Alloy C22
Alloy C22 (UNS N06022) is a nickel-chromium-molybdenum-tungsten alloy with higher chromium content (20–22.5%) than C276. It was developed to extend corrosion resistance into more strongly oxidizing environments. It performs well in mixed acid conditions and in environments that shift between oxidizing and reducing.
1. Chemical Composition
The table below shows the elemental composition for Alloy C22. The higher chromium level and the precise molybdenum-tungsten balance are what separate its performance from other C-family alloys.
| Grade | C | Mn | Si | S | Cr | Mo | Fe | W | Co | Other |
| N06022 (C-22) | 0.015 | 0.50 | 0.08 | 0.02 | 20.0–22.5 | 12.5–14.5 | 2.0–6.0 | 2.5–3.5 | 2.5 | V 0.35 |
2. Mechanical Properties
The following minimum values apply to Alloy C22 in the solution-annealed condition for seamless pipe and tube per ASTM B622.
| Grade (UNS) | Alloy Family | Tensile Min, ksi (MPa) | Yield Min, ksi (MPa) | Elong. Min, % | Fiber Stress, psi |
| N06022 | Low C Ni-Mo-Cr (C-22) | 100 (690) | 45 (310) | 45 | 25,000 |
3. Equivalent Grade
The table below maps Alloy C22 to international equivalent designations for procurement and documentation across different national standards.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| C22 | N06022 | H06022 | NS3308 | - | - | 2.4602 | - |
4. Key Technical Advantages
- Better Performance in Oxidizing Acid Environments: The 20–22.5% Cr content, well above C276's 14.5–16.5% Cr, gives C22 noticeably better resistance to oxidizing acids including nitric acid, chromic acid, and hot concentrated sulfuric acid. This makes C22 the preferred alloy in mixed-acid or oxidizing-to-reducing transition environments.
- Strong Resistance to Wet Chlorine and Hypochlorite: C22 outperforms C276 in chlorine-saturated water and hypochlorite solutions. The higher chromium promotes a more stable passive film that resists breakdown from oxidizing halogen species.
- Resistance to Pitting and Crevice Corrosion: The combination of 12.5–14.5% Mo, 2.5–3.5% W, and 22% Cr produces a PREN above 55 (in the solid solution condition), giving the alloy pitting resistance even in highly concentrated chloride solutions at elevated temperatures.
- Technical Note: C22 and C276 are frequently compared in process engineering. C22 is specified when the process stream contains oxidizing species (ferric chloride, cupric chloride, dissolved oxygen) or when the service environment alternates between oxidizing and reducing conditions. For purely reducing acid service without oxidizing species, C276 may be equally effective and is more widely stocked.
5. Common Manufacturing Standards
ASTM B622: Standard specification for seamless nickel and nickel-cobalt alloy pipe and tube. This is the primary product standard for Alloy C22 (UNS N06022) in tubular form for corrosive process service.
ASTM B619: Standard specification for welded nickel and nickel-cobalt alloy pipe, applicable to C22 in welded pipe form where seamless production is not needed.
ASTM B626: Standard specification for welded nickel and nickel-cobalt alloy tube, covering C22 in welded tube form for heat exchanger and condenser applications.
DIN EN ISO 6207 / DIN 17752: European standards for nickel alloy products. C22 is designated 2.4602 under EN material numbering.
JIS H4552: Japanese Industrial Standard for nickel and nickel alloy tubes, covering NW6022 designation for Alloy C22.
Manufacturing Standards Comparison Table:
| Standard | GB | EN/DIN | JIS | GOST |
| ASTM B622 (Seamless Pipe/Tube) | GB/T 15007 | DIN 2.4602 | JIS H4552 (NW6022) | - |
| ASTM B619 (Welded Pipe) | - | DIN 2.4602 | - | - |
6. Primary Applications
- Nitric and Sulfuric Acid Mixed Streams: Chemical plant piping and heat exchanger tubes in nitration processes where the fluid may contain concentrated nitric acid with sulfuric acid. C22 significantly outperforms C276 in this mixed oxidizing environment.
- Chlorine and Chlorine Dioxide Service: Bleach plant equipment in pulp and paper manufacturing, including chlorine dioxide reactors, piping, and heat exchangers. Hypochlorite and dissolved chlorine at elevated temperatures push material requirements beyond C276.
- Waste Treatment and Incineration: Scrubber components, condensers, and spray headers handling strongly oxidizing wet scrubber liquors containing ferric chloride and high dissolved oxygen in industrial waste treatment plants.
- Pharmaceutical Multipurpose Reactors: Pharmaceutical and specialty chemical reactors that run campaigns across different chemistries, including concentrated nitric acid cleaning cycles. A single alloy must cover the full chemistry range of the facility.
- Hydrometallurgy: Leach reactors and SX-EW piping in copper and precious metal hydrometallurgical plants where hot oxidizing acidic lixiviant solutions containing ferric iron and chlorides exceed the capability of C276.