What Is C-22 Alloy Plate?
C-22 is a low-carbon nickel-chromium-molybdenum alloy, designated UNS N06022 and normally supplied as plate, sheet and strip under ASTM B575 / ASME SB575. Its high chromium content of 20.0–22.5 % gives strong resistance to oxidizing media, while molybdenum at 12.5–14.5 % and tungsten at 2.5–3.5 % provide protection in reducing media. That balance lets the plate survive mixed-acid and chloride-bearing streams that quickly attack 316L or duplex stainless steel.
The alloy is not precipitation hardenable, so it keeps its corrosion resistance in the as-welded condition. Plates are delivered in the solution-annealed state with a uniform austenitic structure and low residual stress, which keeps forming and welding predictable. Typical service environments include wet chlorine, nitric acid, oxidizing acids contaminated with chloride ions, hypochlorite solutions and sulfur dioxide cooling systems.
Chemical Composition of C-22 Alloy (UNS N06022)
| Element | Composition, % | Element | Composition, % |
|---|---|---|---|
| Ni | Remainder | Cr | 20.0–22.5 |
| Mo | 12.5–14.5 | W | 2.5–3.5 |
| Fe | 2.0–6.0 | Co | 2.5 max |
| Mn | 0.50 max | V | 0.35 max |
| Si | 0.08 max | C | 0.015 max |
| P | 0.02 max | S | 0.02 max |
Mechanical and Physical Properties
| Property | Value | Condition |
|---|---|---|
| Tensile strength | 100 ksi (approx. 690 MPa) | Solution annealed plate |
| 0.2 % yield strength | 45 ksi (approx. 310 MPa) | Solution annealed plate |
| Elongation | 45 % | 2 in / 50 mm gauge |
| Physical property | Value | Temperature |
|---|---|---|
| Density | 8.69 g/cm³ | 24 °C |
| Specific heat | 0.093 kcal/kg·°C | 52 °C |
| Melting range | 1357–1399 °C | – |
| Modulus of elasticity | 205.5 kN/mm² | Room temperature |
| Electrical resistivity | 113.7 µΩ·cm | 24 °C |
| Coefficient of expansion | 12.4 µm/m·°C | 24–93 °C |
| Thermal conductivity | 10.1 W/m·K | 48 °C |
Plate Size Range and Supply Condition
Thickness: 0.3 mm to 200 mm, with intermediate gauges produced to order.
Width: 40–600 mm slit coil width, plus 1000 / 1219 / 1500 / 1800 / 2000 / 2500 / 3000 / 3500 mm plates.
Length: 2000 / 2438 / 3000 / 5800 / 6000 / 12000 mm, cut to drawing when required.
Surface finish: BA, 2B, No.1, No.4, 4K, HL, 8K, or as-rolled and pickled.
Production route: hot rolled or cold rolled, followed by solution annealing and pickling.
Order standards: ASTM / ASME, JIS, GB and DIN / EN equivalents.
Inspection: ISO, SGS or BV third-party certificates issued against the mill test report.
Corrosion Performance and Typical Applications
Because chromium, molybdenum and tungsten work together, C-22 plate resists both uniform and localized attack. It is one of the few nickel alloys that tolerates wet chlorine gas and chlorine dioxide bleaching liquors, and it also withstands pitting and crevice corrosion in seawater and chloride salt solutions.
Chemical reactors, columns and storage vessels handling mixed acids.
Plate heat exchangers and shell-and-tube exchangers in pickling lines.
Flue gas desulfurization absorbers, ductwork and SO₂ cooling towers.
Phosphoric acid and sulfonation process equipment.
Chlorination and hypochlorite generation systems.
Pulp and paper bleaching stages, plus selective leaching circuits.
Valve bodies, clad components and pump internals exposed to chlorides.
Fabrication, Welding and Heat Treatment
C-22 plate is readily cold formed and machined with carbide tooling at low cutting speeds. Hot forming is carried out between roughly 1000 °C and 1200 °C, followed by a full solution anneal at about 1120 – 1180 °C and rapid water quenching to restore corrosion resistance. Welding uses matching nickel-base filler and controlled heat input; interpass temperature should stay low and the surface oxide must be removed after welding to avoid preferential attack. Post-weld solution annealing is not normally required for thin sections, but it is recommended for heavy-wall fabrications in severe service.
Frequently Asked Questions
Q: What does the designation UNS N06022 mean?
UNS N06022 is the unified numbering system identifier for the C-22 nickel-chromium-molybdenum alloy, and it is the code used on mill certificates in place of trade names.
Q: Which standard covers C-22 plate?
ASTM B575 and ASME SB575 specify the chemical composition and mechanical properties of low-carbon nickel-chromium-molybdenum plate, sheet and strip, including UNS N06022.
Q: How thick can C-22 plate be supplied?
Standard production covers 0.3 mm to 200 mm; thinner gauges are cold rolled and thicker sections are hot rolled, then solution annealed.
Q: Is C-22 plate resistant to seawater?
Yes. The molybdenum and tungsten additions give good pitting and crevice corrosion resistance in chloride-bearing water, which is why the alloy is used in marine and desalination duty.
Q: Can C-22 plate be welded without losing corrosion resistance?
The alloy remains resistant in the as-welded condition because it is not precipitation hardenable; clean joint preparation and removal of welding oxides are the critical controls.
Q: What is the difference between C-22 and C-276 plate?
Both are nickel-chromium-molybdenum alloys with similar corrosion behaviour, but C-22 contains more chromium and tungsten, giving it better resistance to oxidizing media, while C-276 is often preferred for the most severe reducing acid service.
