**2. Experimental procedure**

The failure analysis was performed on the steam generator tube in different areas with circumferential cracks previously detected using Eddy current and UT technique. Three areas were characterized on the surface of the tube, called: the free tube, transition zone, and expanded zone (**Figure 2**). Using the stereo microscope, the area of interest was identified and isolated by a silicon carbide disc cutter. The cutting procedure was dry, without refrigeration, so as not to disturb the composition of the existing deposits on the external surface.

The characterization of the deposits was performed by EDX, XPS, and XRD. These deposits or sludge frequently accumulate in the area of the tube sheet.

The characterization of the fracture surface was performed by longitudinally cutting the tube into two equal half parts. One of them was tested for fatigue to obtain the fracture surfaces (**Figure 3**).

The fracture surface was examined on SEM to quantify the length and depth of existing cracks. The deposits found on the fracture surface of the two cracks were analyzed by EDX and by Auger spectroscopy, five zones of each crack from OD, (point 1) to inside the crack (point 5) (**Figure 4a**).

Metallographic characterization of the axial sections was carried out on the other half of the tube piece as shown in **Figure 4b**.

In addition, the microstructural characterization of the hard sludge steam generator has been carried out. The samples to be examined were collected during the last cycles and appeared in form of deposits or sludge and accumulate in the area of the tube sheet. All samples were weighed and dried at 60° for 48 hours and then classified by sieve size into three types: dust (D), small size (SS), and large size (LS). Microstructural and physical characterization of each individual particle was also performed using SEM and EDX analysis, stereomicroscope, and microhardness.

#### **Figure 2.** *As-reception segment tube. showing deposits analysis areas.*

**Figure 3.** *Devices used to obtain the fracture surface.*

**Figure 4.**

*a) Points analyzed on the fracture surface by Auger spectroscopy. b) Metallographic specimens from the other half of the tube in the area of interest.*
