Intergranular Corrosion and Weld Decay in Stainless Steels

Intergranular corrosion (IGC) is the selective electrochemical dissolution of a narrow zone immediately adjacent to austenite grain boundaries in stainless steel, driven by localised chromium depletion caused by chromium carbide (Cr₂₃C₆) precipitation during exposure to temperatures between 450 and 850 °C. When this sensitisation occurs in the heat-affected zone of a weld, the resulting attack is called weld decay — one of the most industrially significant and insidious failure modes in austenitic stainless steel fabrications, because the corroded structure can look entirely sound on the outside while grain boundaries have been dissolved through the wall thickness. This article provides a thorough technical treatment of the sensitisation mechanism, chromium depletion kinetics, time-temperature-sensitisation diagrams, weld decay and knife-line attack, ASTM A262 testing methodology, and all available material and process mitigation strategies.

The Sensitisation Mechanism

Austenitic stainless steels owe their corrosion resistance to a self-healing, adherent passive film of hydrated chromium oxide (Cr₂O₃·nH₂O) that forms spontaneously on the metal surface. This passive film requires a minimum bulk chromium content of approximately 10.5–12 wt% to be thermodynamically stable and self-repairing. Sensitisation destroys this condition locally at grain boundaries, creating a microstructural weakness that is invisible to the naked eye and can only be reliably detected by chemical testing.

Chromium Carbide Precipitation Reaction

Standard austenitic stainless steels (e.g., 304 with 0.04–0.08 wt% C) contain carbon in supersaturated solid solution at room temperature after solution annealing. When the steel is re-heated to 450–850 °C — the sensitisation range — the supersaturated carbon becomes thermodynamically unstable and preferentially precipitates with chromium at grain boundaries, forming chromium carbide:

Precipitation reaction:
  23Cr  +  6C  →  Cr₂₃C₆

  Cr₂₃C₆ forms preferentially at:
    · Austenite grain boundaries    (highest diffusion rate path)
    · Twin boundaries               (lower energy; less preferential)
    · Incoherent second-phase boundaries (sigma, delta-ferrite)
    · Dislocation tangles           (cold-worked steel only)

Thermodynamics (Gibbs energy of formation):
  ΔG°_Cr23C6 ≈ −135 kJ/mol at 700°C  (highly stable carbide)
  ΔG°_TiC     ≈ −155 kJ/mol at 700°C  (more stable → basis for 321 stabilisation)
  ΔG°_NbC     ≈ −145 kJ/mol at 700°C  (more stable → basis for 347 stabilisation)

Carbon activity (ac) relative to Cr₂₃C₆ stability:
  High carbon content (0.08%) → high ac → faster precipitation
  Low carbon (0.03%, L-grade) → lower ac → precipitation 10–30× slower

The Chromium Depletion Zone

Because chromium must diffuse from the grain interior to the boundary to supply the growing Cr₂₃C₆ precipitate, a chromium-depleted zone forms on both sides of the boundary. The critical feature is the relative diffusion rates of chromium and carbon in austenite: carbon diffuses approximately 100× faster than chromium in the FCC austenite lattice at sensitisation temperatures. Carbon therefore redistributes almost instantaneously to grain boundaries, while chromium replenishment from the grain interior lags far behind. The result is a sustained chromium depletion zone that can persist for hours at moderate temperatures.

Diffusivity comparison in austenite at 700°C:
  D_C^γ  ≈ 1.5 × 10⁻¹²  m²/s   (carbon in austenite)
  D_Cr^γ ≈ 1.5 × 10⁻¹⁷  m²/s   (chromium in austenite)
  Ratio:  D_C / D_Cr ≈ 10⁵ at 700°C

Chromium depletion zone width (estimate):
  x ≈ 2√(D_Cr × t)
  At 700°C, t = 60 s:  x ≈ 2√(1.5×10⁻¹⁷ × 60) ≈ 60 nm
  At 700°C, t = 1 hr:  x ≈ 2√(1.5×10⁻¹⁷ × 3600) ≈ 470 nm

Minimum chromium for passivity:
  Bulk %Cr ≥ 12 wt% required for stable passive film
  Depletion trough: %Cr can fall to 8–10% in severely sensitised grain boundary zone
  Electrochemical consequence: E_corr of depleted zone ≈ 200–400 mV more negative
  than bulk → strong galvanic driving force for preferential dissolution

Time-Temperature-Sensitisation (TTS) Diagrams

The TTS diagram (also called a sensitisation C-curve or sensitisation nose diagram) maps the boundary between sensitised and unsensitised conditions as a function of temperature and time at temperature. It is the stainless steel analogue of the TTT (time-temperature-transformation) diagram used for steel heat treatment. The shape is always a C-curve because:

• High-temperature arm (above 750 °C): thermodynamic driving force for Cr₂₃C₆ precipitation decreases as the solubility limit for carbon in austenite increases; long times required.
• Nose (~600–700 °C): maximum precipitation rate because both the thermodynamic driving force and the diffusion rate are simultaneously at practical values; sensitisation can occur in seconds for standard-carbon grades.
• Low-temperature arm (below 550 °C): diffusion of both carbon and chromium is too slow for significant carbide precipitation; even weeks of exposure do not cause detectable sensitisation below 425 °C.

The nose of the C-curve shifts to longer times (to the right) as carbon content decreases. For standard 304 (0.06 wt% C), the nose time is approximately 30–60 seconds at 675 °C. For 304L (0.03 wt% C), the same degree of sensitisation requires approximately 10–30 minutes. This shift is why L-grades are acceptable for most welding applications: weld HAZ cooling rates pass through the sensitisation range faster than the L-grade nose time in typical structural thicknesses.

Weld Decay

Weld decay is intergranular corrosion that occurs in the heat-affected zone of a weld in unsensitised standard-grade austenitic stainless steel. It is called “decay” because the corrosion appears to eat away the metal adjacent to the weld during service, often after months or years — long after the fabrication is complete and visually inspected. The term is vivid and historically established; it appears in Fontana and Greene’s Corrosion Engineering (1978) and remains the standard descriptor in engineering practice.

Location and Mechanism in the HAZ

During welding of standard 304 or 316 plate, the temperature distribution across the HAZ is highly non-uniform. Consider a single-pass weld on 10 mm plate with moderate heat input:

• Fusion line (0–2 mm): Peak temperature >1400 °C; all carbides dissolved; metal returns from this temperature through the sensitisation range rapidly due to high thermal gradient; typically insufficient time for sensitisation unless very high heat input is used.
• Inner HAZ (2–4 mm from fusion line): Peak temperature 1000–1400 °C; carbides dissolve; cools rapidly through sensitisation range; generally not sensitised at typical welding heat inputs.
• Weld decay zone (4–10 mm from fusion line): Peak temperature 450–850 °C — exactly the sensitisation range. The metal is heated into the sensitisation range, held there as the weld pool advances and the thermal wave spreads, and then cools slowly enough (especially in thick sections) to allow Cr₂₃C₆ precipitation. This is the weld decay zone.
• Unaffected base metal (>10–15 mm): Temperature never reached 450 °C; no sensitisation.

Weld Decay in Service

Weld decay manifests as a band of corrosion damage parallel to and set back from the weld bead, typically 3–10 mm from the visible fusion line. In aggressive environments (concentrated acids, hot chloride solutions, nitric acid), grain boundaries in the weld decay zone dissolve, causing individual grains to fall out — a phenomenon called grain dropping. Structural integrity of the weld joint is destroyed while the weld metal and unaffected base metal remain sound. Detection requires metallographic examination, ASTM A262 testing, or in-service non-destructive inspection (radiography to detect thickness loss, or electrochemical mapping).

Knife-Line Attack

Knife-line attack (KLA) is a narrow, sharp-edged form of intergranular corrosion that occurs immediately adjacent to the fusion line in stabilised austenitic grades (321 containing titanium, 347 containing niobium). It is effectively the opposite problem to weld decay: the stabilising additions that prevent bulk sensitisation create a new vulnerability immediately at the fusion boundary.

Mechanism of Knife-Line Attack

During welding, the base metal immediately adjacent to the fusion line experiences peak temperatures above approximately 1250–1300 °C. At these temperatures, the titanium carbides (TiC) or niobium carbides (NbC) that normally prevent chromium carbide formation are dissolved back into solid solution — their Gibbs free energy of formation becomes insufficient to maintain stability at these extreme temperatures. The stabilising effect is temporarily destroyed in this narrow zone.

On cooling from the weld, the dissolved titanium and niobium are in solid solution, but they require time to reprecipitate as TiC or NbC. If the steel subsequently passes through or is held in the sensitisation range — either during a post-weld stress relief (PWHT) at 600–750 °C, or during service at elevated temperature — the carbon is free to precipitate as Cr₂₃C₆ in this narrow de-stabilised zone before TiC or NbC can reform. The result is a razor-thin (<1 mm wide) sensitised zone immediately at the fusion boundary that is invisible to visual inspection and can only be detected by ASTM A262 testing or careful metallographic examination with selective etching.

Sensitisation in Other Alloy Systems

While austenitic stainless steels are the most discussed, sensitisation and IGC occur in other alloy systems by analogous mechanisms whenever a precipitate preferentially depletes a passivity-maintaining element from grain boundary regions.

Ferritic Stainless Steels

Ferritic stainless steels (e.g., 430, 444, 446) sensitise far more rapidly than austenitic grades because chromium diffusivity in BCC ferrite is approximately 100× higher than in FCC austenite at equivalent temperatures, but carbide precipitation is also faster, and the sensitisation range is broader (400–900 °C). More critically, ferritic stainless steels also precipitate chromium nitrides (Cr₂N) in addition to Cr₂₃C₆, making sensitisation doubly difficult to prevent. Welding a ferritic stainless steel sensitises the entire HAZ almost instantaneously. Prevention requires either ultra-low interstitial (ULI) grades (combined C + N < 0.03 wt%, e.g., grade 444 UNS S44400) or post-weld annealing above 900 °C with rapid quenching. Ferritic stainless weld metal typically has poor toughness and corrosion resistance in the as-welded condition regardless of grade.

Duplex Stainless Steels

Duplex stainless steels (2205, 2507) are significantly more resistant to sensitisation than austenitic grades for three reasons: (1) their low carbon content (typically <0.02 wt% C) limits the available carbon for carbide precipitation; (2) chromium diffuses approximately 100× faster in the ferrite phase than in austenite, rapidly replenishing any depletion zone; and (3) the two-phase ferritic-austenitic microstructure interrupts grain boundary continuity, preventing the formation of a continuous sensitised network. However, duplex steels are susceptible to sigma-phase precipitation at 600–1000 °C and 475 °C embrittlement, both of which involve chromium partitioning to precipitates and can degrade corrosion resistance, though by different mechanisms from classical IGC.

Aluminium Alloys (Sensitisation of 5xxx Series)

The 5xxx-series aluminium alloys (Al-Mg, e.g., 5083, 5052) experience a form of sensitisation when exposed to temperatures above 65–80 °C for prolonged periods. The magnesium-rich intermetallic Al₃Mg₂ (beta phase) precipitates at grain boundaries, creating a narrow aluminium-depleted, magnesium-rich zone that is anodic to the grain interior and preferentially corrodes in chloride environments — a failure mode called stress corrosion cracking (SCC) in sensitised aluminium-magnesium alloys. This is a significant concern for marine structures (ship hulls, offshore platforms) using thick 5083 plate that has experienced temperature excursions above the sensitisation threshold during service or storage.

ASTM A262 — Standard Testing for Sensitisation

ASTM A262 (Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels) is the primary standard for qualifying material, welding procedures, and post-weld heat treatments for sensitisation resistance. It specifies six practices, each targeting a different service environment or alloy system.

Interpretation of ASTM A262 Results

Prevention and Mitigation Strategies

Four complementary approaches prevent intergranular corrosion in austenitic stainless steel. Selection depends on service temperature, section thickness, weld geometry, applicable codes, and cost.

Strategy 1 — Low-Carbon (L) Grades

The simplest and most economical prevention approach is to use 304L (UNS S30403) or 316L (UNS S31603) with a maximum carbon content of 0.03 wt%. Reducing carbon reduces the total amount of Cr₂₃C₆ that can form and shifts the TTS nose to longer times. For single-pass welds on plate up to approximately 25 mm thick with conventional heat input, 304L and 316L are resistant to sensitisation because the HAZ cooling rate is fast enough to pass through the sensitisation range before significant precipitation occurs.

Limitations: the L designation controls only carbon content at delivery. If the material is subsequently exposed to extended time in the sensitisation range (e.g., during furnace brazing, thick-section multi-pass welding with high interpass temperature, or stress relief PWHT), even 304L can eventually sensitise. The 0.03 wt% C limit simply buys more time, not infinite immunity.

Strategy 2 — Stabilised Grades (321 and 347)

Stabilised grades contain strong carbide-forming elements — titanium in 321 (UNS S32100) and niobium in 347 (UNS S34700) — that preferentially combine with carbon in preference to chromium. Because TiC and NbC are thermodynamically more stable than Cr₂₃C₆ over most of the sensitisation temperature range, the carbon is captured before it can form chromium carbide, and chromium remains uniformly distributed in solid solution. Stabilised grades are preferred for long-term service at 450–850 °C where even L-grades would eventually sensitise.

Stabilisation requirements:
  Grade 321 (Ti-stabilised):
    Minimum Ti content: Ti ≥ 5 × (C + N) wt%   [ASTM A240 requirement]
    Typical Ti: 0.40–0.70 wt% for C ≤ 0.08 wt%
    TiC precipitation range: 400–900°C (more stable than Cr₂₃C₆ at T < 1250°C)

  Grade 347 (Nb-stabilised):
    Minimum Nb content: Nb ≥ 8 × C wt%          [ASTM A240 requirement]
    Typical Nb: 0.60–1.00 wt% for C ≤ 0.08 wt%
    NbC precipitation range: 400–900°C

  Stabilisation ratio (SR):
    SR = (%Ti or %Nb) / (%C + %N for Ti-grades; %C only for Nb-grades)
    SR ≥ 5 for Ti (321): satisfactory resistance to sensitisation
    SR ≥ 8 for Nb (347): satisfactory resistance

  Stabilisation anneal (optional, improves KLA resistance):
    Heat to 900°C for 2h → allows TiC/NbC to precipitate uniformly
    before any Cr₂₃C₆ can form in the weld HAZ

Strategy 3 — Solution Annealing

Solution annealing (resolution annealing) dissolves all chromium carbide precipitates back into solid solution, restoring a uniform chromium distribution and full passive film integrity across all grain boundaries. It is the only remediation available for already-sensitised material.

Solution annealing procedure for austenitic SS:
  Temperature:  1050–1100°C
  Hold time:    1 h per 25 mm of section thickness (minimum 30 min)
  Quench:       Water quench immediately after hold
                (must cool through 850→450°C faster than TTS nose time)

  Minimum quench rate to avoid re-sensitisation:
    304 (0.06%C):  must cool from 850 to 450°C in < 3 min
    304L (0.03%C): must cool from 850 to 450°C in < 30 min
    316 (0.06%C):  must cool from 850 to 450°C in < 4 min

  Limitations:
    · Distortion risk on thin-wall or precision components from rapid quench
    · Scale formation on surface must be removed by pickling (HNO₃/HF bath)
      or mechanical abrasion before return to service
    · Not practical on large completed assemblies;
      prevention (L-grade or stabilised) is preferred at design stage
    · Verify restoration of corrosion resistance by ASTM A262 testing
      after solution annealing

Strategy 4 — Welding Procedure Controls

When standard 304/316 must be used (e.g., to match existing equipment for a repair), welding procedure controls can reduce but not eliminate sensitisation risk:

• Minimise heat input: use the lowest amperage consistent with fusion quality; stringer beads (no weave); maximise travel speed within code limits. Lower heat input reduces HAZ width and the time spent in the sensitisation range.
• Control interpass temperature: do not begin the next pass until the previous pass has cooled to <150 °C (maximum). This limits cumulative thermal cycling in the sensitisation zone.
• Use matching filler metal of L-grade: ER308L for 304/304L base; ER316L for 316/316L base. Even when joining standard-grade base metal, L-grade filler prevents the weld metal itself from becoming a sensitisation source.
• Back purge with argon: prevents oxidation of the root pass and maintains chromium content at the inside surface, which is critical for corrosion-exposed pipe internals.
• Consider back-step welding sequence: for multi-pass welds, a back-step sequence reduces cumulative heat soak at any given location in the HAZ.

Composition Requirements — Key ASTM A240 Grade Comparison

Failure Analysis — Identifying Intergranular Corrosion

When a stainless steel component fails in service and IGC is suspected, the following diagnostic approach is used:

1. Visual and macroscopic examination: Look for characteristic cracking or loss-of-wall pattern parallel to and set back from welds (weld decay), or razor-thin cracking immediately at the fusion line (knife-line attack). Grain dropping (individual grains falling from the surface) leaving a rough, granular texture is pathognomonic of advanced IGC.
2. Optical metallography with selective etching: Electrolytic etch in 10% oxalic acid (Practice A); examine for ditch structure. Glyceregia etch (HCl:HNO₃:glycerol) reveals sensitised grain boundaries as dark networks. Grain boundary attack depth can be measured at 200–500×.
3. Scanning electron microscopy (SEM) and EDS/WDS: High-magnification imaging of grain boundary precipitates; energy-dispersive or wavelength-dispersive X-ray spectroscopy to confirm chromium depletion in grain boundary zones. A chromium content below 12 wt% in the depleted zone on the EDS line scan confirms active sensitisation.
4. ASTM A262 corrosion testing on archive material or a section cut from the failed component (if uncorroded material remains): provides standardised quantitative data for failure analysis report and insurance/legal purposes.
5. Review fabrication records: Was the correct grade used (304L vs. 304)? Was interpass temperature controlled? Was post-weld heat treatment performed and at what temperature? Was solution annealing after fabrication required by the applicable code but not performed?

Frequently Asked Questions

Recommended References

Further Reading