Figure 6 summarizes the result of analyzing samples of
corrosion product removed from the bottom surface of the pipe
using x-ray diffraction. The observed diffraction pattern is
compared with standard patterns of FeCl3, FeCl2, Fe2O3, and
FeO(OH). An example illustrating the result of analyzing the
corrosion product by energy dispersive x-ray spectroscopy in a
scanning electron microscope is shown in Fig. 7. Combining
the above results could lead to the conclusion that the corrosion
product consisted of mixed iron chlorides and oxides indicating
that the pipe was subjected to corrosion attack by chloride ions
in defective regions of the insulating coating. However, the
problem was further compounded by intergranular cracking
suggesting that the pipe was subjected to both pitting corrosion
and stress corrosion cracking. It is known that stress corrosion
cracking of austenitic stainless steels is initiated at the grain
boundaries (Ref 1). As pointed out earlier, this behavior could
be related to localized deformation alongside grain boundaries.
In this regard, examination of the deformation substructure in
the vicinity of the perforation by transmission electron
microscopy revealed features typical of a material with low
stacking fault energy as described below.
Bright-field TEM images illustrating characteristic deformation
substructures in various regions in the vicinity of the
perforation are shown in Fig. 8a. A common feature was the
observation of slip lines (traces of {111} slip planes) with
dislocations mostly confined to their original slip planes as
shown in Fig. 8a. This reflects a lower tendency for cross-slip
associated with low stacking fault energy. At higher magnifications,
extended nodes resulting from the intersection of three
partial dislocations of the type 1/6Æ112æ were observed as
shown in the example of Fig. 8b. These observations are
indicative of a material with high strain hardening rate, which is
expected in many austenitic stainless steels. Detailed microstructural
characterization down to the scale of transmission
electron microscopy showed no evidence for sensitization
particularly the presence of carbide precipitates at grain
boundaries. Therefore, it is possible that as a result of high
strain hardening rate, the grain boundaries became weaker than
the matrix leading to localized intergranular separation in the
presence of chloride ions.