What Is Stainless Steel Sensitization?
Sensitization is one of the most significant concerns when welding austenitic stainless steels. It refers to the precipitation of chromium carbides at grain boundaries, which depletes the surrounding matrix of chromium and renders the steel susceptible to intergranular corrosion (IGC). This phenomenon occurs when the material is held in or passes through a critical temperature range during welding, making it a primary concern for welded stainless steel equipment in chemical processing, marine, and food industry applications.
At CoreMetal Steel, we supply stabilized and low-carbon stainless steel products specifically designed to resist sensitization. This guide covers the mechanism, detection, prevention, and remediation of sensitization in welded stainless steel.
The Sensitization Mechanism
Chromium Carbide Precipitation
In austenitic stainless steels, chromium is dissolved in the solid solution to provide corrosion resistance. When heated to a specific temperature range, chromium atoms migrate to grain boundaries and combine with carbon to form chromium carbide (Cr23C6):
- At temperatures above 425C (800F), carbon becomes mobile in the austenite matrix
- Chromium diffuses to grain boundaries faster than carbon can redistribute
- Chromium carbide (Cr23C6) precipitates form at grain boundaries
- The adjacent zone becomes depleted of chromium (below 12% – the minimum for passivity)
- This chromium-depleted zone is anodic relative to the cathodic grain interior
- The result is selective intergranular corrosion along the chromium-depleted zones
Temperature-Time Relationship
| Temperature Range | Effect | Time to Sensitization |
|---|---|---|
| 425-500C (800-930F) | Slow precipitation | Hours to days |
| 500-650C (930-1200F) | Rapid precipitation (critical range) | Minutes to hours |
| 650-850C (1200-1560F) | Peak sensitization rate | Seconds to minutes |
| 850-1100C (1560-2010F) | Carbides dissolve | Dissolution occurs |
The peak sensitization temperature for most austenitic stainless steels is approximately 675C (1250F). During welding, the Heat-Affected Zone (HAZ) passes through this critical range, creating a narrow band of sensitized material adjacent to the weld.
Detecting Sensitization
Straight-Line Test (ASTM A262 Practice A)
Oxalic acid etch test for screening:
- Electrolytic etching in 10% oxalic acid solution
- Microscopic examination reveals three structures:
- Step structure: No carbide precipitation – PASS
- Dual structure: Some carbide precipitation – requires further testing
- Ditch structure: Extensive carbide precipitation – FAIL
Streicher Test (ASTM A262 Practice E)
Boiling ferric sulfate-sulfuric acid test:
- 120-hour exposure to boiling Fe2(SO4)3 + H2SO4 solution
- Measure corrosion rate (mil/month or mm/year)
- Acceptance criteria vary by grade
Huey Test (ASTM A262 Practice C)
Boiling nitric acid test – most severe:
- 5 x 48-hour periods in boiling 65% HNO3
- Measure weight loss per 48-hour period
- Used for highly corrosive service conditions
- Most demanding of the common sensitization tests
Electrochemical Potentiokinetic Reactivation (EPR) Test
- ASTM C1070 / G108
- Quick (typically under 1 hour) electrochemical test
- Measures degree of sensitization (DOS) quantitatively
- Non-destructive variant available for field use
Consequences of Sensitization
Intergranular Corrosion (IGC)
The primary consequence is intergranular attack:
- Corrosion progresses along grain boundaries
- Material can lose structural integrity without significant weight loss
- Appears as sugaring on the surface or grain fallout
- Common in heat exchanger tubes, pipe welds, and tank welds
Weld Decay
Also known as knife-line attack in extreme cases:
- Narrow band of corrosion adjacent to the weld bead
- Can lead to through-wall leaks in piping
- Often appears months or years after installation
- Particularly dangerous because the rest of the component appears sound
Stress Corrosion Cracking (SCC) Susceptibility
Sensitized material is more susceptible to SCC:
- Chromium-depleted grain boundaries provide crack initiation sites
- Combined with residual stress from welding
- Particularly dangerous in chloride environments
Prevention Methods
1. Use Low-Carbon Grades
The simplest and most effective solution:
| Standard Grade | Carbon Content | Low-Carbon Alternative | Carbon Content |
|---|---|---|---|
| 304 | 0.08% max | 304L | 0.03% max |
| 316 | 0.08% max | 316L | 0.03% max |
| 321 | 0.08% max + Ti | 321 (stabilized) | Ti greater than or equal to 5x C% |
| 347 | 0.08% max + Nb | 347 (stabilized) | Nb greater than or equal to 10x C% |
Low-carbon grades have insufficient carbon to form significant chromium carbide, even when held in the sensitization temperature range.
2. Use Stabilized Grades
Stabilized grades contain elements that preferentially combine with carbon:
- Titanium (321): Ti combines with C to form TiC instead of Cr23C6. Titanium content greater than or equal to 5x carbon content
- Niobium (347): Nb combines with C to form NbC. Niobium content greater than or equal to 10x carbon content
- Both TiC and NbC are stable at welding temperatures and do not deplete chromium
3. Solution Annealing After Welding
Heating the entire component to dissolve carbides and restore chromium distribution:
- Temperature: 1040-1120C (1900-2050F) for austenitic grades
- Cooling: Rapid quench in water to prevent re-precipitation
- Effectiveness: Restores full corrosion resistance
- Limitations: May not be practical for large assemblies, causes distortion
4. Control Heat Input During Welding
- Use minimum heat input consistent with sound welds
- Stringer beads preferred over weave beads
- Interpass temperature control (typically below 150C/300F)
- Fast cooling through the sensitization range
- Back-purging with argon for root passes
5. Post-Weld Pickling and Passivation
While this does not reverse sensitization, it removes surface chromium depletion and contamination:
- Removes heat tint and oxide scale from welding
- Restores passive film on surface
- Does not address internal grain boundary carbide precipitation
- Should be used in conjunction with other prevention methods
Material Selection Guide for Welded Service
| Service Condition | Recommended Grade | Rationale |
|---|---|---|
| Mild corrosive, no post-weld heat treatment | 304L / 316L | Low carbon prevents sensitization |
| Severe corrosive, post-weld solution annealing impractical | 304L / 316L | Low carbon grade essential |
| High temperature service (500-800C) | 321 / 347 | Stabilized grades resist sensitization at temperature |
| Cryogenic service | 304L / 316L | Low carbon + good low-temperature toughness |
| Food/pharmaceutical (frequent sanitization) | 316L | Low carbon + molybdenum for cleaning chemical resistance |
Conclusion
Sensitization of stainless steel during welding is a well-understood phenomenon with proven prevention methods. The most reliable approach is selecting low-carbon (L grades) or stabilized (321, 347) materials for welded constructions that will operate in corrosive environments.
CoreMetal Steel supplies all stainless steel grades including 304L, 316L, 321, and 347 in plate, sheet, pipe, tube, bar, and fittings. Contact our technical team at tracy@coremetalsteel.com for material recommendations to prevent sensitization in your welded applications.