**3.5 Sludge characterization**

After sub-samples separation, M (magnetite layered), C (collars), and F (flakes) pieces were identified visually to characterize macroscopic features such as size, shape, and color. **Figure 11** shows macroscopic photographs of deposit sub-samples. Typical magnetite pieces were present as small "gravel," generally on the order of a few millimeters in length. The colors of these pieces were grey with areas of orange tone and showed geometries and curvature compatible with the triangular shape of the space between tubes in SG tube sheet (**Figure 12**).

**Figure 11.** *Different morphology of deposits collected from steam generator.*

*Failure Analysis of Steam Generator Tubes DOI: http://dx.doi.org/10.5772/intechopen.106607*

#### **Figure 12.**

*Individual particle from tube-sheet SG. Dimensions fit the space between the tubes of tube sheet.*

#### **Figure 13.**

*Cross-section of magnetite layered particle. Optical and SEM images.*

Magnetite layered pieces of several mm of thickness presented very high hardness values (876 Hv maximum). On the other hand, "flakes" present moderate hardness values (533 Hv maximum). "Collars" showed hardness slightly lower and heterogeneous values.

Most of the magnetite deposits are part of the oxide layer formed on the low carbon steel alloy and its composition is Fe3O4. **Figure 13** shows the morphology in the cross-section of magnetite pieces by optical microscopy. The structure consists of alternate layers with greater or lesser porosity. In the layers with higher porosity is easy to distinguish magnetite crystals with loss of cohesion. SEM/EDX measurements were performed on the cross-section of the magnetite layered in order to quantify the concentration of the local elements at the different layers.

**Figure 14** shows the elemental distribution on the magnetite layers with enriched bands in Mn. Dot analysis shows only Fe and O in the compact layers while in the porous layers exists an enrichment of Mn, Ni, Si, and other elements in a lower concentration (**Figure 15**).

The total of these elements forming the enriched bands is only 2%, the rest is Fe and O. Additional analyses performed detected an increase in S and Ni with isolated precipitates of Cu (**Figure 16**).

As discussed at the start of this section, "collars" pieces were detected Spanish NPPs (**Figure 17**). Cross-sections of these samples are formed by a number of metallic inclusions embedded in a matrix of Al and Si species (**Figures 18** and **19**). These last elements were enriched preferably near the interface with SG tube surface. Si/Al ratio is higher when the silicates are the binding agents.

Regarding "flakes" or tube scales is only observed in some NPPs before CC treatment. These particles are not detected with subsequent FFA additions and after chemical cleaning treatment.

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

#### **Figure 14.** *Mapping of elements distribution in the cross-section of magnetite layered.*

#### **Figure 15.** *Dot EDX analysis in the cross-section of magnetite layered sample.*

Normally, flakes are formed by an inner layer in contact with the tubing surface and an outer layer with high porosity (**Figure 20**).
