**3.4 Fracture Surface characterization**

The tube segment with the cracks was cut into axial two halves. In one of them, a fatigue process for obtaining the fracture surfaces was performed. This fracture was examined by SEM in order to measure the length and depth of the open cracks. Two fractures were identified and these were formed by small cracking linked between them by ductile ligaments. Dimensions of both cracks are summarized in **Table 7**.

Morphology of the fracture is shown in **Figure 7**. Crack A presents an intergranular morphology with several ductile ligaments. Maximum depth of the intergranular crack was 937 mm, 82% of the tube thickness. Crack B, also of intergranular


**Table 7.**

*Dimensions of the fracture surface of the two cracks.*

**Figure 7.** *Fracture surface with details of the intergranular crack and deposits morphology (A and B).*

morphology, is not complete, due to the initial cut to obtain the two halves. Maximum depth of this crack B is 55% of the tube thickness.

Intergranular morphology of the two fractures showed some different aspects between them (**Figure 7**). Fracture surface of crack B was covered by a continuous gelatinous film, while the fracture surface of crack A showed corrosion deposits only in the initial area, near the outer surface, while the remainder of the fracture was completely clean.

Significant differences in the EDX analyses of the two areas (A and B) of the fracture were not observed. Crack B has a higher sulfur average concentration than crack A. Occasional sulfur concentrations can reach 29.1% at crack B. Silicon concentrations are similar in both cracks while the chlorine concentration is higher at crack A. It should be noted that Na was not detected in any of the cracks. On the other hand, Zn is more frequently found at crack A than at crack B where only traces of this element were present. Small concentrations of barium have also been detected in the two fractures.

Metallographic characterization of selected cracks by stereoscopy microscopy was prepared in the thickness plane of the other half segment tube. **Figure 8** shows the morphology of the crack in the metallographic samples. The maximum depth detected

**Figure 8.** *Intergranular cracks in metallographic samples.*

was 210 μm, being in all cases of intergranular morphology. In one of them, it can be observed that the crack initiation is parallel to OD surface, possibly due to the existence of a hardened surface (shot peening process). In other areas, the cracks propagated with normal orientation to OD surface.

**Figure 9a** shows the sulfur distribution inside the cracks by x-ray mapping. In some cases, S seems to be associated with Ni or Cu and not with oxygen. It is very possible that these elements are in sulfide form. A Fe and Ni (Cr depleted) rich layer

can be observed in **Figure 9b**. Besides, Si and oxygen have been detected in very internal areas of the crack. Also, the presence of S is remarkable in this area.

Deposits located on the fracture surface of cracks A and B were analyzed by Auger spectroscopy. Analyses were performed in five areas of each crack from OD to the tip of the crack. Sputtering process was performed after prior analysis in these five areas in order to observe the concentration depth profile from fracture surface to base metal. **Figure 10** shows the profiles corresponding to point 1 (near OD surface) and point 5 (bottom or tip of the crack). **Table 8** summarizes the results of these analyses where it is indicated (in nm and with arrows) the enrichment and depletion of some elements at different depths.

**Figure 10.** *Auger profile concentration for O, Si, Cr, Fe, and Ni and Cr, Fe, and Ni. Areas 1 and 5 of crack A.*


#### **Table 8.**

*Profile concentration summary by auger spectroscopy on the five areas of crack a. up arrow (↑) indicates enrichment and down arrow (↓) depletion, relative to base material concentration.*

Concentration profiles at crack A (area 1), show a thicker silicate layer around 400 nm. This point is the closest to OD. In the other points, far from OD (area 5), the silicate layer is thinner (<100 nm). Ni enrichment is always detected at the five points of the analyses. Fe is always depleted in all cases, even though in the first layers this depletion is less pronounced. Cr is enriched in the first nm and after it is stabilized with a concentration corresponding to the base material or slightly depleted.
