How sensitization works
Austenitic stainless steels resist corrosion because of a chromium-rich passive film. When the steel is held in the sensitizing temperature range — during welding, slow cooling after heat treatment, or high-temperature service — carbon combines with chromium and precipitates as chromium carbides along the grain boundaries. The thin band of metal next to each boundary is left depleted in chromium and can no longer maintain a passive film. In a corrosive environment, those bands dissolve while the grains stay passive, and whole grains can eventually fall out.
On welded equipment, the attack typically appears as weld decay: a band in the heat-affected zone a short distance from the fusion line, where the metal spent enough time in the sensitizing range. Stabilized grades can suffer a narrower form, knife-line attack, immediately adjacent to the weld.
Where it shows up in plants
Intergranular corrosion matters in acidic and oxidizing process streams, in chemical and pulp equipment, and wherever sensitized stainless is exposed to an aggressive fluid. Sensitized grain boundaries also give a preferred path for intergranular stress corrosion cracking, for example in polythionic acid exposure during refinery shutdowns. Other alloy families, including aluminum alloys and nickel alloys, have their own intergranular mechanisms.
How it is prevented
| Approach | Principle |
|---|---|
| L grades (304L, 316L) | Low carbon content leaves too little carbon to form harmful carbide networks during welding |
| Stabilized grades (321, 347) | Titanium or niobium ties up carbon as their own carbides, protecting the chromium |
| Solution annealing | Heating to dissolve carbides, then rapid cooling, restores a sensitized part |
| Weld procedure control | Limiting heat input and interpass temperature reduces time in the sensitizing range |
How it is tested
ASTM A262 sets the practices for detecting susceptibility to intergranular attack in austenitic stainless steels. Practice A, an oxalic acid etch examined under the microscope, is a rapid screening test: an acceptable structure clears the material, while a suspect structure calls for one of the immersion tests in the same standard, such as the boiling nitric acid (Huey) test or the copper–copper sulfate–sulfuric acid (Strauss) test. The purchase specification names which practice applies and its acceptance criteria.