Photographs illustrating a section of the pipe where perforation occurred are shown in Fig. 1. Corrosion product was observed at the outer bottom surface in the vicinity of the perforation; however, the internal surface was relatively clean.
This is further illustrated in the optical macrographs of Figure 2 indicating that the pipe was corroded by interaction between the bottom surface and the surrounding environment particularly in those regions where the insulating coating was damaged.
Figure 3 summarizes typical microstructure and composition of a sound section of the pipe. As shown in the secondary electron image of Fig.
3(a), the microstructure revealed by etching was comparable to that of 304 stainless steel in the annealed condition (Ref 10).
A corresponding energy dispersive spectrum illustrating the elemental composition of the pipe
is shown in Fig. 3(b) consistent with an alloy based upon the Fe-Cr-Ni system.
Figure 3(c) summarizes the results of measuring the chemical composition by inductively coupled plasma atomic energy spectroscopy (ICP-AES) in comparison with the nominal composition of 304 stainless steel verifying that the pipe was manufactured according to specifications.
Consistent with the above results, examination of a cross section of the pipe indicated that the tube wall thinning eventually leading to perforation was initiated from the outer
pipe surface as demonstrated in the secondary electron image of Fig. 4(a).
Evidence for intergranular cracking at the outer surface was observed in the vicinity of the perforation as shown in Fig. 4(b).
In contrast, there was no evidence for such cracking at the inner surface (Fig. 4c) suggesting that the cracking was also initiated at the outer surface of the pipe.