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.