below. Its precise elemental balance is the foundation of its excellent corrosion resistance.
| Element | Content Range (Min - Max, %) | Primary Function |
|---|---|---|
| Nickel (Ni) | Balance (approx. 56-63%) | Forms a stable austenitic matrix, providing the fundamental basis for overall corrosion resistance. |
| Chromium (Cr) | 20.0 - 22.5 | Key element; provides excellent resistance to oxidizing media (e.g., nitric acid, wet chlorine) and enhances high-temperature oxidation resistance. |
| Molybdenum (Mo) | 12.5 - 14.5 | Key element; works synergistically with tungsten to provide excellent resistance to reducing acids (e.g., sulfuric acid, hydrochloric acid) as well as pitting and crevice corrosion in chloride-containing media. |
| Tungsten (W) | 2.5 - 3.5 | Enhances the effect of molybdenum; further improves corrosion resistance in reducing environments and chloride-containing media, particularly pitting resistance. |
| Iron (Fe) | 2.0 - 6.0 | Controlled within a specific range to balance cost and performance. |
| Cobalt (Co) | ≤ 2.5 | Impurity element; strictly controlled. |
| Carbon (C) | ≤ 0.015 | Extremely low content; effectively prevents carbide precipitation at grain boundaries during welding or high-temperature service, avoiding intergranular corrosion. |
| Manganese (Mn) | ≤ 0.50 | Impurity element; improves hot workability. |
| Silicon (Si) | ≤ 0.08 | Extremely low content; reduces the risk of forming silicate scale in oxidizing media and minimizes hot embrittlement. |
| Vanadium (V) | ≤ 0.35 | Impurity element. |
| Phosphorus (P) | ≤ 0.02 | Harmful impurity; strictly controlled. |
| Sulfur (S) | ≤ 0.02 | Harmful impurity; strictly controlled. |
2. Hastelloy C-22 alloy Physical Properties
| Property | Typical Value / Range | Unit |
|---|---|---|
| Density | 8.69 – 8.90 | g/cm³ |
| Melting Range | 1325 – 1400 (typically 1350-1380) | °C |
| Thermal Conductivity (20°C) | 9.4 – 11.2 | W/(m·K) |
| Specific Heat Capacity (20°C) | 398 – 456 | J/(kg·K) |
| Mean Coefficient of Thermal Expansion (20-200°C) | 12.4 – 14.0 | ×10⁻⁶/°C |
| Modulus of Elasticity (room temperature) | 203 – 207 | GPa |
| Electrical Resistivity (room temperature) | approx. 1.21 | μΩ·m |
3. Hastelloy C-22 alloy Room Temperature Mechanical Properties (Solution Annealed, Typical Values)
| Property | Typical Range (Minimum) | Unit |
|---|---|---|
| Tensile Strength (Rm) | 690 – 950 (commonly ≥ 690) | MPa |
| 0.2% Yield Strength (Rp0.2) | 310 – 365 (commonly ≥ 310) | MPa |
| Elongation (A) | 45 – 65% (commonly ≥ 45%) | % |
| Hardness | approx. 89 – 93 HRB | - |
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4. Hastelloy C-22 alloy High Temperature Mechanical Properties
Alloy 22 maintains good strength at elevated temperatures, but its primary advantage lies in corrosion resistance rather than high-temperature strength. Long-term structural use is recommended below 400°C to avoid precipitation of detrimental phases. Short-term exposure can be at higher temperatures.
| Temperature (°C) | 0.2% Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|---|
| 20 | ≥ 310 | ≥ 690 | ≥ 45 |
| 100 | ≥ 270 | - | - |
| 200 | ≥ 225 | - | - |
| 300 | ≥ 195 | - | - |
| 400 | ≥ 175 | - | - |
5. Hastelloy C-22 alloy Key Corrosion Resistance Properties
The most prominent characteristic of Alloy 22 is its comprehensive and excellent corrosion resistance, performing exceptionally well in both oxidizing and reducing environments.
| Corrosive Media / Type | Description of Corrosion Resistance |
|---|---|
| Oxidizing Acids | Excellent corrosion resistance. Outstanding resistance to nitric acid (all concentrations up to boiling), wet chlorine gas, and mixed acids containing oxidizing chloride ions (e.g., aqua regia). |
| Reducing Acids | Good to excellent corrosion resistance. Performs well in sulfuric acid (wide concentration and temperature range) and hydrochloric acid (low to moderate concentrations and temperatures). |
| Localized Corrosion | Excellent resistance to pitting and crevice corrosion, especially in chloride-containing solutions. Very high pitting resistance equivalent number (PREN). |
| Stress Corrosion Cracking (SCC) | Highly resistant to chloride-induced stress corrosion cracking, superior to many other nickel-based alloys. |
| Mixed Acids | Particularly suitable for handling complex mixed acid environments such as nitric-hydrofluoric acid (pickling solutions). |
| Seawater & Halides | Excellent corrosion resistance in stagnant and flowing seawater and brine. |
| Organic Acids | Good corrosion resistance to organic acids such as formic acid and acetic acid. |
Limitations: Not recommended for extremely reducing environments such as oleum, hot concentrated phosphoric acid, or high-temperature, high-concentration hydrochloric acid. Long-term exposure above 400°C may lead to precipitation of detrimental phases, affecting corrosion resistance and ductility.
6. Hastelloy C-22 Welding and Heat Treatment
Heat Treatment (Solution Annealing):
Purpose: To obtain a uniform austenitic structure, dissolve any harmful precipitated phases, and achieve optimal corrosion resistance.
Temperature: 1105 – 1135°C (2021 – 2075°F)
Cooling: Rapid water quenching (rapid air cooling may be used for thin materials less than about 1.5 mm thick) to avoid the precipitation of harmful phases during slow cooling through the 400-980°C range.
Welding:
Solution Annealing: Hold at 1065 – 1120°C followed by water quenching to dissolve carbides and homogenize the microstructure.
Stress Relief Annealing: Hold at 590 – 650°C followed by air cooling, primarily used to reduce residual stress.
Weldability: Good. All conventional welding methods can be used, such as GTAW/TIG, GMAW/MIG, SMAW, and SAW.
Filler Metal Selection: Matching filler metals are recommended, such as ERNiCrMo-10 (AWS A5.14) wire.
Process Control: Use low heat input, low amperage, and a no-weave or minimal-weave technique. Interpass temperature should be strictly controlled below 100°C. Back-and-face argon shielding is recommended to prevent oxidation.
7. hastelloy C22 Application Areas
Alloy C22 is widely used in extremely corrosive environments where high material reliability is required:
| Industry | Applications |
|---|---|
| Chemical & Petrochemical Processing | Reactors, heat exchangers, scrubbers, piping systems, agitator shafts for handling sulfuric acid, hydrochloric acid, nitric acid, mixed acids, and chloride-containing media. |
| Pollution Control | Scrubbers, dampers, flue ducts, and chimney liners in flue gas desulfurization (FGD) systems, handling high-temperature, humid, chlorine-containing corrosive gases. |
| Pharmaceutical & Fine Chemicals | Process equipment and lined reactors requiring high purity and resistance to aggressive cleaning agents such as nitric-hydrofluoric acid mixtures. |
| Nuclear Industry | Nuclear waste treatment containers, nuclear fuel reprocessing equipment. |
| Marine & Desalination | Seawater cooling systems, desalination plant components, offshore platform equipment. |
| Pulp & Paper | Digesters, bleaching plant equipment in contact with chlorine-containing compounds. |
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FAQ
Q：Are Alloy 22, Hastelloy C-22, and Inconel 22 the same?
A：Yes, they generally refer to the same material (UNS N06022). Hastelloy C-22 is a registered trademark of Haynes International, and Inconel 22 is a registered trademark of Special Metals Corporation.
Q：What is its maximum recommended operating temperature?
A：For structural components intended for continuous long-term use, a maximum recommended operating temperature is 400°C to avoid the precipitation of harmful phases. Higher short-term exposure temperatures are permissible, but should be evaluated based on the specific application.
Q：Is its corrosion resistance better than C-276?
A：Yes. Alloy 22, through its optimized chromium, molybdenum, and tungsten ratio, generally outperforms C-276 in resistance to pitting, crevice corrosion, and stress corrosion cracking, especially in oxidizing and mixed acid environments.
Q：Is post-weld heat treatment required?
A：For most applications, no. Its extremely low carbon content makes it insensitive to post-weld sensitization (intergranular corrosion). Post-weld solution treatment is recommended only when it is necessary to eliminate high residual stress or treat areas that have become sensitized.
Q：What are the international standards for it?
A：Common standards include: UNS N06022, W.Nr. 2.4602, ASTM B574 (bars), ASTM B575 (plates/strips), ASTM B622 (seamless tubes), ASTM B619/B626 (welded tubes), ASME SB-574/SB-575, etc.
