Intergranular Corrosion vs Pitting in Stainless Steel: Two Different Failure Modes, Two Different Remedies
Specifying the wrong stainless steel grade often results from conflating two distinct failure modes: intergranular corrosion (IGC) — caused by chromium depletion at grain boundaries after sensitisation — and pitting corrosion — caused by local passive film breakdown in the presence of chloride ions. The remedy for each is different. Solving one does not solve the other. Misunderstanding this distinction leads to over-specification in one dimension and under-protection in another.
Intergranular Corrosion: The Sensitisation Mechanism
When austenitic stainless steel is heated to 425–850°C — the sensitisation range — carbon migrates to grain boundaries and combines with chromium to form Cr₂₃C₆ carbides. The adjacent metal is depleted of chromium below the ~12% Cr threshold needed for passivation. In corrosive service the grain boundary becomes anodic to the grain interior — the metal corrodes preferentially along grain boundaries, eventually causing entire grains to fall out. This is intergranular corrosion (IGC), also called knife-line attack when it occurs in the narrow HAZ directly adjacent to a weld.
The remedies for IGC are: (1) use low-carbon grades (316L, C ≤0.030%, limits carbide formation); (2) use stabilised grades (321, Nb-stabilised 347, or Ti-stabilised 1.4571) where a more stable carbide-former ties up the carbon before Cr₂₃C₆ can form; or (3) post-weld solution anneal to redissolve carbides and restore uniform chromium distribution. Low-carbon and stabilised grades solve IGC — they do not improve pitting resistance.
Pitting Corrosion: The Chloride Mechanism
Pitting requires chloride ions, a passive film, and a sufficiently aggressive combination of temperature and Cl⁻ concentration to exceed the pitting potential. Once a pit initiates, the local chemistry inside the pit becomes acid and depleted in oxygen — creating an autocatalytic cell that drives further pitting. The metric for pitting resistance is PRE = %Cr + 3.3×%Mo + 16×%N. Pitting resistance is improved by increasing Cr, Mo, and N content — not by reducing carbon. A stabilised grade like 321 (C≤0.08%, no Mo) has exactly the same pitting resistance as standard 316 (PRE ~24) — switching from 316L to 321 does not improve chloride pitting performance at all.
Grade Selection Clarity
| Problem | Correct Remedy | Incorrect Substitution |
|---|---|---|
| IGC / sensitisation in HAZ | 316L (low C), 321 (Ti-stabilised), 347 (Nb-stabilised) | Duplex 2205 — higher PRE but still sensitises |
| Pitting in chloride service | Duplex 2205 (PRE ≥35), 904L (PRE ~33), Super Duplex 2507 (PRE ≥42) | 321 or 347 — stabilised but PRE same as 316 |
| Continuous sensitisation-range service (425–850°C) | 321 or 347 only — low C insufficient above 425°C long-term | 316L — insufficient for long-term high-temp exposure |
| Both IGC risk AND chloride pitting | 316L + high PRE (904L, 825) — address both independently | 321 alone — solves IGC only |
Polythionic Acid SCC — A Related IGC Mechanism
PTA-SCC (polythionic acid stress corrosion cracking) is a special form of intergranular attack that occurs in sensitised stainless steel during plant shutdown — sulphide deposits on the pipe surface react with oxygen and moisture to form polythionic acid (H₂SₓO₆), which attacks sensitised grain boundaries. The remedy is not low-carbon grade — it is stabilised grades (321, 347) that are immune to sensitisation even after extended service in the sensitisation range. 316L is not immune to PTA-SCC after long service at 450–600°C because Cr₂₃C₆ still forms slowly even at low carbon contents.