## 3 Results and Discussion
### SEM Analysis of the Pitting Defects  
Two sub-samples from the section of the pipe received in the laboratory were obtained in order to carry out a scanning electron microcopy (SEM) analysis of the micromechanisms taking part in the pitting process analyzed here. One sub-sample contained the main trough-wall pitting (Figure 2a) and its surrounding area, and another sample contained an area where generalized pitting in an earlier stage of the process could be observed (Figure 3). Figure 7a shows an image obtained by means of SEM of the through-wall pitting of the sample (inner face). In this image, the areas in which the process has advanced in depth, connecting with the outer surface, can be clearly distinguished. Additionally, a second region adjacent to the perforations can also be observed (see Figure 7b), where a loss of material was clear to the naked eye. In this region the tube has lost part of the surface material and grain boundaries are noticeable, pointing to a process that advances preferentially along such grain boundaries. Finally, Figure 7c shows an area attached to the through-wall pitting where no metal loss was observed by the naked eye, but where an advanced level of damage at grain boundaries can be seen through the SEM image, supporting the previous observation about the preferential attack at the grain boundaries. Meanwhile, Figure 8a presents another pitting from the second sub-sample, which is in an earlier stage of the process. It can be observed that it is not a through-wall pitting. In this case, the observation at higher magnification, shown in Figure 8b, makes it possible to once again confirm the existence of the same type of processes that advances preferentially through the grain boundaries. As a first hypothesis to explain the corrosion processes progressing along the grain boundaries, it is worth considering the existence of chromium carbides precipitates at the grain boundary. However, the low carbon content of grade AISI 316L stainless steel makes the formation of these precipitates very difficult in practice (chemical composition analysis revealed 0.022% C content). In any case, to rule this out, a semi-quantitative chemical composition scan was carried out using SEM, with an example of the results found being shown in Figure 9 (chemical composition scan in a non-damaged area) and Figure 10 (chemical composition in a particular grain boundary within a damaged area). In all cases, no peaks of Cr were observed. If these analyses had detected abnormally low Cr percentages, they would have been indirectly indicative of chromium carbide precipitation at the grain boundary (the zone close to the precipitates becomes depleted of Cr to feed the carbides). With all this, steel sensitization (precipitation of chromium carbides at the grain boundary) can be ruled out as the main cause of the corrosion phenomena being analyzed. Finally, a SEM semi-quantitative analysis of the chemical composition of the corrosion products deposited in the sample was carried out in the area presented in Figure 3, where onset pitting and corrosion deposits can be observed. The results, which are presented in Figure 11, show the presence of sulfur (S) in non-negligible percentages, which cannot come \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Test** & **Required [22]** \\ \hline C & 0.022 \(\pm\) 0.005 & \(<\)0.03 \\ Si & 0.450 \(\pm\) 0.072 & \(<\)1.00 \\ Mn & 1.507 \(\pm\) 0.080 & \(<\)2.00 \\ P & 0.035 \(\pm\) 0.005 & \(<\)0.045 \\ S & \(<\)0.008 & \(<\)0.03 \\ Cr & 16.22 \(\pm\) 0.48 & 16.0–18.0 \\ Ni & 9.86 \(\pm\) 0.37 & 10.0–14.0 \\ N & 0.037 \(\pm\) 0.009 & – \\ Mo & 1.944 \(\pm\) 0.091 & 2.0–3.0 \\ Cu & 0.319 \(\pm\) 0.028 & – \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the stainless-steel sample (% weight). from the small amount of sulfur existing in the steel (see Table 1 with a <0.008% S content). Thus, necessarily, sulfur has been provided externally, quite likely by the corrosive agent.