Alloy C-22, designated UNS N06022 and W.Nr. 2.4602, is a nickel-chromium-molybdenum alloy that combines the oxidising-acid resistance of high-chromium nickel grades with the reducing-acid resistance of high-molybdenum grades. It is one of the most widely specified materials for chemical process equipment handling mixed, contaminated or strongly oxidising streams.
What Is Alloy C-22 (UNS N06022)?
Alloy C-22 contains nickel as the balance at about 56% minimum, 20.0-22.5% chromium, 12.5-14.5% molybdenum and 2.5-3.5% tungsten, with iron held to 2.0-6.0% and cobalt to 2.5% maximum. The tungsten addition, together with tight limits on carbon and silicon, produces a combination of uniform and localised corrosion resistance that exceeds that of earlier nickel-chromium-molybdenum grades in oxidising media.
The alloy resists pitting, crevice corrosion and stress corrosion cracking, and it performs well in wet chlorine, nitric acid and mixed acids that contain both oxidising species and chloride ions. It also resists grain-boundary precipitation in the weld heat-affected zone, which means welded vessels and piping can be placed in service without a post-weld solution anneal.
Typical uses include chemical reactors, heat exchangers, scrubbers, flue-gas desulphurisation ducting, pickling equipment, pulp and paper bleaching circuits, and sour-service oil and gas components. Because the alloy is ductile and readily formed and welded, it is supplied as sheet, plate, strip, bar, seamless tube and welded tube.
Chemical Composition and Physical Properties
The composition limits below follow the nickel-chromium-molybdenum alloy specifications for plate, sheet and strip, expressed in mass percent.
| Element | Limit |
|---|---|
| Nickel | balance, 56.0 min |
| Chromium | 20.0-22.5 |
| Molybdenum | 12.5-14.5 |
| Tungsten | 2.5-3.5 |
| Iron | 2.0-6.0 |
| Cobalt | 2.5 max |
| Carbon | 0.015 max |
| Silicon | 0.08 max |
| Manganese | 0.50 max |
| Vanadium | 0.35 max |
| Phosphorus and sulphur | 0.02 max each |
| Property | Typical value |
|---|---|
| Density | 8.69 g/cm³ |
| Melting range | 1357-1399 °C |
| Thermal conductivity at 25 °C | about 10 W/m·K |
| Modulus of elasticity | about 205 GPa |
| Tensile strength, annealed sheet | 690 MPa min |
| 0.2% yield strength, annealed sheet | 310 MPa min |
| Elongation, annealed sheet | 45% min |
Corrosion Resistance in Oxidising and Reducing Media
The practical strength of alloy C-22 is that it does not force a choice between oxidising and reducing duty. Common media and the expected behaviour are summarised below.
Wet chlorine and chlorine dioxide: excellent resistance, including moist gas streams where many nickel alloys pit.
Nitric acid and mixed acids containing oxidising species plus chlorides: excellent resistance across a wide concentration range.
Ferric chloride, cupric chloride and other strong oxidants: high resistance to pitting and crevice attack.
Sulphuric and phosphoric acid, including contaminated grades: good performance in both dilute oxidising and reducing conditions.
Formic acid, acetic acid and acetic anhydride: good resistance in the concentrations and temperatures typical of organic synthesis.
Seawater and chloride brines: high resistance to pitting and crevice corrosion, with no chloride stress corrosion cracking in normal service.
Hydrochloric acid: suitable only in dilute, low-temperature conditions; concentrated or hot hydrochloric acid requires a high-molybdenum nickel-molybdenum grade.
Microstructural Stability and Welded Fabrication
Alloy C-22 is resistant to the formation of continuous grain-boundary precipitates in the heat-affected zone, so welded equipment retains its corrosion resistance as fabricated. Gas tungsten arc, gas metal arc, shielded metal arc and submerged arc processes are all used, with matching nickel-chromium-molybdenum filler classified in the ERNiCrMo family.
Workshop practice matters as much as filler selection. Joints should be clean and free from oil, marking paint and iron contamination; interpass temperature should be controlled; and completed welds should be ground, brushed with stainless tools and pickled or electropolished to restore the passive film. Heat treatment is not normally required, and prolonged exposure to the 600-900 °C range should be avoided because it can precipitate intermetallic phases.
Standard Family and Equivalent Grades
Supply is governed by a family of product-form specifications, which should always be quoted on the purchase order together with the required condition and test schedule.
| Product form | Specification |
|---|---|
| Plate, sheet and strip | ASTM B575 / ASME SB575 |
| Rod, bar and forging stock | ASTM B462 / ASME SB462 |
| Seamless pipe and tube | ASTM B622 / ASME SB622 |
| Welded pipe | ASTM B619 / ASME SB619 |
| Welded tube | ASTM B626 / ASME SB626 |
| Sour service qualification | ISO 15156-3 / NACE MR0175 |
| European designation | NiCr21Mo14W, W.Nr. 2.4602 |
Within the same alloy family, the C-series has evolved from the original C grade through C-276 (UNS N10276) and C-4 (UNS N06455) to C-22 (UNS N06022) and C-2000 (UNS N06200). C-22 is generally preferred over C-276 and C-4 in oxidising media and offers better localised corrosion resistance; C-2000 extends the range further into reducing acid duty at higher cost. The B-series, represented by UNS N10665 and UNS N10675, remains the choice for hot concentrated hydrochloric acid, while the G-series, including UNS N06030 and UNS N06035, targets phosphoric acid and strongly oxidising mixed acids with high chromium content.
Frequently Asked Questions
Q: What is alloy C-22 used for?
It is used for chemical reactors, heat exchangers, scrubbers, FGD ducting, pickling lines, bleaching circuits and other process equipment exposed to chlorides and mixed oxidising acids.
Q: How does alloy C-22 differ from C-276?
C-22 has higher chromium and a tungsten addition, giving better resistance to oxidising media and stronger localised corrosion performance than C-276 in most process streams.
Q: Does welded C-22 require post-weld heat treatment?
No. The alloy resists grain-boundary precipitation in the heat-affected zone, so normally only cleaning, pickling or electropolishing is needed after welding.
Q: Is alloy C-22 resistant to seawater?
Yes. It offers high resistance to pitting, crevice corrosion and chloride stress corrosion cracking in seawater and concentrated brines.
Q: Can alloy C-22 handle hydrochloric acid?
Only dilute, low-temperature hydrochloric acid. Hot or concentrated hydrochloric acid calls for a high-molybdenum nickel-molybdenum grade from the B-series.
Q: Which standard covers C-22 sheet and plate?
ASTM B575 with the equivalent ASME SB575 covers plate, sheet and strip; bar and forging stock are covered by ASTM B462.
