Sensitization happens when a stainless steel is held at an
intermediate temperature (typically ~425 - 815 degrees Celsius) and
chromium carbides precipitate at the grain boundaries. This
precipitation of chromium carbides at the grain boundaries happens
because the carbides are insoluble at these temperatures. In order
for the carbide to precipitate, it must get chromium from the
surrounding metal. This means that there is a chromium depleted
zone around the grain boundaries. It should be apparent that this
chromium depleted zone will be less corrosion resistant,
specifically to intergranular corrosion.
Sensitization is particularly important in metals that are
welded. This is because the welding zone experiences temperatures
in the range that can cause sensitization. This can cause localized
intergranular corrosion at the weld site.
