Hastelloy C-22 (UNS N06022) Technical Guide | Superior Corrosion Resistance in Oxidizing Environments
Date: 2024年10月24日 Categories: All Products、Hastelloy Views: 2184
Excerpt:
Hastelloy C-22 (UNS N06022) technical guide from Hangbo Alloy. Details the chromium/molybdenum/tungsten alloy design (Cr 20.0–22.5%, Mo 12.5–14.5%, W 2.5–3.5%) that beats C-276 in oxidizing acid and chloride service, PREN and ASTM G48 CPT/CCT data, Green Death / Yellow Death mixed-media results, ERNiCrMo-10 welding practice, ASTM B575/B574/B622 compliance, and an acceptance checklist plus FAQ.
Hastelloy C-22 (UNS N06022) — Precision Metallurgy for Oxidizing Acid and High-Chloride Service | Hangbo Alloy
Alloy Import Pitfalls Series — Technical Bulletin for Chemical Process, Flue-Gas, and Pharmaceutical Buyers
Why "One of the C-Alloys" Is Not a Specification
The C-family of nickel-chromium-molybdenum alloys looks deceptively uniform on a datasheet: C-276 (UNS N10276), C-22 (N06022), and C-2000 (N06200) are all nickel-base alloys with chromium and molybdenum as the principal additions. Yet each was developed for a different corner of the corrosion map, and the differences are commercially decisive. Specify "C-276 or equivalent" and a trader may offer you C-22 — or the reverse — believing the family resemblance is good enough. It is not. In strongly oxidizing acid service — hot sulfuric acid with oxidizing contaminants, wet chlorine, chlorine dioxide, bleach-plant and flue-gas environments — C-22 routinely outperforms C-276 precisely because its alloy design is different.
Hangbo Alloy (Shanghai Hangbo Alloy Group, nickel-alloy.com) supplies C-22 plate (ASTM B575), rod and bar (ASTM B574), pipe, fittings, and forgings with certified chemistry and, on request, G48 pitting/crevice test documentation. This page gives the precision buyer the comparison framework the datasheets omit.
1. Composition and the Role of Tungsten
The C-family balances three alloying elements against each other. Chromium drives resistance to oxidizing media; molybdenum drives resistance to reducing media and localized attack; tungsten behaves like a slow-diffusing, heavier cousin of molybdenum. The differences among C-276, C-22, and C-2000 are deliberate shifts of that balance.
| Element (wt %) | C-276 (N10276) | C-22 (N06022) | C-2000 (N06200) |
|---|---|---|---|
| Nickel | Balance (~57) | Balance (~56) | Balance (~59) |
| Chromium | 14.5–16.5 | 20.0–22.5 | 22.0–24.0 |
| Molybdenum | 15.0–17.0 | 12.5–14.5 | 15.0–17.0 |
| Tungsten | 3.0–4.5 | 2.5–3.5 | — |
| Iron | 4.0–7.0 | 2.0–6.0 | 3.0 max |
| Cobalt | 2.5 max | 2.5 max | 2.0 max |
| Carbon | 0.01 max | 0.015 max (0.01 typical) | 0.01 max |
| Copper | — | 0.5 max | 1.3–1.9 |
| Manganese / Silicon / Vanadium | Mn 1.0, Si 0.08, V 0.35 | Mn 0.5, Si 0.08, V 0.35 | Mn 0.5, Si 0.08, V 0.2 |
The tungsten story — why the tables lie to the casual reader. Tungsten is not present to make C-22 "more like C-276." In C-276, tungsten's slow diffusivity improves resistance in specific reducing, hot hydrochloric and sulfuric services and retards precipitation of grain-boundary phases during welding. In C-22, tungsten is held to 2.5–3.5% while chromium is raised to 20–22.5% — a chromium level far above C-276's. The result is an alloy whose matrix stays stable and precipitation-resistant through welding and thermal exposure, while the elevated chromium confers exceptional resistance to oxidizing acids, ferric and cupric chloride, wet chlorine, and chloride-bearing oxidizing media. C-2000 completes the evolution by adding copper (for sulfuric acid resistance) and dropping tungsten entirely — but copper additions bring their own phase-stability compromises.
| Characteristic | C-276 | C-22 | C-2000 |
|---|---|---|---|
| Design center | Universal reducing + mild oxidizing | Oxidizing acids + chlorides, weld-zone stability | Oxidizing + sulfuric acid, chloride extremes |
| Oxidizing acid performance (hot H₂SO₄ with oxidizers, wet Cl₂) | Good | Superior to C-276 | Comparable/superior |
| Reducing acid performance (hot HCl) | Excellent | Good | Good |
| Phase stability in weld HAZ / long-term aging | Good (improved over old C alloys) | Excellent (very low topologically-close-packed phase precipitation) | Good |
| Hot hydrochloric acid | Best of the three | Good | Good |
| Cost driver | Mo + W | Balanced Cr/Mo | Mo + Cu |
2. PREN — A Useful Indicator, Not a Verdict
The Pitting Resistance Equivalent Number is the shorthand buyers quote to compare localized-corrosion resistance. The standard formula weights chromium, molybdenum, tungsten, and nitrogen:
PREN = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N
Because these alloys carry essentially zero nitrogen, the nitrogen term drops out. Using mid-range compositions:
| Alloy | Cr (%) | Mo (%) | W (%) | PREN (mid-range) | G48 FeCl₃ Behavior (typical ranking) |
|---|---|---|---|---|---|
| 316L stainless | 17 | 2.2 | — | ~24 | Baseline; pits near ambient in FeCl₃ |
| Alloy 625 | 21.5 | 9 | — | ~51 | CPT far above 316L |
| C-22 (N06022) | 22 | 13 | 3 | ~70 | CPT > 150 °C in G48; CCT ~80 °C |
| C-276 (N10276) | 15.5 | 16 | 3.5 | ~73 | CPT > 150 °C; CCT ~55 °C |
| C-2000 (N06200) | 23 | 16 | — | ~76 | Extremely high CPT/CCT |
Read the trap: by PREN arithmetic, C-276 (~73) and C-2000 (~76) outrank C-22 (~70). Yet in the standardized ferric-chloride tests that matter, C-22's critical crevice temperature exceeds C-276's by a wide margin, and in Green Death / Yellow Death testing C-22 is the standout of the family. Why? Because PREN is a composition-weighted index calibrated on stainless steels; it cannot capture how chromium and molybdenum are partitioned in a fully austenitic nickel matrix, how stable the microstructure remains after welding, or how the alloy behaves in the mixed oxidizing-chloride media of real plants. When the process stream contains chlorides plus an oxidizer, the actual G48 and mixed-acid rankings — not the PREN spreadsheet — should govern material selection.
3. Localized Corrosion Data: Pitting and Crevice Resistance
Localized attack, not general corrosion, is what kills C-family alloys in chloride service. The standardized test is ASTM G48 (ferric chloride), which produces a Critical Pitting Temperature (CPT) and Critical Crevice Temperature (CCT). Higher is better. Representative published values from alloy producers' corrosion programs are shown below to rank the alloys; actual numbers vary with test lot and crevice geometry.
| Alloy | CPT, ASTM G48 FeCl₃ | CCT, ASTM G48 FeCl₃ | Comment |
|---|---|---|---|
| 316L | ~15 °C (59 °F) | ~0 °C (32 °F) | Baseline stainless |
| 254SMO | ~60 °C (140 °F) | ~30 °C (86 °F) | Super-austenitic |
| Alloy 625 | ~100 °C (212 °F) | ~40 °C (104 °F) | High Mo, moderate Cr |
| C-276 | >150 °C (>302 °F) | ~55 °C (131 °F) | High Mo/W, moderate Cr |
| C-22 | >150 °C (>302 °F) | ~80 °C (176 °F) | High Cr + Mo/W balance |
The crevice gap between C-276 (≈55 °C) and C-22 (≈80 °C) is the single most useful number for the process engineer specifying gasketed joints, tube-to-tubesheet crevices, and flanged connections in chloride service: C-22 tolerates crevice service roughly 25 °C hotter than C-276 before stable crevice attack initiates.
Two aggressive mixed-media tests confirm the trend in oxidizing chloride conditions:
| Test Medium | C-22 Result | C-276 Result |
|---|---|---|
| "Green Death" — 11.5% H₂SO₄ + 1.2% HCl + 1% FeCl₃ + 1% CuCl₂ | First pitting only near 120 °C | Pitting at the boiling point (~103 °C solution) |
| "Yellow Death" — 4% NaCl + 0.1% Fe₂(SO₄)₃ + 0.021 M HCl | No pitting to the 150 °C test maximum | Resists to a lower temperature ceiling |
| Yellow Death — crevice (CCT) | ~75 °C | ~60 °C |
| Boiling 45% MgCl₂ (SCC screening) | No cracking in >1000 h exposure | No cracking in >1000 h exposure |
In stress-corrosion screening (boiling 45% magnesium chloride, ASTM G36 practice), both C-22 and C-276 survive beyond 1000 hours where 316L cracks within hours — but that test is a severity benchmark, not a service simulation. Seawater and brackish-water crevice exposures run by independent laboratories (e.g., LaQue Center programs) repeatedly show C-22 with zero attacked crevice sites where lesser alloys initiate attack — the empirical basis for C-22's dominance in offshore, FGD, and bleach-plant crevice service.
4. Oxidizing Acid Service: The Precision Buyer's Map
| Service Environment | C-276 | C-22 | C-2000 |
|---|---|---|---|
| Sulfuric acid, dilute + oxidizing impurities | Good | Superior | Superior |
| Sulfuric acid, mid-concentrations with chlorides | Good | Superior | Superior (Cu helps) |
| Hydrochloric acid, hot & reducing | Superior | Good | Good |
| Hydrochloric acid + oxidizing chlorides | Limited | Superior | Superior |
| Wet chlorine / chlorine dioxide | Good | Superior | Superior |
| Flue-gas desulfurization (FGD) scrubber | Good | Superior | Excellent |
| Phosphoric acid with chlorides/fluorides | Moderate | Good | Good |
| Seawater heat exchangers & crevice hardware | Good | Superior | Excellent |
The message for importers is consistent: if your service is reducing (hot deaerated HCl, reducing sulfuric), C-276's extra molybdenum earns its keep. If your service is oxidizing or mixed oxidizing-chloride — the majority of modern FGD, bleach, agrochemical, and pharmaceutical waste streams — C-22 is the precision choice, and substituting C-276 to "save cost" or because "it's the classic alloy" is a corrosion-engineering downgrade.
5. Welding and Fabrication — Where C-22 Earns Its Specification
C-family alloys fail in welds when the heat-affected zone precipitates topologically close-packed phases (μ, σ, P, and Laves) that deplete the matrix of molybdenum and tungsten. C-22's composition was balanced specifically to minimize this precipitation, giving it exceptional weld-zone and long-term thermal stability relative to older C-type alloys.
| Fabrication Consideration | C-22 Practice | Hangbo Alloy Note |
|---|---|---|
| Solution anneal | ~2050 °F (1121 °C) minimum, water quench | Confirm after hot forming |
| Welding filler | Matching ERNiCrMo-10 / ENiCrMo-10 (alloy 22 filler) | Never "any C-type filler" |
| Heat input control | Moderate, with interpass ≤ ~200 °F (93 °C) | Weld procedure per ASME IX |
| Post-weld heat treatment | Not required; avoid if possible | PWHT degrades properties |
| Hot forming | Above ~1850 °F, then re-solution anneal | Below ~1850 °F may strain-harden |
| Cleaning | Chloride-free pickling; avoid iron contamination | Iron contamination initiates pitting |
Import pitfall: a "C-22" vessel fabricated with C-276 filler metal is not C-22 construction, no matter what the plate certificate says. Specify the AWS filler classification (ERNiCrMo-10 / ENiCrMo-10) in the purchase order and weld-procedure qualification record.
6. ASTM Compliance and the Acceptance Checklist
| Product Form | ASTM | ASME | Hangbo Alloy Supply |
|---|---|---|---|
| Plate, sheet, strip | B575 | SB-575 | 1.5–50 mm plate, 3000 mm width max |
| Rod and bar | B574 | SB-574 | Ø 3–350 mm, straightened & cut |
| Seamless pipe & tube | B622 | SB-622 | Up to 12" NB |
| Welded pipe & tube | B619 / B626 | SB-619 / SB-626 | On application |
| Fittings | B366 (WPNC22-class) | SB-366 | Buttweld & socketweld |
| Acceptance Check | Criterion |
|---|---|
| UNS on MTC | N06022 — not "C-22 type," not N10276 |
| Chromium | 20.0–22.5% (the C-276 discriminator is ~14.5–16.5%) |
| Tungsten | 2.5–3.5% |
| Carbon | ≤0.015% (typ. ≤0.01%) |
| Anneal condition | Solution annealed + water quenched, no PWHT |
| Optional testing | ASTM G48 crevice test report; PMI on every piece |
| Branding/marking | Each plate/bar stamped with UNS + heat number |
Technical FAQ — Hastelloy C-22 (UNS N06022)
1. What is the difference between Hastelloy C-22 and C-276?
C-22 (N06022) raises chromium to 20–22.5% while moderating molybdenum (12.5–14.5%) and tungsten (2.5–3.5%), which makes it superior in oxidizing acids and chloride-oxidizing media. C-276 (N10276) carries more molybdenum and tungsten with only ~15% chromium, favoring hot reducing acids. In G48 crevice testing, C-22's critical crevice temperature is roughly 25 °C higher than C-276's.
2. How do you calculate PREN for nickel alloys, and why does C-22 outperform C-276 despite a lower PREN?
PREN = %Cr + 3.3(%Mo + 0.5%W) + 16(%N). C-22 computes to roughly 70 versus about 73 for C-276, yet C-22 has the higher G48 crevice temperature. PREN is an index calibrated on stainless steels; it cannot capture chromium-molybdenum partitioning in the nickel matrix, weld-zone phase stability, or real mixed-media effects. Standardized pitting/crevice tests and plant experience should govern selection.
3. Which ASTM specifications cover Hastelloy C-22 plate and bar?
Plate, sheet, and strip are covered by ASTM B575; rod and bar by ASTM B574; seamless pipe and tube by B622, with ASME SB parallels. Hangbo Alloy certifies each product form against the correct specification and provides PMI on request.
4. Is Hastelloy C-22 resistant to hydrochloric acid?
C-22 resists hydrochloric acid well in many concentrations, but in hot, strongly reducing HCl the higher-molybdenum C-276 remains the traditional first choice. When the HCl stream contains oxidizing species (ferric, cupric, chlorine), C-22 is the better selection because its elevated chromium counters the oxidizer that accelerates C-276's attack.
5. What is the role of tungsten in Hastelloy C-22?
Tungsten (2.5–3.5%) contributes to solid-solution strength and localized-corrosion resistance and, like molybdenum, is present at a level that keeps the alloy phase-stable through welding and aging. C-22 deliberately moderates tungsten relative to C-276 and invests the compositional budget in chromium instead — the source of its oxidizing-acid superiority.
6. Does C-22 resist pitting and crevice corrosion in seawater?
Extremely well. Independent crevice-test programs show C-22 with zero attacked crevice sites in seawater, and its ASTM G48 critical crevice temperature (~80 °C) is far above any seawater operating temperature. It is a preferred metallurgy for offshore chloride and crevice hardware where lesser alloys crevice-corrode within months.
7. What filler metal should be used to weld Hastelloy C-22?
AWS ERNiCrMo-10 (GTAW/GMAW) and ENiCrMo-10 (SMAW). Fabrication of C-22 with C-276-type filler produces an off-composition weld and voids the alloy's corrosion credentials; Hangbo Alloy supplies filler-matched weld procedures for pressure-boundary construction.
8. Is Hastelloy C-22 suitable for flue-gas desulfurization (FGD) service?
Yes. FGD environments combine sulfuric acid, chlorides, fluorides, and oxidizing conditions — precisely the media where C-22's high chromium outperforms C-276. It is widely specified for scrubber absorbers, outlet ducts, and internals, often as thin weld-overlay or lined construction.
9. Can Hastelloy C-22 be substituted for C-2000, or vice versa?
Only with a corrosion review. C-2000 (N06200) adds copper for sulfuric-acid resistance and drops tungsten, giving a different balance in hot sulfuric and mixed acids. C-22 and C-2000 both beat C-276 in oxidizing chloride media, but "C-276 or equivalent" clauses should be rewritten as explicit UNS callouts to avoid unintended substitution.
10. Why does the C-22 certificate show a high chromium number, and how can I verify grade integrity on arrival?
The 20.0–22.5% chromium window is C-22's fingerprint; C-276's is 14.5–16.5%. Verify UNS N06022, chromium, molybdenum, tungsten, and carbon against the chemistry table on this page, and request positive material identification (PMI) on every plate or bar. Hangbo Alloy performs in-house PMI and offers third-party witness testing at the Shanghai mill.
Alloy Import Pitfalls Series — Hangbo Alloy (Shanghai Hangbo Alloy Group, nickel-alloy.com). Corrosion rankings and values are compiled from recognized producer and laboratory literature (including ASTM G48 and mixed-acid test programs) for material selection guidance; they are not warranties of performance in specific chemistries. Contact Hangbo Alloy for C-22 stock, G48 test documentation, and application engineering support.
