Sensitization (stainless steel)

Sensitization is a material condition created by thermal history, not the corrosion damage itself. In susceptible austenitic stainless steel, chromium-rich carbides can precipitate at grain boundaries and leave nearby material depleted in chromium. A weld can create that condition in part of its heat-affected zone, depending on the alloy and the thermal cycle.

The closest neighbouring term is intergranular corrosion. Sensitization means the microstructure has become susceptible; intergranular corrosion is attack that actually develops along grain boundaries when that susceptible material meets a relevant environment. ASTM A262 is explicitly scoped to practices for detecting susceptibility to intergranular attack in austenitic stainless steels. Its scope also warns that results do not predict other corrosion forms or performance in every service environment, so an A262 result must not be stretched beyond the specified alloy, practice and service decision.

Worked example: condition or damage?

Take two sections from beside an austenitic stainless weld. Metallographic and specified corrosion testing show that section A has grain-boundary chromium depletion, but the joint has remained dry and shows no metal loss. Section A is sensitized; it does not yet exhibit intergranular corrosion.

Section B from the same thermal history has operated in an environment capable of attacking the depleted boundary regions, and examination shows material loss following those boundaries. Section B is sensitized and has intergranular corrosion. Calling both observations “corrosion” would erase the difference between susceptibility and damage; calling section B only “sensitized” would omit the damage already present.

For welding control, the heat input and interpass temperature records describe parts of the thermal history, but they do not by themselves prove absence of sensitization. The material grade, full heat history and the specified test or engineering assessment have to support that conclusion.