**4. Failure analysis. Discussion**

Alloy 800NG steam generator tubes are a composition modified austenitic stainless steel for use as steam generator tubes in PWRs. The main modifications concern the carbon content, C<0.03% by weight, and the Ti/C>12 ratios to avoid sensitization of

**Figure 16.** *Mapping of elements distribution in the cross-section of Magnetite layered.*

**Figure 17.** *Morphology of "collars" removed from NPPs.*

the material. The final stage during the tube manufacturing process is a shot-peening treatment of its external surface in order to prevent stress corrosion cracking mechanisms. The shot-peening process creates an outer layer with high plastic deformation of the surface that prevents crack propagation under SCC conditions. Circumferential cracks in the expansion transition were detected before extraction of the analyzed tube by Eddy's current inspection besides "denting" degradation and a height of sludge, about 46mm.

Destructive examination of the expansion transition zone has revealed a 5 mm wide band with multiple circumferential cracks separated by small ductile ligaments. These cracks were located 3 mm from the last point of contact between the tube and the tube sheet. No other type of damage such as "wastage," pitting or others has been detected.

#### **Figure 18.** *Cross-section collar. Spatial distribution of chemical elements by SEM-EDX.*

Two cracks have been characterized during the fractographic examination both of intergranular morphology, which at first glance presents different aspects. Crack B's fracture surface was completely covered by a viscous morphology film while crack A was covered with deposits in the initial area, near the external surface of the tube, the rest of the fracture was apparently clean.

OD-initiated corrosion of circumferential cracks in SG tubes has been described by different authors [7]. Destructive examination performed by EDF [8] found circumferential and longitudinal cracks in forty tubes examined. Circumferential cracks were located below the top of the tube sheet, while axial cracks were detected above the tube sheet under a sludge layer. The existence of "denting" was not explicitly mentioned.

Extraction of deposits from the expansion transition zone and free tube, next higher area, were compared by EDX analysis. These results indicated a process of impurities concentration in the gap between the tube sheet and the tube. Si, Mg, Ca, S, Cu, Zn, Na, and Cl were detected in this area but these concentrations were lower in the upper part of the sludge zone. It should be noted that traces of Pb have been found in some of the EDX analyses carried out on the initial area of only one of the analyzed cracks. Some authors suggest that alloy 800 could be more susceptible to SCC in alkaline solutions with lead (Pb) [9].

XPS and DRX of the extracted deposits revealed the existence of metallic Cu and FeS2, typical water chemistry of secondary side SG. Likewise, reduced sulfur species such as sulfides or sulfites have been detected in the expansion transition zone and

**Figure 19.** *Cross-section collar. Spatial distribution of chemical elements by SEM-EDX.*

under the sludge. On the other hand, XPS analyses detected the presence of Fe and Cr metallic in the free tube deposits under the sludge. In addition, there is a need to clarify the species reduction that could be produced due to ionic sputtering carried out for the analysis. However, metallic Fe has also been identified in the existing deposits in the expansion transition zone, in which ion sputtering was not carried out.

EDX analyses performed on the fracture surface of the A and B cracks confirmed the presence of impurities inside the cracks. Crack B fracture surface was completely covered by a film rich in silicates with discrete particles very rich in sulfur, besides the impurities mentioned above. However, the analyses performed in crack A revealed only deposits in the initial area of the fracture, which did not differ from those analyzed in crack B, except that they had a lower concentration of sulfur. The presence of impurities inside the cracks is also confirmed by the EDX analyses carried out inside the cracks prepared for metallographic characterization.

Silicon compounds, like silica or silicates, were continuously detected on different areas of the external surface tube and over fracture surfaces of the cracks. Many scientific papers describe the silicate's role influence in the SCC susceptibility through intergranular mechanism on the SG secondary side. Failure analyses in tubes of French plants described Al-silicate deposits over a chromium-rich brittle layer associated with corrosion damage. The presence of Al-silicates confirms that cracks formation has not been produced in strong alkaline media where this silicate is not stable

**Figure 20.** *Cross-section of "flake."*

(pH>10 at 300°C). EPRI (Rev.7 of the secondary guides) notes that when Si/Al ratio decreases the response of alloy 600 to IGA/SCC is worse [10]. In addition, a threshold value 2 of Si/Al ratio could produce an IGA/IGSCC increase for alloy 600 MA.

In the case of alloy 800NG, deposits formation of silicates depends on microstructure. Tubes 800 NG are manufactured of Fe-base alloy. Oxide layer of this alloy expose to secondary water of PWR present a strong affinity for silica [11] and iron, even nickel can perform a cation exchange reaction with this element. Other elements such as Ca, Mg, and other alkalines can form mixed silicates producing local acid in the environments and contributing to increasing corrosion mechanisms due to the formation of protons. This mechanism could perform under the deposits in contact with an external surface of the tube due to the inhibitory capacity of silica as is demonstrated in the results of plants.

However, the silicate's role is unknown in the SCC of Incoloy 800. In fact, laboratory results show that the formation of the Al-silicate layers depends on the microstructure. These layers are very thin when they are formed on rich-chromium film, thicker on rich-nickel film, and 20–30 times thicker on rich-Fe film. Iron is the major element in Incoloy 800 it has a strong affinity for silica. In this way, iron can migrate with the Al-silicate by cation exchange reaction. Similar behavior can be attributed to nickel but this mechanism is not observed for the chromium. Meanwhile, calcium is fixed on the surface of silicates. This type of reaction can produce local acid in the environment under the deposits and contribute to material dissolution [8].

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

Furthermore, silica is considered an element with inhibitory capacity. This remark is based on the results of plants where lower levels of IGA/SCC were produced when silica above 40 ppb was detected during the blowdown [5].

On the other hand, many experiments have been carried out related to the effect of sulfur in different alloys for steam generator tubes in the middle of the secondary with the aim of carrying out comparative tests between alloys 600, 690, and 800. Most of the tests in the laboratory cannot be extrapolated to plant conditions, but they provide a study of the resistance of these alloys in a certain medium with sulfur species, such as sulfates in an acid medium. For example, alloy 800 was susceptible to IGSCC in a sulfate acid environment (pH = 4, at room temperature). This susceptibility was increased when the pH was lower. On the other hand, the alloy 600 produced wastage in an acid sulfate environment as was demonstrated in tests performed at CIEMAT labs in "model boiler test" [12].

The behavior of alloy 800 NG secondary water chemistry of PWR with sulfates in a neutral or slightly alkaline environment has been studied by Bouvier of EDF [13]. The dilution used was intended to be representative of existing environments in flow restricted areas and consisted of deionized water with additions of sulfuric acid and caustic soda to achieve pH between 5 and 9.5 at 320°C and sulfate concentrations of 100ppm, 5000ppm (0.05M), and 57000ppm (0.6M). Alloy 800 exhibited IGSCC / IGA in the 0.05 and 0.6 M sulfate solutions at pH=5, while no damage was observed at pH 6. Cracking was only observed in slightly polarized specimens (+100mV/Ecorr), contrary to what was observed in caustic media [14], where copper significantly increases the aggressiveness of the solution for alloy 800. Results obtained by Westinghouse also indicate that the presence of 0.6M sulfate concentrations has no accelerating effect on the stress corrosion cracking of alloy 800 at pH 6 to 8 at 320°C, while it has a significant effect at pH 5 [15]. In summary, one would only expect a significant contribution of sulfur species to the cracking detected in an acidic pH medium, which if present should have been identified by the presence of chromium-rich oxide layers in the corroded zones or some type of damage, as dissolution of material in typical of acidic media [16].

The results of the EDX analyses and Auger analyses carried out on the fracture surfaces of cracks A and B allow us to infer the medium in which the cracks have been formed. For this purpose, the concentration profiles obtained on cracks A and B with those obtained on samples tested in the laboratory were compared (**Figure 21**). In acidic media, a first layer rich in iron and nickel is observed, which is identified as a layer deposited on the specimen during cooling, and below it a layer with enrichment in chromium and depletion in nickel and iron is visible. In caustic media, an oxide

#### **Figure 21.**

*Auger analyses. Concentration profile of deposits formed on the external surface of a C-ring specimen tested in acid sulfates at pH =4 at room temperature and in caustic media.*

layer with clear nickel enrichment, slight chromium enrichment in its outermost part, and iron impoverishment is observed.

**Figure 22** shows the concentration profiles obtained on the crack fracture surface formed in an Incoloy 800 tube tested in a "model boiler" in the laboratory, in acid sulfate media, with pH=4, at room temperature.

If we compare these results with the concentration profiles obtained at the bottom of cracks A and B (**Figure 21**), we observe that the main difference between these curves and those of the laboratory sample is the existence of nickel enrichment in the latter, which, as shown in **Figure 23**, is the typical trace left by alkaline environments.

Therefore, we can conclude that the cracks have been formed in a neutral or slightly alkaline environment. Alkaline environments with pH> 10 can be discarded due to the presence of silicates. Sulfur presence would suggest acidic environments, although this evidence is not fully conclusive in all performed analyses. However,

**Figure 22.**

*Auger analyses. Concentration profiles of the oxides at the bottom of a crack in an Incoloy 800 tube were tested in a model boiler in acid sulfate media.*

**Figure 23.**

*Concentration profiles obtained in the segment of the tube on the fracture surface of cracks A and B in the area near the bottom of the crack.*

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

since at pH =5, sulfates considerably increase the susceptibility of alloy 800 to IGSCC, although in significant concentrations, we cannot rule out the widespread presence of sulfur in the deposits and oxide layers on the external surface and inside the cracks did not enhance the stress corrosion cracking process to some extent. In addition, it must be considered that, as a consequence of the expansion tube and shot peening process, the susceptibility to SCC in the expansion transition zone would have been higher than the tube-free zone contributing to an increase in the denting mechanism [17].

Regarding sludge characterization of particles collected on the tube-sheet as a result of secondary water chemical used, it should be noted that a root or contributing cause for the degradation mode by denting is produced by magnetite deposition over the tube-sheet surface (known as steam generator fouling). Flow accelerated corrosion (FAC) is the main source of iron from carbon steels and low alloy steels, which causes deposition in the secondary cycle. FAC is an electrolytic process where a mass transfer is produced between oxide/fluid interfaces. The most dominant variables are temperature, fluid velocity, fluid pH, the water amine, oxygen content, steam quality, void fraction of the fluid, piping geometry, and the pipe material composition [18]. Deposition process can be approached as a chemical and physical process where crystallization mechanisms and adhesion of solid particles to the surface are carried out. Soluble iron is deposited on the surfaces when the solubility of the iron decreases. It has been suggested that soluble iron can bind magnetite particles (consolidation) and reduce their re-entrainment [19, 20].

Magnetite solubility is very strongly dependent on both pH and temperature (**Figure 24**). For 300°C there is a minimum in solubility (about 510–9 M) found at about pH300=7. However, the solubility increases one order of magnitude with the change of one unit of pH at 300ºC in both directions [21]. As a rule, the magnetite solubility presents a minimum value for each temperature and pH, diminishing for lower temperatures and higher pH. This is why efforts to avoid the deposition of magnetite by ETA injection before shutdown and, thus, reduce the magnetite solubility and FAC. Obviously, if the amount of iron in solution is high the magnetite deposition will occur during the shutdown, especially in the cold legs. Magnetite layer is formed by a competitive process between deposition and solubility due to the small variations around this minimum of the solubility.

A protective layer of magnetite can increase during normal operation of the plants, by these previous arguments, up to a certain thickness from which magnetite layer spallation is produced. This would explain the structure found in samples named magnetite layered where alternate layers are formed during the oxidation process of low carbon steels (tube sheet). A first oxide layer is initially composed mainly of iron and oxygen and other alloying elements that depending on their different diffusion coefficients will form enriched layers in the oxide microstructure. The thickness of this first oxide layer can increase during operation up to a limit depending on several factors: temperature, environment, and time. This process will lead to a rise in the stress between a metal surface and oxide layer. When the stress in the scale increases to the limit accommodated by elastic strain, it will deform or fracture. The spallation of the scale removes its protective function allowing direct access of the environment to the metal beneath and leads to a rapid increase in oxidation forming a second oxide layer. During the oxidation, the stresses can grow due to the different specific volumes of the oxide and the volume of the metal being consumed in the reaction, which can also be influenced by specimen geometry. Moreover, thermal stresses caused by differential thermal expansion or contraction in the oxide and the substrate during temperature change can also lift the oxide layer. FFA additions can increase the

#### **Figure 24.**

*Calculated solubilities of magnetite as a function of pH over the temperature range 25°C–300°C. p{H2} = 1 atm [20].*

spallation process due to hydrophobic behavior over the low carbon steel surface. This is one of the main reasons why the magnetite layered samples represent the highest fraction of the deposits collected.

Low Si/Al ratios have been associated with an increased risk of degradation of Alloy 600 MA tubing but no evidence for Alloy 800 tubing because this alloy is much more resistant to SCC. However, binding agents may promote the sludge consolidation, particularly in the "collars," increasing the concentration of minor species as potential impurities for the tubing degradation [10]. On the other hand, metallic inclusions embedded in the collar matrix have been attributed to denting process when they are oxidized, although not all inclusions detected were associated with alloys with a low resistance to corrosion. Some metallic inclusions came through stainless steel or stellites. Moreover, scales formed during operation on the external tubing surface are composed of a silicate inner layer in contact with the tube surface and a magnetite outer layer with high porosity. In this way, magnetite is always present in the composition of the different types of collected samples in the sludge and its consolidation.

As is seen in the previous figures, the formation of the "collars" depends mainly on the silicon and aluminum concentration. These elements are initially in form of

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

colloids or aggregates of colloids and they may be oxides, oxyhydroxides, or mixed. The size of the colloids depends mainly on the environmental conditions: pH and temperature. Colloids with a large surface area produce a strong interaction with surface oxides, such as tube-sheet or SG tubing. This interaction is subjected to two main types of forces: "van der Waals" and "electrostatic repulsion" due to the presence of their surface charges. There are several theories that exist to explain the interaction between colloids and the coolant ingredients, or otherwise, the sum of the van der Waals and electrostatic interaction potentials between particle pairs. This mechanism forms the basis of the well-known Derjaguin–Landau–Verwey–Overbeek (DLVO) theory for colloid stability [22, 23]. The final result of this mechanism is a consolidation process of the deposits over the tube sheet that leads to the formation of hard sludge or "collars," which are responsible mainly for the mechanism of denting.

Since denting needs a sludge accumulation plus contaminants and ODSCC is closely related to that, some strategies of NPP are focused on: a) Reducing the contaminants intake into SG; b) Decreasing the entry of erosion-corrosion products into SGs by means of FFA injection before shutdown and ETA injection during normal operation; c) Removing the accumulated sludge, especially hard sludge from the SG's during outages by lancing and inner bundle lancing.

On the other hand, chemical cleaning process is used to remove magnetite from the gtube scale and hard tube-sheet deposits accumulated on the tube-sheet surface. However, a high concentration of silicon and aluminum was found, due to the higher temperatures at the interface of the collar and tube or tube sheet precluded the removal of the hard sludge with chemical cleaning at the dented areas in the hot leg sludge piles. Moreover, dissolution rates vary with the surface area of the deposit available to the solvent and, therefore, tube-sheet and crevice deposits will dissolve slowly if the surface area deposits are low [24].
