Understanding Sensitization in Stainless Steel
Sensitization is the phenomenon where chromium carbides (Cr₂₃C₆) precipitate along the grain boundaries of austenitic stainless steels during exposure to temperatures in the range of 425–815°C (800–1,500°F). This precipitation depletes the adjacent grain boundary zone of chromium, reducing the local corrosion resistance and making the steel susceptible to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC).
Mechanism of Sensitization
- Carbon atoms diffuse to grain boundaries (carbon has high diffusivity)
- Chromium combines with carbon to form Cr₂₃C₆ precipitates at grain boundaries
- Chromium depletion zones (typically 0.1–10 μm wide) form adjacent to the precipitates
- The depleted zone falls below the ~12% Cr needed for passivation
- The depleted zones become anodic relative to the grain surfaces
- Corrosion propagates preferentially along grain boundaries
Factors Affecting Sensitization
- Carbon content: Higher carbon = faster sensitization. “L” grades (304L, 316L with ≤0.03% C) were developed for this reason.
- Temperature: Peak sensitization occurs at 650–750°C.
- Time: Standard grades (0.08% C) may sensitize in minutes at 700°C; L grades may take hours to days.
- Stabilizing elements: Titanium (321) and niobium (347) tie up carbon as TiC or NbC, preventing chromium carbide formation.
- Cold work: Accelerates sensitization by increasing dislocation density.
- Grain size: Larger grains reduce grain boundary area, slowing the overall rate of sensitization.
Standard Test Methods
1. Oxalic Acid Etch Test (ASTM A262 Practice A)
Purpose: Screening test to determine if further testing is required.
Procedure: Electrolytic etching in 10% oxalic acid solution at 1V DC for 90 seconds. Examine at 100–250× magnification.
Classification:
- Step structure: No chromium carbide precipitates visible. Material passes.
- Dual structure: Some grain boundary grooving. Further testing required.
- Serrated structure: Significant chromium carbide precipitation. Material fails.
2. Strauss Test (ASTM A262 Practice E)
Purpose: Detects susceptibility to intergranular corrosion.
Procedure: Immersion in boiling solution of sulfuric acid + copper sulfate + copper turnings for 4 × 24-hour periods (96 hours total). Bend specimens after each period and examine at 10× magnification.
Acceptance: No intergranular cracks visible at 10× after bending.
3. Huey Test (ASTM A262 Practice C)
Purpose: The most severe standard test. Evaluates corrosion rate in strongly oxidizing acids.
Procedure: Immersion in boiling 65% nitric acid (HNO₃) for 5 × 48-hour periods (240 hours total). Measure weight loss after each period.
Acceptance: Maximum corrosion rate typically ≤ 0.002 ipm (≤ 1.3 mm/year).
4. Streicher Test (ASTM A262 Practice B)
Purpose: Evaluates susceptibility to IGC in ferritic and nickel-bearing austenitic grades.
Procedure: Immersion in boiling 50% sulfuric acid + ferric sulfate solution for 120 hours. Measure corrosion rate.
5. Electrochemical Potentiokinetic Reactivation (EPR) Test
Purpose: Rapid, quantitative measurement of sensitization degree.
Procedure: Potentiodynamic polarization in 0.5M H₂SO₄ + 0.01M KSCN solution.
Interpretation:
- I_r/I_a < 0.01: Unsensitized
- I_r/I_a 0.01–0.05: Low degree of sensitization
- I_r/I_a > 0.05: Sensitized
Test Method Selection Guide
| Test Method | Duration | Type | Best For |
|---|---|---|---|
| Oxalic Acid Etch (A262-A) | 30 min | Screening | Quick go/no-go |
| Strauss (A262-E) | 96 hours | Qualitative | General service |
| Huey (A262-C) | 240 hours | Quantitative | Oxidizing acid service |
| Streicher (A262-B) | 120 hours | Quantitative | Ferritic grades |
| EPR | 30 min | Quantitative | Rapid assessment |
Prevention of Sensitization
- Use low-carbon grades: 304L, 316L (≤0.03% C)
- Use stabilized grades: 321 (Ti-stabilized), 347 (Nb-stabilized)
- Solution annealing: Heat to 1,040–1,120°C followed by rapid quench
- Control heat input during welding: Lower heat input reduces time in sensitization range
- Post-weld heat treatment: Solution anneal after welding for critical service
CoreMetal Steel — Sensitization-Resistant Stainless Steel
CoreMetal Steel stocks a full range of sensitization-resistant stainless steel products including 304L, 316L, 321, and 347 in plate, sheet, pipe, tube, bar, and fittings. All materials can be tested to ASTM A262 Practices A, B, C, and E upon request.
Email: tracy@coremetalsteel.com | Phone: +86 18291910632