**3. Results**

#### **3.1 Visual inspection**

Circumferential cracks and denting were detected by Eddy's current technique before the extraction in the expanded transition zone. Three different areas were identified for subsequent tests: free tube, transition zone, and expanded zone (**Figure 5a**). Zero is also marked for recording observations collected along the tube perimeter.

Multiple circumferential small cracks separated by ductile ligaments in an area of 5 mm width are shown in **Figure 5b**. These cracks were located at 3 mm from the last contact point between the tube and the tube sheet. Damage as "wastage," pitting or other has not been detected. Different color is observed among these areas. The expanded region shows a red coloration (tube sheet contact) while the transition zone presents a dark gray color because this area has been covered by sludge.

#### **Figure 5.**

*a) As-reception segment tube extracted in the area of the sketch. b) Image analysis to detect the expansion end of the tube by stereoscopy microscopy.*

## **3.2 Deposits characterization on free tube under sludge.**

**Table 5** shows a comparative analysis of the deposits removed from the free tube surface. EDX results of sludge deposits, located in the expansion transition and extracted in the free tube, indicate a process of impurities concentration on the crevice area formed between the tube sheet and the tube. Significant concentrations of Si, Mg, Ca, S, Cu, Zn, Na, and Cl were detected. This process of impurity concentration is not as evident in the top of the sludge area where EDX analysis indicates that the detected impurities are present only in part of the analyses. Moreover, Pb has been found in low concentrations.

XPS and XRD results of these deposits showed the existence of metallic Cu and FeS2, which correspond to reducing environment of the typical operation secondary side in SG. Likewise, reduced sulfur species, such as sulfides or sulfites, were detected in the expansion transition zone and the area under sludge. Moreover, XPS has identified the presence of metallic Fe and Cr in the free tube under the sludge.

Other compounds identified are a mixture of silicates, mainly of Al, and Mg. Na is sometimes associated with Cl and S, although this association is qualitative by EDX analysis. XPS analysis has identified the presence of Ca (as Ca chlorides) and a mixture of metallic oxides with K and Mg. S is detected in both oxidized and reduced forms, like sulfates and sulfides.

A comparative XPS analysis of deposits at both free tube and expansion transition zone is summarized in **Table 6**. Fe is detected as magnetite and mixed spinels. Metallic Fe is present in the free tube deposits of the surface and is not detected in the transition area. Al-silicates with Mg, Na, CaCl2, and S, like sulfates and sometimes as sulfites, are detected in both areas.

### **3.3 Cracking characterization on OD side**

**Figure 6a** shows the cracks detected on the OD side of the tube. Cracks have a circumferential orientation with corrosion deposits and they were detected in an area around the tube located above the last point of contact between the tube and tube sheet. The highest density of the cracks was located at 5 mm of this last point. **Figure 6b** shows the intergranular morphology of the cracks at high magnification and the aspect of the corrosion deposits on the tube surface. EDX analyses show important amounts of Na, Si, S, and Cl in these areas. Also, the presence of Cu and traces of Pb and Zn were also detected in five analyses.


#### **Table 5.**

*Overview of EDX, XPS and XRD analyses on deposits removed from the free surface of the tube.*

#### *Failure Analysis – Structural Health Monitoring of Structure and Infrastructure Components*


*\*\* minority phase (mixed oxides)*

#### **Table 6.**

*XPS analysis on free tube under sludge and transition area.*

**Figure 6.**

*a) Circumferential cracking on OD transition zone. b) Detail of the intergranular morphology of the cracks.*
