Environmental Engineering Reference
In-Depth Information
Figure 3.17
Photomicrograph illustrating intergranular corrosion in 316 stainless
steel.
ly more active than the grain interior. This is reflected in the controlled etching
of the metallographic specimens whereby the grain boundaries are revealed.
An ''overetching'' equalizes the attack. However, under certain conditions the
grain boundaries remain very reactive and under corrosive conditions the attack
along
the
grain
boundary
is
sustained
resulting
in
intergranular
corrosion
(IGC).
The factors that contribute to the increased reactivity of the grain boundary
area are as follows:
1.
Segregation of specific elements or compounds at the grain boundary, as in
aluminum alloys or nickel-chromium alloys
2.
Enrichment of one of the alloying elements at the grain boundary, as in brass,
and
3.
Depletion of the corrosion-resisting constituent at the grain boundary, as in
stainless steels
All of these factors contributing to intergranular corrosion have their origin in
the thermal processing of the materials, such as welding, stress relief, and other
heat treatments.
3.6.1 Austenitic Stainless Steels
Intergranular corrosion of austenitic stainless steels poses a big industrial prob-
lem. Many failures have occurred in welded components exposed to environ-
