Sensitization and Intergranular Corrosion in Stainless Steel
Weld a piece of standard 304 or 316 — and in the heat-affected zone (HAZ) next to the weld, the microstructure changes permanently. Chromium carbides precipitate at grain boundaries, depleting the surrounding metal of chromium. This sensitized zone is vulnerable to intergranular corrosion (IGC) — grain-by-grain attack that can cause catastrophic failure. Here's how it happens and how to prevent it.
The Mechanism: Chromium Depletion
When austenitic stainless steel is heated to 425-870°C (the "sensitization range"), carbon atoms diffuse to grain boundaries where they combine with chromium to form Cr₂₃C₆ (chromium carbide) precipitates. Each carbon atom consumes approximately 16 chromium atoms. The chromium-depleted zone adjacent to the grain boundary drops below the ~12% Cr threshold needed to maintain the passive film — making it susceptible to selective corrosion attack.
Why Welding Triggers Sensitization
During welding, the HAZ experiences exactly the 425-870°C temperature range where carbide precipitation is fastest. The time at temperature during a typical weld pass (seconds to minutes) is sufficient for sensitization in standard (non-L-grade) 304 and 316 with carbon content above 0.03%.
IGC Failure Mode
Intergranular attack is especially dangerous because it penetrates deep into the metal with minimal surface evidence. A pipe may look sound externally while grain boundaries throughout the HAZ have been corroded to a depth of several millimeters. Under stress, the pipe can separate along these weakened grain boundaries — a failure that looks brittle on examination but is actually corrosion-driven.
Three Solutions to Prevent Sensitization
| Solution | Mechanism | Examples | Best For |
|---|---|---|---|
| Low Carbon (L-grade) | Reduce C ≤ 0.03% — insufficient carbon to form significant Cr₂₃C₆ | 304L, 316L, 317L | General welded pipe: standard practice |
| Stabilization | Add Ti or Nb — these elements form carbides preferentially, leaving Cr in solution | 321 (Ti), 347 (Nb) | Elevated temp service above 425°C where L-grade strength drops |
| Solution Annealing | Heat to 1050-1150°C + water quench — dissolves carbides back into solution | Post-weld heat treatment | Heavy-section welds; when L-grade alone insufficient |
ASTM A262: The IGC Verification Standard
ASTM A262 provides five practices for detecting susceptibility to intergranular attack:
Key takeaway for procurement: When specifying stainless steel pipe, always require ASTM A262 Practice E if the pipe will be welded in service. If the pipe arrives pre-welded (spool fabrication), request Practice E on a welded coupon. L-grade alone (304L/316L) is usually sufficient — but verify with testing.