Hastelloy C-276 (UNS N10276) and C-22 (UNS N06022) are two of the most widely specified nickel-molybdenum-chromium alloys for corrosive chemical service. Both offer exceptional resistance to pitting, crevice corrosion, and stress corrosion cracking, but their compositional differences create distinct performance advantages in specific environments. Understanding these differences is critical for selecting the optimal alloy for your process equipment.
Chemical Composition Comparison
Hastelloy C-276 (UNS N10276)
- Nickel: Balance (≈57%)
- Molybdenum: 15.0-17.0%
- Chromium: 14.5-16.5%
- Tungsten: 3.0-4.5%
- Iron: 4.0-7.0%
- Cobalt: ≤2.5%
- Carbon: ≤0.01%
- Silicon: ≤0.08%
Hastelloy C-22 (UNS N06022)
- Nickel: Balance (≈56%)
- Chromium: 20.0-22.5%
- Molybdenum: 12.5-14.5%
- Tungsten: 2.5-3.5%
- Iron: 2.0-6.0%
- Cobalt: ≤2.5%
- Carbon: ≤0.015%
- Silicon: ≤0.08%
The key differences: C-22 has higher chromium (20-22.5% vs 14.5-16.5%) and lower molybdenum (12.5-14.5% vs 15-17%) compared to C-276. This compositional shift significantly affects the balance between oxidizing and reducing acid resistance.
Corrosion Resistance Performance
Oxidizing Acid Resistance
The higher chromium content in C-22 provides superior resistance to oxidizing environments:
- Nitric acid: C-22 shows better resistance across all concentrations, particularly above 30% HNO₃
- Chlorine-containing media: C-22’s higher Cr content provides better resistance to wet chlorine and hypochlorite solutions
- Ferric/ferric chloride: C-22 resists attack better in concentrated FeCl₃ solutions
- Chromic acid: C-22 preferred for chromic acid concentrations above 10%
Reducing Acid Resistance
The higher molybdenum content in C-276 provides superior resistance to reducing environments:
- Hydrochloric acid: C-276 resists HCl better at concentrations above 10% and temperatures above 50°C
- Sulfuric acid: C-276 preferred for H₂SO₄ concentrations above 20% at moderate temperatures
- Phosphoric acid: Both perform well, with C-276 showing marginally better performance in contaminated (wet-process) acid
- Formic acid: C-276 slightly better in concentrated formic acid at elevated temperatures
Mixed Acid Environments
In mixed oxidizing-reducing conditions (common in real process environments), C-22 generally shows superior overall performance due to its balanced composition that addresses both corrosion mechanisms simultaneously.
Pitting and Crevice Corrosion
The critical pitting temperature (CPT) and critical crevice corrosion temperature (CCCT) in 6% FeCl₃ solution:
- C-276: CPT ≈ 50°C, CCCT ≈ 25°C
- C-22: CPT ≈ 70°C, CCCT ≈ 40°C
C-22’s significantly higher critical temperatures reflect its superior localized corrosion resistance, primarily due to higher chromium content.
PREN Comparison
- C-276 PREN: ~59 (15.5%Cr + 3.3×16%Mo)
- C-22 PREN: ~65 (21.5%Cr + 3.3×13.5%Mo)
While both alloys have exceptionally high PREN values compared to stainless steels, C-22’s higher PREN indicates superior resistance to localized corrosion in chloride-containing environments.
Mechanical Properties
| Property | C-276 | C-22 |
|---|---|---|
| Yield Strength (MPa) | ≥355 | ≥310 |
| Tensile Strength (MPa) | ≥790 | ≥690 |
| Elongation (%) | ≥40 | ≥45 |
| Hardness (HRC) | ≤37 | ≤34 |
C-276 offers higher strength, while C-22 provides better ductility. Both alloys maintain excellent toughness at cryogenic temperatures.
Welding Considerations
Filler Metal Selection
- C-276 welding: AWS A5.14 ERNiCr-4 (matching composition) or ERNiCr-3 for dissimilar joints
- C-22 welding: AWS A5.14 ERNiCr-6 (matching) or ERNiCr-3 for dissimilar joints
Weld Decay Resistance
C-22 shows superior resistance to weld heat-affected zone (HAZ) corrosion due to its lower tendency for grain boundary precipitation of intermetallic phases. The ultra-low carbon content of both alloys (<0.01-0.015%) minimizes carbide precipitation, but C-22's balanced composition provides additional margin against weld zone attack.
Application Selection Guide
Choose C-276 When:
- Primary exposure is to reducing acids (HCl, dilute H₂SO₄)
- Higher strength is required for pressure vessel design
- Process involves mixed oxidizing-reducing conditions with reducing dominance
- Existing installations already use C-276 (proven track record)
- Cost is a significant factor (C-276 is typically 10-15% less expensive)
Choose C-22 When:
- Primary exposure includes oxidizing acids or mixed oxidizing-reducing conditions
- Chloride pitting or crevice corrosion is the primary concern
- Wet chlorine or chlorine dioxide service
- Flue gas desulfurization (FGD) systems
- Waste incineration scrubber systems
- Chemical processes with variable oxidizing-reducing conditions
Cost Comparison
Both alloys are expensive compared to stainless steels, but:
- C-276 is typically 10-15% less expensive than C-22 due to lower chromium content
- Fabrication costs are similar for both alloys
- Total lifecycle cost should be evaluated based on expected equipment life in the specific service environment
Industry Application Summary
| Industry | Preferred Alloy | Application |
|---|---|---|
| Chemical Processing | Both – case by case | Reactors, heat exchangers, piping |
| FGD Systems | C-22 | Absorber towers, ductwork |
| Petroleum Refining | C-276 | Sour gas equipment |
| Waste Treatment | C-22 | Acid waste handling |
| Pulp & Paper | C-22 | Digesters, bleach plant |
| Pharmaceutical | C-276 | Reactor vessels |
CoreMetal Supply for Nickel Alloy Equipment
CoreMetal Steel supplies Hastelloy C-276 and C-22 in plate, sheet, pipe, tube, fittings, and round bar form. All products carry full EN 10204 3.1 mill test certificates and comply with ASTM B574 (plate/sheet), B622 (seamless pipe/tube), and B619/B626 (welded pipe) specifications.
Contact CoreMetal Steel for competitive pricing on Hastelloy C-276 and C-22 products, with material selection guidance from our technical team for your specific corrosive service application.