Sensitization is one of the most important failure mechanisms in austenitic stainless steels. It occurs when chromium carbides precipitate at grain boundaries during exposure to elevated temperatures, depleting adjacent regions of chromium and making them susceptible to intergranular corrosion (IGC). Understanding sensitization and the available countermeasures is essential for anyone specifying or fabricating stainless steel equipment.
What Is Sensitization?
Sensitization occurs when austenitic stainless steel is held in the temperature range of 425°C to 850°C. In this range, carbon combines with chromium at grain boundaries to form chromium carbide (Cr₃₃C₆) precipitates. Each particle consumes approximately 20 times its weight in chromium, depleting local chromium below the critical ~12% needed for passivation.
The Sensitization Mechanism
- Carbon diffusion: Carbon atoms diffuse rapidly to grain boundaries at sensitization temperatures
- Chromium carbide nucleation: Cr₃₃C₆ nucleates at grain boundaries
- Chromium depletion: Growing carbides consume chromium faster than it can diffuse back (Cr diffuses ~100,000 times slower than carbon in austenite)
- Sensitized condition: Adjacent matrix has less than 12% chromium, unable to maintain passive oxide film
Time-Temperature-Sensitization (TTS) Diagram
Key features: nose temperature approximately 650-700°C (sensitization fastest), minimum time at nose as little as 1-2 minutes, below 425°C diffusion is too slow, above 850°C carbides dissolve.
Weld Decay
The most common form of sensitization in practice. During welding, the HAZ passes through the sensitization range. Weld decay appears as a narrow line of corrosion along the fusion line, potentially causing through-wall leakage in pipes and tanks.
Prevention Methods
1. Low-Carbon Grades (L-Grades)
| Standard Grade | Carbon Content | L-Grade | Carbon Content |
|---|---|---|---|
| 304 | ≤ 0.08% | 304L | ≤ 0.03% |
| 316 | ≤ 0.08% | 316L | ≤ 0.03% |
With carbon ≤ 0.03%, insufficient carbon is available for harmful chromium carbide precipitation. L-grade steels are resistant to sensitization during welding.
2. Stabilized Grades (Ti or Nb)
Type 321 (Ti-stabilized): Titanium forms TiC instead of Cr₃₃C₆, keeping chromium in solution. Minimum Ti = 5 × (C+N)% up to 0.70%.
Type 347 (Nb-stabilized): Niobium forms NbC. Preferred for high-temperature applications above 550°C because TiC can coarsen. Minimum Nb = 8 × C% up to 1.10%.
3. Solution Annealing
Heating to 1050-1100°C dissolves all chromium carbides, then rapid cooling (water quench) returns the steel to a fully austenitic, corrosion-resistant condition. Effective but often impractical for large structures.
Testing for Sensitization
- ASTM A262 Practice A (Oxalic Acid Etch): Rapid screening — “ditch” structure indicates sensitization
- ASTM A262 Practice B (Strauss Test): 24-hour boil in copper sulfate + sulfuric acid
- ASTM A262 Practice C (Huey Test): 5 × 48-hour boils in 65% nitric acid. Most severe test.
- EPR (Electrochemical): Fast test, results in 30 minutes. Measures degree of sensitization (DOS).
Practical Recommendations
- For welded equipment in corrosive service, always specify L-grade or stabilized grades
- For high-temperature service above 400°C, use 321 or 347
- For welding of standard grades, use solution annealing if corrosion resistance is critical
- Test using ASTM A262 or EPR methods when material integrity is in question
Summary
By understanding the sensitization mechanism and specifying the appropriate material grade (L-grade, 321, or 347), engineers can prevent intergranular corrosion and ensure long service life in demanding environments.
CoreMetal Steel stocks 304, 304L, 316, 316L, 321, and 347 stainless steel in sheet, plate, pipe, and bar forms with certified chemical composition and corrosion resistance testing.