Alloy C-22, UNS N06022, is frequently specified for the most aggressive duties in chemical processing: chlorinated and oxidising streams that also contain chlorides, where most corrosion-resistant alloys either pit or suffer stress corrosion cracking. This review explains where that resistance comes from, how it behaves in welded construction and where the practical limits lie.
Why Alloy C-22 Resists Aggressive Media
Three mechanisms work together. Chromium at 20.0-22.5% forms a thin, self-repairing passive oxide film that controls uniform corrosion in oxidising environments. Molybdenum at 12.5-14.5% and tungsten at 2.5-3.5% extend that protection into reducing conditions and strongly improve resistance to pitting and crevice attack. Finally, very low carbon and silicon limits prevent the continuous grain-boundary precipitation that would otherwise destroy corrosion resistance in the heat-affected zone of a weld.
The nickel-rich matrix, roughly 56% minimum, keeps the alloy ductile and gives it the austenitic structure needed to resist chloride stress corrosion cracking. The result is a material that tolerates the swings between oxidising and reducing conditions found in real process streams, where a single stream may contain oxidising acids, chlorides and organic species at the same time.
Composition, Physical Data and Localised Corrosion Resistance
Composition limits are set out below in mass percent, followed by the physical and mechanical data used in design calculations.
| 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 |
| 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 plate | 690 MPa min |
| 0.2% yield strength, annealed plate | 310 MPa min |
| Elongation, annealed plate | 45% min |
Localised corrosion resistance can be ranked with the pitting resistance equivalent number, calculated as Cr plus 3.3 times the sum of Mo and half of the W content. Using mid-range composition values of 21.25% chromium, 13.5% molybdenum and 3.0% tungsten, the number comes to approximately 71, which is why the alloy resists pitting in chloride media well beyond the capability of conventional stainless steels.
Behaviour in Oxidising, Reducing and Mixed Acids
Performance depends on the balance between oxidising and reducing species in the stream, and the table below summarises the practical picture.
| Medium | Expected behaviour |
|---|---|
| Wet chlorine, chlorine dioxide and hypochlorite | Excellent; suited to bleach and chlorine handling equipment |
| Nitric acid and mixed oxidising acids with chlorides | Excellent across normal process concentrations |
| Ferric chloride and cupric chloride solutions | Excellent; high resistance to pitting and crevice attack |
| Sulphuric and phosphoric acid, including contaminated grades | Good in both dilute oxidising and mildly reducing conditions |
| Formic acid, acetic acid and acetic anhydride | Good at the concentrations and temperatures used in organic synthesis |
| Seawater and concentrated brines | Excellent; no chloride stress corrosion cracking in normal service |
| Hot concentrated hydrochloric acid | Not recommended; a high-molybdenum nickel-molybdenum grade performs better |
Welded Structures and Grain-Boundary Stability
Alloy C-22 was developed partly to remove the post-weld heat treatment that earlier nickel-chromium-molybdenum grades required. Because carbon and silicon are held at very low levels, carbide and intermetallic precipitation at grain boundaries is slow, and welded joints in the as-welded condition retain corrosion resistance close to that of the base metal.
Fabrication control still matters. Joints must be free from oil, grease, marking paint and embedded iron; stainless steel brushes and dedicated grinding wheels should be used; interpass temperature should be kept low; and finished welds should be cleaned and pickled or electropolished to restore the passive film. Prolonged exposure to the 600-900 °C range should be avoided, because it can precipitate intermetallic phases that reduce toughness and corrosion resistance in the affected zone.
Selection Pitfalls, Standards and Quality Verification
Several mistakes recur when alloy C-22 is specified or purchased, and each of them can be avoided with a clear purchase specification.
Assuming that more molybdenum is always better: very high molybdenum grades lose resistance in oxidising media, so C-22 is a better balance for mixed streams than a nickel-molybdenum grade.
Ignoring mill condition: the alloy must be supplied in the correct solution-annealed and quenched condition, because an incorrect cooling practice damages localised corrosion resistance.
Ignoring the medium: where both strongly reducing and strongly oxidising extremes occur, a later C-type grade with higher overall alloying may be justified.
Neglecting fabrication quality: iron contamination, weld oxides and poor pickling cause more field failures than the base metal itself.
| 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 and tube | ASTM B619 and B626 / ASME SB619 and SB626 |
| Sour service qualification | ISO 15156-3 / NACE MR0175 |
| European designation | NiCr21Mo14W, W.Nr. 2.4602 |
Verification should combine a mill certificate with the specified heat treatment condition, positive material identification on delivery, dimensional and surface inspection, and, for critical equipment, an intergranular corrosion test such as the ferric sulphate test commonly applied to high-molybdenum nickel alloys.
Frequently Asked Questions
Q: What makes alloy C-22 resistant to pitting and crevice corrosion?
High chromium combined with 12.5-14.5% molybdenum and 2.5-3.5% tungsten, plus very low carbon and silicon, produces a stable passive film and resists grain-boundary precipitation.
Q: Does alloy C-22 suffer stress corrosion cracking in chlorides?
No, it is highly resistant to chloride stress corrosion cracking in normal service, which is one reason it is used in seawater and brine systems.
Q: Is welded C-22 as corrosion resistant as the base metal?
Yes, in the as-welded condition, provided the joint is properly cleaned, protected from iron contamination and pickled or electropolished afterwards.
Q: Is post-weld heat treatment required for alloy C-22?
Normally not. The alloy resists grain-boundary precipitation in the heat-affected zone, and heat treatment may itself introduce unwanted phases.
Q: Can alloy C-22 be used in hydrochloric acid service?
Only in dilute, low-temperature conditions; hot concentrated hydrochloric acid requires a high-molybdenum nickel-molybdenum grade instead.
Q: Which specifications apply to C-22 sheet and plate?
ASTM B575 and ASME SB575 cover plate, sheet and strip, with ASTM B462 applicable to bar and forging stock.
