Sensitization in Stainless Steel Explained

Sensitization is the metallurgical precondition behind most intergranular corrosion failures in austenitic stainless steel: chromium carbide precipitation at grain boundaries that locally strips away the chromium responsible for passivation. This guide explains the precipitation mechanism, the sensitizing temperature range and its time dependence, where sensitization concentrates in welded structures, how it is tested per ASTM A262 and EPR methods, and how grade selection and heat treatment prevent it.

What Is Sensitization?

Austenitic stainless steel relies on a minimum of roughly 10.5–12% chromium in solid solution to sustain the thin, self-healing passive chromium oxide film responsible for its corrosion resistance. Sensitization compromises this locally, at grain boundaries, without necessarily affecting the bulk composition of the steel at all: it is a grain-boundary phenomenon, not a bulk metallurgical change, which is exactly why sensitized material can appear completely normal by standard mill certification chemistry and mechanical testing.

Mechanism: Chromium Carbide Precipitation and Chromium Depletion

When austenitic stainless steel is held within the sensitizing temperature range, carbon — present even at low levels in standard grades — diffuses to grain boundaries and combines with chromium to precipitate chromium-rich M₂₃C₆ carbides (predominantly Cr₂₃C₆). Because chromium diffuses far more slowly through the austenite lattice than carbon does at these temperatures, the carbide draws its chromium from the immediately adjacent matrix faster than chromium from further away can diffuse in to replenish it. This creates a narrow zone directly bordering the grain boundary that is depleted below the passivation threshold, even though the bulk chromium content of the steel, measured away from the boundary, remains fully within specification. The result is a continuous, corrosion-susceptible network that traces the entire grain boundary structure.

23 Cr (from adjacent matrix) + 6 C (diffusing to boundary)
                     →  Cr23C6 (grain boundary precipitate)

Net effect: local Cr depletion in matrix bordering the
boundary, below the ~12% threshold required to sustain
the passive chromium oxide film

The Sensitizing Temperature Range

Sensitization is strongly time-and-temperature dependent, following a classic time-temperature-sensitization (TTS) C-curve: at any given temperature within the sensitizing band, there is a minimum exposure time below which insufficient carbide forms to create a continuous depleted network, and this minimum time varies sharply with temperature.

Where Sensitization Occurs: Weld Decay and Knife-Line Attack

Welding is the single most common cause of sensitization in service, because the thermal cycle of every fusion weld in austenitic stainless steel inevitably drives some portion of the surrounding material through the sensitizing range.

Weld Decay

The heat-affected zone immediately adjacent to the fusion line reaches very high peak temperatures, well above the carbide dissolution temperature, and cools quickly enough to avoid significant resensitization. A band located somewhat further from the fusion line, however, reaches only the 425–815°C sensitizing range and can dwell there long enough during cooling to sensitize. This produces the classic weld decay pattern: a corroded band running parallel to, but visibly offset from, the weld itself, while the fusion zone and immediately adjacent HAZ, and the unaffected base metal further out, remain unattacked.

Knife-Line Attack

Stabilized grades such as 321 (Ti-stabilized) and 347 (Nb-stabilized) are formulated specifically to resist weld decay, but they remain vulnerable to a narrower, more localized failure mode called knife-line attack. Immediately at the fusion line, peak welding temperature is high enough to dissolve the stabilizing titanium or niobium carbides back into solution. If the material is subsequently exposed to the sensitizing range again — during a multi-pass weld, a stress-relief heat treatment, or service exposure — before the stabilizing elements can reprecipitate their more stable carbides, ordinary chromium carbides can form instead, sensitizing a narrow band precisely at the fusion line.

Consequences: Intergranular Corrosion and Cracking

Sensitized material is not itself failed material; the chromium-depleted network only becomes a problem when exposed to an environment capable of preferentially attacking it. In such environments, sensitization is the metallurgical precondition for intergranular corrosion, where the continuous depleted network at grain boundaries corrodes selectively while the grain interiors remain largely intact, and for intergranular stress corrosion cracking, where an applied or residual tensile stress combines with the weakened boundary network to produce cracking that follows the grain boundary path. A well-documented industrial variant, polythionic acid stress corrosion cracking, affects sensitized austenitic components during shutdown when sulfide scale on the surface reacts with air and moisture to form polythionic acids that attack the chromium-depleted boundaries under residual or applied stress.

Preventing Sensitization

Testing for Sensitization: ASTM A262

ASTM A262 defines a family of standardized practices for detecting susceptibility to intergranular attack, selected according to grade, expected service environment, and whether a quick screening result or a rigorous quantitative measurement is required.

EPR (Electrochemical Potentiokinetic Reactivation) Testing

The EPR test, including the widely used double-loop DL-EPR variant, offers a nondestructive alternative to acid immersion testing. Rather than a pass/fail result, EPR measures an electrochemical reactivation charge directly related to the area fraction and continuity of chromium-depleted grain boundary, yielding a quantitative degree of sensitization (DOS) value. This makes EPR particularly useful for comparing relative sensitization severity across specimens, tracking the effectiveness of a repair solution anneal, or assessing in-service components where destructive sample removal is impractical.

Practical Guidance for Welding Engineers

• Default to L-grade base metal and matching low-carbon filler metal for any welded austenitic stainless steel component intended for aggressive or intergranular-attack-prone service.
• For stabilized grades, verify post-weld thermal history does not reintroduce a knife-line attack risk, particularly for multi-pass welds or subsequent stress-relief heat treatments.
• Control interpass temperature and heat input to minimize time spent by the HAZ within the 425–815°C range during multi-pass welding.
• Specify the correct ASTM A262 practice, matched to actual service environment, in welding procedure qualification requirements rather than defaulting to a generic screening test.
• Where destructive sampling is impractical on an in-service component, consider EPR testing as a quantitative, nondestructive alternative.

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