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Sensitization of Stainless Steel
STAINLESS SALES CORPORATION
Stainless Science Training
Why is stainless steel corrosion resistant?
Stainless steel is known for its corrosion resistance in many environments in which carbon and low alloy tool steels would corrode. During the fabrication of stainless steel components or structures it is possible to degrade the corrosion resistance of austenitic stainless steels, such as 304, by a metallurgical phenomenon called sensitization.
Sensitization occurs when stainless steels with high chromium content and more than 0.02% carbon are exposed to temperatures between about 425 °C (797 °F) and °870 °C (1598 °F). This can occur during welding or annealing after cold-working. If the exposure time is too long, the areas near the metal’s grain boundaries lose their corrosion resistance due to the formation of chromium carbide precipitates on the grain boundaries. If a sensitized alloy is exposed to a corrosive environment the areas near the grain boundaries will be preferentially attacked. As the corrosion proceeds the grains fall out and the metal loses strength.
Sensitization can be prevented by reducing the carbon content, adding stabilizers such as niobium (347 SS) or titanium (321 SS), or reducing the time of exposure to the critical temperature range. Sensitization can also occur in ferritic and duplex stainless steels, though the thermal profile that results in the formation of precipitates is different than for austenitic stainless steels.
Michael Pfeifer, Ph.D., P.E.
President, Industrial Metallurgists, LLC