As shown in the example of Fig. 5, three types of grain structures could be distinguished throughout the pipe. A grain structure typical of annealed 304 stainless steel was observed in
sound sections as demonstrated in Fig.
5(a) (specimen was polished and etched). Figure 5(b) shows a grain structure of a section subjected to corrosion attack near the perforation.
Although the specimen was examined in the as-polished condition, the structure appears to be typical of a heavily etched material.
Near the edges of the perforation at the outer surface, a structure such as that shown in Fig. 5(c) was observed in the as-received condition (unpolished and unetched), which typifies a fracture surface produced by intergranular separation.
This observation is consistent with that of Fig. 4(b) showing intergranular cracking around the perforation at the outer pipe surface. Near the perforation, the average microhardness was measured to be HV 185, however, in far removed regions; the average microhardness was HV 162
comparable to that of annealed 304 stainless steel.
Evidently,