Sensitization in the heat affected zone (HAZ) of stainless steel weldments remains one of the most insidious failure mechanisms in corrosion-resistant piping and equipment. When chromium carbides precipitate along grain boundaries in the HAZ, the surrounding material becomes depleted of chromium, making it vulnerable to intergranular corrosion that can cause sudden, catastrophic failures in service.

CoreMetal Steel supplies stabilized and low-carbon stainless steel products specifically engineered to resist sensitization in welded fabrication.

The Sensitization Mechanism

Chromium Carbide Precipitation

Austenitic stainless steels contain carbon in solid solution at high temperatures. As the material cools slowly through the critical temperature range of 425C to 850C (800F to 1560F), carbon atoms combine with chromium atoms to form chromium carbide (Cr23C6) precipitates along grain boundaries.

The Chromium-Depleted Zone

Each chromium carbide precipitate consumes chromium from the surrounding matrix, creating narrow zones (1-5 micrometers wide) where chromium content drops below the minimum 10.5% required for passivation. These chromium-depleted zones become anodic relative to cathodic grain surfaces, creating galvanic cells that drive intergranular corrosion.

Why the HAZ Is Most Vulnerable

During welding, the HAZ experiences a temperature profile that peaks below melting point but remains within the sensitization range long enough for chromium carbide precipitation, creating a unique vulnerability zone.

Detecting Sensitization

ASTM A262 Practice A – Oxalic Acid Etch Test

A rapid screening test revealing step structure (acceptable), dual structure (requires further testing), or ditch structure (fail).

ASTM A262 Practices B, C, and E

• Practice B (Strauss Test): 16-hour boiling in acidified copper sulfate. Suitable for Mo-bearing grades.
• Practice C (Huey Test): Five 48-hour boiling periods in 65% nitric acid. Most severe test.
• Practice E (Streicher Test): 24-hour boiling in sulfuric acid + ferric sulfate.

Electrochemical Potentiokinetic Reactivation (EPR)

A non-destructive test measuring the degree of sensitization expressed as charge per unit area (C/cm2).

Prevention Strategies

Low-Carbon Grades (L Grades)

• 304L: Maximum 0.030% carbon prevents sensitization in most welded applications
• 316L: Maximum 0.030% carbon with molybdenum for pitting resistance
• Limitation: L grades have lower high-temperature strength than standard grades

Stabilized Grades (Ti/Nb)

• Type 321 (Ti-stabilized): Titanium carbides form preferentially, leaving chromium available
• Type 347 (Nb-stabilized): Niobium carbides provide similar stabilization
• Preferred for high-temperature service above 400C where both sensitization resistance and creep strength are required

Solution Annealing After Welding

Heating to 1050-1100C and rapidly cooling dissolves chromium carbides. Restores full corrosion resistance but often impractical for large assemblies.

Welding Process Controls

• Use low heat input welding parameters
• Employ multi-pass welding with interpass temperature control
• Apply water cooling or copper backing bars to increase cooling rate
• Avoid excessive preheating of austenitic stainless steels

Sensitization in Different Stainless Steel Types

• Austenitic grades: Most susceptible due to FCC crystal structure
• Ferritic grades: Susceptible through chromium nitride and carbide precipitation
• Duplex grades: Inherently resistant due to lower carbon and two-phase structure

Design Considerations

• Specify L grades or stabilized grades for all welded construction in corrosive service
• Include weld decay testing requirements in material specifications
• For sour service, specify NACE MR0175/ISO 15156 compliance
• Document welding procedures with heat input limits

Why Choose CoreMetal Steel

CoreMetal Steel provides comprehensive stainless steel solutions: full inventory of L grades (304L, 316L, 317L) and stabilized grades (321, 347), material certified to ASTM A240, A312, A269, A213, and A276, sensitization testing per ASTM A262, and complete traceability from mill to delivery.

Contact CoreMetal Steel today for sensitization-resistant stainless steel products.