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Stainless Steel - Iron-Based, Chromium-Defined
Stainless steel is a family of iron-based (Fe) alloys defined by a minimum chromium content of 10.5% by mass (per ISO 15510). This chromium threshold triggers the formation of an invisible, self-healing passive film of chromium oxide (Cr₂O₃) on the surface - the mechanism behind its corrosion resistance. Without this film, stainless steel would corrode like carbon steel.
The most widely specified stainless grades in process piping are: 304/304L (general purpose, 18% Cr / 8% Ni), 316/316L (added 2–3% Mo for improved chloride resistance), 317L (higher Mo, 3–4%), and duplex 2205 (22% Cr / 5% Ni / 3% Mo - high strength and chloride resistance combined).
Hastelloy - Nickel-Based, Engineered for Aggression
Hastelloy is a registered trade name for a series of nickel-based superalloys developed by Haynes International. Unlike stainless steel, the base metal is nickel (>50% Ni), not iron. This fundamental difference gives Hastelloy a qualitatively different corrosion resistance mechanism - the nickel matrix itself is highly resistant to both oxidising and reducing environments, augmented by high molybdenum (14–16%) and chromium additions.
The most common grades in chemical process piping are Hastelloy C-276 (UNS N10276) and Hastelloy C-22 (UNS N06022). Both are classified as Corrosion Resistant Alloys (CRA) - the industry term for alloys selected specifically for extreme corrosion environments rather than mechanical performance.
Chemical Composition: What's Inside Each Alloy?
| Element | 316L Stainless | Duplex 2205 | Hastelloy C-276 | Hastelloy C-22 | Function |
|---|---|---|---|---|---|
| Fe (Iron) | Balance (~63%) | Balance (~69%) | 4–7% | 2–6% | Base metal |
| Ni (Nickel) | 10–14% | 4.5–6.5% | Balance (~57%) | Balance (~56%) | Matrix corrosion resistance; austenite stability |
| Cr (Chromium) | 16–18% | 21–23% | 14.5–16.5% | 20–22.5% | Passive film formation; oxidation resistance |
| Mo (Molybdenum) | 2–3% | 3–3.5% | 15–17% | 12.5–14.5% | Pitting & crevice corrosion resistance; reduces PREN gap |
| W (Tungsten) | - | - | 3–4.5% | 2.5–3.5% | Enhanced pitting resistance; localized corrosion resistance |
| Mn (Manganese) | ≤2% | ≤2% | ≤1% | ≤0.5% | Deoxidation |
| C (Carbon) | ≤0.03% (L grade) | ≤0.03% | ≤0.01% | ≤0.01% | Low C prevents sensitisation during welding |
| Co (Cobalt) | - | - | ≤2.5% | ≤2.5% | High-temp stability |
Mechanical Properties Comparison
| Property | 316L (annealed) | Duplex 2205 | Hastelloy C-276 (annealed) | Hastelloy C-22 (annealed) |
|---|---|---|---|---|
| Tensile Strength (UTS) | 485–690 MPa | 620–900 MPa | 690–800 MPa | 690–795 MPa |
| Yield Strength (0.2%) | 170–310 MPa | 450–530 MPa | 280–350 MPa | 310–380 MPa |
| Elongation | 40% | 25% | 40–50% | 45–55% |
| Hardness | ~79 HRB | ~28 HRC max | ~90 HRB | ~100 HRB |
| Max Service Temp. | 870 °C (oxidising air) | 300 °C (limited) | 1,040 °C | 1,093 °C |
| Cryogenic Service | ✔ Excellent (FCC structure) | Limited | ✔ Excellent (FCC structure) | ✔ Excellent |
| Weldability | Excellent - no PWHT | Good (ferrite control critical) | Good - no PWHT required | Good - no PWHT required |
| Work-Hardening Rate | High | Moderate | High | High |
| Machinability | Fair (work-hardens) | Difficult | Difficult - requires carbide tooling | Difficult |
FAQ
Q: What is the main difference between stainless steel and Hastelloy?
Stainless steel is iron-based with ≥10.5% chromium, Hastelloy is nickel-based with >50% nickel. This makes Hastelloy far more resistant to reducing acids and chlorides, but costs 6–10× more than 316L.
Q: When should I use Hastelloy C-276 instead of 316L stainless steel?
Use C-276 when: Cl⁻ >200 ppm at elevated temp, HCl or H₂SO₄ present, wet H₂S/sour gas service, mixed oxidising/reducing conditions (e.g., FGD), or 316L has failed by pitting/SCC.
Q: What is the PREN of Hastelloy C-276 vs 316L stainless steel?
316L: PREN ≈ 24. Hastelloy C-276: PREN ≈ 72. C-276's pitting resistance is nearly 3× higher, critical for chloride-rich environments.
Q: Is Hastelloy stronger than stainless steel?
At ambient temps, C-276 and 316L have similar strength (~690 MPa UTS). Hastelloy's advantage is high-temp performance - maintains strength to 1,040°C vs 316L's 870°C. Duplex 2205 actually has higher yield strength than both.
Q: What are the different grades of Hastelloy?
C-276 - most common, all-around corrosion resistance, C-22 - better in oxidising acids (HNO₃), B-3 - HCl specialist, not for oxidising media, X - high-temp gas turbines.
Q: Does Hastelloy rust?
No. Its >50% nickel base resists corrosion without relying solely on a passive film. Immune to chloride SCC and HIC. Only vulnerable to hot concentrated oxidising acids (e.g., HNO₃ + HF), where C-22 performs better.
Q: Can stainless steel replace Hastelloy to save costs?
Yes - 316L handles most general service. Upgrade to Hastelloy only when corrosion data shows >0.1 mm/year attack on stainless. Super duplex 2507 (PREN 43) is a cost-effective intermediate option.
Q: What standards cover Hastelloy pipe products?
ASTM B622/SB-622 (seamless), ASTM B619/SB-619 (welded pipe), ASTM B626/SB-626 (heat exchanger tube). Dimensions per ASME B36.19/B36.10. Specify UNS number (N10276 for C-276, N06022 for C-22).
