What intergranular corrosion is
Intergranular corrosion (IGC) is a localized attack that follows the grain boundaries of a metal while leaving the grain interiors largely intact. In austenitic stainless steel it is most often triggered by welding, which is why the field name is weld decay. The attack runs parallel to the weld in the heat-affected zone, undercutting the surface and, in severe cases, letting whole grains drop out. It is one of the principal failure modes covered in our overview of the common types of corrosion, and it is distinct from the chloride-driven pitting and stress-corrosion cracking that affect the same alloys.
Sensitization and chromium-carbide precipitation
The root cause is a metallurgical change called sensitization. Stainless steel resists corrosion because dissolved chromium forms a passive oxide film. When the steel is held in the critical range of 425 to 815 degrees C, carbon diffuses to the grain boundaries and reacts with chromium to form chromium carbides, chiefly Cr23C6. Because those carbides are rich in chromium, they draw chromium out of the thin band of metal next to the boundary.
The result is a chromium-depleted zone hugging each grain boundary. If that band falls below roughly 11 to 12 percent chromium, it can no longer sustain a stable passive film, so it corrodes preferentially in service while the grain centers stay protected. The higher the carbon content and the longer the time in the critical range, the more carbide forms and the deeper the depletion. This is why a weld, which heats the adjacent base metal straight through the sensitizing range as it cools, is the classic trigger. Welding filler choice interacts with this behavior, which we cover in weld decay and filler selection.
How carbon content drives the problem
Carbon is the single variable that decides whether a standard austenitic grade will sensitize during normal fabrication. Reduce the available carbon, or tie it up so it cannot reach the grain boundaries, and chromium stays in solution where it belongs.
| Grade type | Examples | Carbon approach | Sensitization resistance |
|---|---|---|---|
| Standard austenitic | 304, 316 | Normal carbon | Susceptible in the welded or heat-affected condition |
| Low-carbon (L) grade | 304L, 316L | Carbon held to 0.03% maximum | Resists sensitization in most welded fabrication |
| Stabilized grade | 321, 347 | Titanium (321) or niobium (347) addition | Resists sensitization, including sustained high-temperature service |
How L-grades prevent weld decay
The low-carbon grades solve the problem at the source. By capping carbon at 0.03 percent maximum, grades such as 304L and 316L leave too little carbon available to form a continuous, chromium-depleting carbide network during the short thermal cycle of welding. The grain boundaries stay chromium-rich, and the fabrication keeps its corrosion resistance without a post-weld heat treatment.
This is exactly why specifiers step down from a standard grade to its L variant for welded work. The trade-off is a small reduction in room-temperature strength, which is rarely limiting. The 304 vs 304L and 316 vs 316L comparisons lay out where each pays off. For the welding practice itself, see our guides to welding 304L and welding 316L. A fuller treatment of the prevention toolbox sits in how to prevent sensitization.
How stabilized grades prevent weld decay
Stabilized grades take a different route. Instead of starving the steel of carbon, they add a strong carbide-forming element that grabs the carbon before chromium can. In 321 that element is titanium, and in 347 it is niobium. These elements form stable titanium or niobium carbides, leaving chromium free to maintain the passive film.
The practical advantage is at elevated temperature. An L-grade protects against the brief thermal excursion of welding, but if a component spends long periods in the 425 to 815 degrees C range in service, low carbon alone may not be enough. Stabilized grades hold up under that sustained exposure, which is why they are favored for hot exhaust, boiler, and high-temperature chemical equipment. The same stabilizing logic appears in nickel alloys such as titanium-stabilized Incoloy 825.
Solution annealing: the recovery option
Sensitization is not always permanent. Solution annealing reheats the steel above the carbide-formation range, typically to dissolve the chromium carbides back into solution, then quenches it fast enough to prevent them reforming. This restores a uniform chromium distribution and recovers full intergranular resistance. It is the standard supply condition for austenitic plate and sheet to ASTM A240, and the mechanism is covered in detail in heat treatment and solution annealing. For large welded assemblies, however, a full post-weld solution anneal is often impractical, which is why choosing an L-grade or stabilized grade up front is usually the better engineering decision.
Testing for susceptibility: ASTM A262
ASTM A262 is the reference method for detecting susceptibility to intergranular attack in austenitic stainless steel. It defines several practices, from a fast screening etch to severe multi-hour immersion tests, so a buyer can match the acceptance test to the service environment.
| Practice | Medium | Role |
|---|---|---|
| A | Oxalic acid etch | Rapid screening of grain-boundary structure |
| B | Ferric sulfate, sulfuric acid | Quantitative weight-loss evaluation |
| C | Nitric acid (Huey) | Severe test for highly oxidizing service |
| E | Copper, copper sulfate, sulfuric acid (Strauss) | Common bend-test acceptance for welded material |
Specify the required practice on the purchase order so the acceptance criterion is unambiguous, and ask for the result to appear on the test documentation. Every order from Nandini Steel ships with an EN 10204 3.1 or 3.2 mill test certificate, and IGC testing to the agreed practice can be included where the application calls for it. Related documentation standards are set out under EN 10204.
Choosing the right grade
For most welded fabrication in ambient and moderate-temperature service, a low-carbon L-grade is the straightforward, cost-effective defense against weld decay. Where the part will see sustained high temperature, a stabilized 321 or 347 is the better fit. When corrosion is driven by aggressive chlorides or acids rather than carbide precipitation alone, the decision moves toward higher-alloy austenitic and super-austenitic grades. Our grade selector and how to prevent sensitization guide help narrow the choice before you request material.
Frequently asked questions
What is weld decay in stainless steel?
At what temperature does sensitization occur?
Do L-grades completely prevent intergranular corrosion?
What is the difference between L-grades and stabilized grades like 321 and 347?
Which ASTM A262 practice should I specify?
Can sensitized stainless steel be repaired?
Request a quote
Nandini Steel holds 304L, 316L, 321, 347, and the full austenitic range ex-stock for immediate dispatch, each supplied with an EN 10204 3.1 or 3.2 mill test certificate. For grade advice, ASTM A262 testing, or live pricing, contact our team or send an RFQ on WhatsApp at +91 99202 09184.