What causes intergranular corrosion? This type of attack
results from local differences in composition, such as
coring commonly encountered in alloy castings. Grain
boundary precipitation, notably chromium carbides in
stainless steels, is a well recognized and accepted
mechanism of intergranular corrosion. The precipitation of
chromium carbides consumed the alloying element - chromium
from a narrow band along the grain boundary and this makes
the zone anodic to the unaffected grains. The chromium
depleted zone becomes the preferential path for corrosion
attack or crack propagation if under tensile stress. The
photos above show the microstructure of a type 304 stainless
steel. The figure on the left is the normalized
microstructure and the one on the right is the "sensitized"
structure and is susceptible to intergranular corrosion or
intergranular stress corrosion cracking.
